# Amps, Watts, Volts - how you figure?



## LastMinuteMarvel

I need to figure out how many fixtures I can plug into each channel on my dimmer packs, and what my minimum amp/watt requirements are for purchasing new dimmer packs. If you can help by providing a formula for calculating these things, that would be great, as well as some guidance on where to find the numbers I need for the calculations on my fixtures.

I currently have some elation DMX-20L dimmer packs. They say 10A per channel, total 20A max.

As for fixtures, I have some ETC S4 Pars, Par 64's (I think - can't find any information stamped on the light anywhere) and some old Altman Fresnels. I've opened them up to find a variety of voltage and wattage numbers on the bulbs themselves, but I don't know what they mean, especially since there is more than one watt rating inside the par 64's.

BTW, this is my first post here and I am hardly a lighting expert. I am a second year middle school drama teacher and the closest I came to lighting a show before this was hanging some lights for college credits. Thanks in advance for any help you can give!


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## MarshallPope

The formula for this is Watts=Volts x Amps. In most cases when using basic building power supply, you will keep Volts at 110. There can be different wattages of your fixtures depending on the lamp used. Your S4s are most likely 575 Watts, but the wattage should be printed on any lamp you are using.


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## xander

Ohm's Law: V=IR, where V is voltage in volts, I is current in amps, and R is resistance in ohms.

and

Power formula: P=IV, where P is power in watts, I is current in amps, and V is voltage in volts.

should get you started. As far as how many units you can put on a dimmer, if the information you have is correct (10A per channel, 20A per pack), and assuming you are at 120VAC, then you can have up to 1200w per channel up to 2400w total. This means that you can put 1200w on channel 1 and 1200w on channel 2 (although I wouldn't recommend it) but then you cannot use channels 3 or 4 because you have maxed out the total capacity. Or you could put 750w on each of the first 3 channels and that would leave 150w for channel 4 (in theory, in actuality, I would try to avoid putting more than 2kw on a single pack). This also assumes all of the components in the pack are actually rated for 20A. Most of them aren't. They say 20A maximum but only install a 15A connector and cheap internal components that don't stand up to it. But even if your pack is ready for a 20A load, you still have to make sure it is plugged into a 20A circuit. 

If you ask more specific questions then we could probably be of more assistance.

Welcome to CB!
-Tim


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## LastMinuteMarvel

Thanks, that gets me started. So if I bring up 4 S4s at the same time, that is (575*4=) 2300 watts. And if the dimmer pack is 20A total max, then that means it can handle (20A*110V=) 2200 watts. Is that going to blow a fuse or the circuit breaker, trying to run 2300 watts of light with a pack rated for 2200 watts? Is that how it works?

And, if I'm considering purchasing some dimmer packs that are only rated at 5A per channel, that's 550W and not enough to run a single S4 at 575W, correct? Can I avoid the overload by only running the fixture at, say 90%? Does that decrease the actual wattage that the light needs?

thanks again!


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## LastMinuteMarvel

xander said:


> Ohm's Law: V=IR, where V is voltage in volts, I is current in amps, and R is resistance in ohms.
> 
> and
> 
> Power formula: P=IV, where P is power in watts, I is current in amps, and V is voltage in volts.
> 
> should get you started.
> 
> If you ask more specific questions then we could probably be of more assistance.
> 
> Welcome to CB!
> -Tim


 
That is very helpful general information. I'll have to get the exact specs on my different fixtures so I can ask more specific questions when I need to. I appreciate the caution about maxing out the channels. These cheap dimmer packs are almost certainly not built with the highest quality components.

Thanks!


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## Les

LastMinuteMarvel said:


> Can I avoid the overload by only running the fixture at, say 90%? Does that decrease the actual wattage that the light needs?


 
Unfortunately, it doesn't quite work out that way. I don't remember the exact threshold, but if you have a 500w fixture, it will generally consume 500w well before you get to 100% output (more like +/- 50%, and then you have to deal with everything looking dim due to amber drift. For safety and reliability, I don't recommend this practice). 

With the Elation dimmers, I am skeptical that they could handle anywhere near the 2200w cited. I wouldn't load them down with anything higher than 500w/channel. They don't like to be within throwing distance of their rating-at all. 

On your par cans, to find the size, measure them. If the can is 8" diameter, it is a par 64 and will run either 500 or 1,000 watt lamps. Par 56's are 7" diameter and can be lamped from 300-500w. The Altman fresnels can be 500 or 750 watts if they're sheet metal. The diecast models will accept up to 1,000 watt lamps. I have come across unmarked lamps before, so if you want to be _sure_ of your wattages, you might just want to do a relamp. It sounds like you have a relatively small space, so you don't likely need a lamp higher than 500w in your fresnels (or par cans, for that matter). Run all Source Four instruments at 575 (or even 375 if you can) and you're in a decent position for negotiating with most shoebox dimmer packs.

Voltages generally shouldn't matter if you're using off-the-shelf dimmer packs. You're dealing with 120v, so all you really have to worry about is your wattages. With these packs that say "20A total per pack", it is best to not run above 1800w total. These aren't exactly rated for continuous duty at maximum output (as I hinted at above), so take 80% of their total and try not to exceed that. I tend to err on the conservative side and say that "10A=1,000w; 15A=1500w". The real numbers are a little more liberal, but under estimating is better than over estimating in this case .


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## TimMiller

The trick with theatrical fixtures is that most are not set to just one lamp. It will hold only one type of lamp but you can get that lamp in different wattages and styles (styles applies to par lamps, you have very narrow "VN", narrow "N", medium "M", and Wide "W") now you cannot put in a lamp that exceeds the maximum printed on the fixture. Also the rule of thumb I always go by is that a 20 amp circuit is equal to 2400 watts. You need to leave some headroom though. Most standard magnetic circuit breakers will not run all day at their maximum wattage they will heat up and eventually trip, normally at the worse times. You can safely put 3 575 lamps on a circuit or 2 750's I have not had a problem running 2 1000w fixtures on a circuit as long as the cabling can safely handle the load. Also keep in mind most cheap dimmer packs and lights for that matter are NOT UL LISTED and fire marshals do not like that. Also the elation dimmer packs are some of the better ones. I like the other versions better that are 6 channel 2400 watts per every 3 channels Max and they can support 20 amps a channel if needed but you could only use 2 channels. Also keep in mind when they wire buildings there is usually more than one plug on a circuit so finding separate circuits gets tricky which is normally done by trial and error and lots of resetting breakers.


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## DuckJordan

Les said:


> Unfortunately, it doesn't quite work out that way. I don't remember the exact threshold, but if you have a 500w fixture, it will generally consume 500w well before you get to 100% output (more like +/- 50%, and then you have to deal with everything looking dim due to amber drift. For safety and reliability, I don't recommend this practice).
> 
> With the Elation dimmers, I am skeptical that they could handle anywhere near the 2200w cited. I wouldn't load them down with anything higher than 500w/channel. They don't like to be within throwing distance of their rating-at all.
> 
> On your par cans, to find the size, measure them. If the can is 8" diameter, it is a par 64 and will run either 500 or 1,000 watt lamps. Par 56's are 7" diameter and can be lamped from 300-500w. The Altman fresnels can be 500 or 750 watts if they're sheet metal. The diecast models will accept up to 1,000 watt lamps. I have come across unmarked lamps before, so if you want to be _sure_ of your wattages, you might just want to do a relamp. It sounds like you have a relatively small space, so you don't likely need a lamp higher than 500w in your fresnels (or par cans, for that matter). Run all Source Four instruments at 575 (or even 375 if you can) and you're in a decent position for negotiating with most shoebox dimmer packs.
> 
> Voltages generally shouldn't matter if you're using off-the-shelf dimmer packs. You're dealing with 120v, so all you really have to worry about is your wattages. With these packs that say "20A total per pack", it is best to not run above 1800w total. These aren't exactly rated for continuous duty at maximum output (as I hinted at above), so take 80% of their total and try not to exceed that. I tend to err on the conservative side and say that "10A=1,000w; 15A=1500w". The real numbers are a little more liberal, but under estimating is better than over estimating in this case .



Sorry but practical experience has proven the whole running dimmers only at 90% still uses the full amount as being false. I would also like to point out running all the fixtures at 80% is going to also help you with not blowing the internal fuses on those packs as well as possibly helping with making sure you don't blow the breakers.

Les think about it this way, You have two constants in the formula, Voltage and Resistance. If neither of those change for dimming then you are going to be using watts. Lower watts = lower amps in this case


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## rochem

Les said:


> Unfortunately, it doesn't quite work out that way. I don't remember the exact threshold, but if you have a 500w fixture, it will generally consume 500w well before you get to 100% output (more like +/- 50%, and then you have to deal with everything looking dim due to amber drift.


 
Sorry, but that's just not true. Assuming a perfect world, if a lamp is rated for 500w at 120V, and you feed it 120V exactly, it will consume exactly 500w. The complication that can arise is that dimming a load does not decrease the supplied voltage in a linear fashion, so dimming a 120V line to 50% will not result in exactly 60V being sent to the lamp. The only way that the lamp will consume more than the rated wattage is if the supplied voltage is higher than the rated voltage of the lamp - for example, if our 500w 120V lamp were supplied with 125VAC, it would actually consume about 520w. 

This is important to remember when you're trying to put four Source Fours with 575w/115V lamps onto the same 20A dimmer - if the dimmer is run at full and each lamp is receiving 115V, you will be drawing exactly 20A on the circuit (575w*4=2300w, 2300w/115V=20A). If the mains voltage is any higher than 115V, let us say 118V for example, assuming a constant resistance, you would now be drawing about 20.5A, exceeding the breaker specification of 20A, and each 575w lamp would be consuming around 605w of power.

However, Les is absolutely right in that you should not keep your lights dimmed to avoid exceeding the breaker capacity. Even if you were able to do it correctly, there's always the potential for someone else to run the lamps at full, and besides the safety factors associated with intentionally overloading a circuit breaker, it's just a bad habit to get in to.


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## Les

DuckJordan said:


> Sorry but practical experience has proven the whole running dimmers only at 90% still uses the full amount as being false. I would also like to point out running all the fixtures at 80% is going to also help you with not blowing the internal fuses on those packs as well as possibly helping with making sure you don't blow the breakers.
> 
> Les think about it this way, You have two constants in the formula, Voltage and Resistance. If neither of those change for dimming then you are going to be using watts. Lower watts = lower amps in this case


 
Well, I didn't say you couldn't; more like you _shouldn't_. Yes, you can make a little more "headroom" by lowering your levels, but the exact threshold you need to avoid is a huge trial and error exercise to find, and can be a very slippery slope, often resulting in lots of blown fuses and potentially damaged components. A dimmer set at 90% intensity doesn't mean it is supplying 90% of the power demanded by the fixture. It is a non-linear relationship.

Source:
http://www.controlbooth.com/forums/lighting-electrics/9449-dimmer-profiling-avoid-over-wattage.html

More related info:http://www.controlbooth.com/forums/lighting-electrics/17525-power-realities.html



I think the verdict was "lamp down or don't do it". It's a very risky game to play.


rochem said:


> Sorry, but that's just not true. Assuming a perfect world, if a lamp is rated for 500w at 120V, and you feed it 120V exactly, it will consume exactly 500w. The complication that can arise is that dimming a load does not decrease the supplied voltage in a linear fashion, so dimming a 120V line to 50% will not result in exactly 60V being sent to the lamp. The only way that the lamp will consume more than the rated wattage is if the supplied voltage is higher than the rated voltage of the lamp - for example, if our 500w 120V lamp were supplied with 125VAC, it would actually consume about 520w.



This is pretty much the point I (not-so-eloquently) tried to make. Hey, I said I didn't know the exact numbers! Guess I shouldn't have made some up .


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## DuckJordan

Les said:


> Well, I didn't say you couldn't; more like you _shouldn't_. Yes, you can make a little more "headroom" by lowering your levels, but the exact threshold you need to avoid is a huge trial and error exercise to find, and can be a very slippery slope, often resulting in lots of blown fuses and potentially damaged components. A dimmer set at 90% intensity doesn't mean it is supplying 90% of the power demanded by the fixture. It is a non-linear relationship.
> 
> Source:
> http://www.controlbooth.com/forums/lighting-electrics/9449-dimmer-profiling-avoid-over-wattage.html
> 
> I think the verdict was "lamp down or don't do it". It's a very risky game to play.


 
I will agree with that, Can't tell you how many times I've had to reset breakers because of little shoe box dimmers accidentally placed on the same circuit.


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## David Ashton

One thing you absolutely cannot "assume" is a constant resistace, the resistance in a theatre lamp increases by a factor of 10 from cold to hot, this is a exerpt from an old thread
"Please consider, a 1000 watt 240 volt T19 on a dimmer{for 120v thinkers just double the current on all the figures} It has a cold resistance of 3.8 ohms and a hot resistance of 57.6 ohms. A typical dimmer will feed 1.65v at "0" and drive .35A putting 2.24 W into the lamp of preheat.
@ 13V you can see the barest glow in the filament and are drawing .96A and 12.6W
@30V you can see a real glow and are drawing 1.5A and 45W
at 60V you begin to get some output but at 1/4 voltage you are drawing 2A or nearly half the "full" current
@80V which is 1/3 voltage you are drawing 2.3A which is more than half the full load current and is also the point at which the dimmer starts to interact with the other dimmers on other phases.
This is why running all your dimmers at 1/3 is the worst thing you can do to your neutrals."
.
This is why you cannot oversimplify wattage/voltage calculations, they become totally meaningless.


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## church

Two replies have made comments about the dimmer packs available to the OP which are personal opinions and as such are valid but can be mis understtod by others. Specifically about the capability/quality of the components and not being listed.

First the Elation pack is UL listed, it is not cUL listed and its use in Canada requires the appropriate provincial electrical field approval. 

I have repaired a number of shoebox dimmers by Elation, NSI etc. including the DP20 and it is worth mentioning that the supply cord is 12 gauge even though it has a standard 15A U ground on the end. The PCB tracks are the correct width and thickness to handle a load of 10A and the TRIACS, opto isolators and other components are from reputable manufacturers. As with any equipment at this end of the market the biggest problem is that it often ends up being used by people who do not know the limitations - this does not mean the equipment is "bad". The original OP took the correct approach by coming to this site to ask questions to help understand how to use the equipment.

These dimmers work well provided: the total load is kept within the capability of the supply and the maximum load is kept below 1000W on any channel. In practice this means if you have a dedicated 15A supply you can operate simultaneously: three channels with a 500W load on each, or one channel with a 500W load and 1 channel with a 1000W load or three channels with a 575W load or four channels with a 375W load or two channels with a 750W load on each. With a dedicated 20A supply you can operate simultaneously: 2 channels with a 1000W load on each, or four channels with a 500W load on each or four channels with a 575W load on each or four channels with a 375W load on each or three channels with a 750W load on each. there are other lamps options but the OP was asking about specific fixture options.


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## JD

So, when is 10 amps at 120 volts NOT 1200 watts? When you are talking about shoebox dimmers!  

Here's something you will not find in any book. If a manufacturer rates their dimmer at 1200 watts, you can be pretty sure you could safely put a 1000 watt lamp on the dimmer with no problem. If the manufacturer rates their dimmer at 10 amps, don't be so sure! Shoebox dimmers rated at 10 amps usually use 15 amp triacs, so there is a chance the inrush will take the triac out before the fuse has a chance to blow. The fuse is there more to protect against a fire than protect the circuit. If a dimmer is rated at 1200 watts, it's a good indication that it is equipped to handle the inrush generated by a 1200 watt load. I am going to use Elation as an example, but this is true across many brands, I am not singling them out. Their 10 amp dimmer does indeed use 15 amp triacs, but their 12 channel 1200 watt (per) rack dimmer uses 40 amp triacs. 

The shoebox packs that are 10 amps per channel, 20 amps per pack should be thought of as 600 watt dimmers, up to the limitation of the pack. If the pack is equipped with a 12/3 power cord, you could probably load four 575 watt S4's on it. If it is equipped with a 14/3 power cord, three is the limit. 

Most of these dimmers are more for DJ work, where the load duty cycle is lower. In theater, you always have to think about the consequences of all channels being on for a sustained period.


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## Blackfaer

There's also the matter of power supply and over-current protection. Depending on the brand and model, and depending on how the dimmer is tied into the power supply (via an outlet on the wall, 2 edison plugs or 1 3-phase twist lock, etc) you will frequently run into a problem with running the maximum rated wattage at full power over time.

Trying to keep it simple so as not to confuse schoolteachers who might be feeling overwhelmed..

A traditional theatrical dimmer tied directly into the main power supply is protected with circuit breakers that will trip only when the amperage/wattage exceeds the maximum. So assuming 120v, a 20amp dimmer (20amps x 120v = 2400watts) will trip when more than 2400 watts is drawn through the breaker. And it will stay on unless that limit is exceeded, indefinitely. 

A typical household or commercial circuit breaker for branch circuits, like what you plug your computer into, is rated to trip when the heat increases too much that it becomes dangerous. That heat is generated by power being greater than that 20amp limit. BUT household breakers are intentionally more cautious - and will trip eventually if you're only running, say, 2100 watts, but running a full 2100 for some time, and it's a warm day to begin with.

If your shoebox dimmer is rated for, say 15amps at 115-120v, but is plugged into a standard edison-style receptacle, you may cause your breakers to trip with only 3 source four units; it just might not happen right away. If it's for 20amps and is plugged into 2 separate circuits, you can probably run 3 lights without trouble, but a fourth would be a problem.

If it's plugged into a large twisting plug that has 5 pins on the plug, or plugged with two edison plugs into 2 separate circuits, you're probably safe to run right at the maximum rated wattage. Unless you can measure your voltage at the place the dimmers plug in, it's usually best to use 120v to calculate your total amperage for determining if you're over or not; it's also always good to have a little headroom as others have mentioned - the more inexpensive the dimmer packs are, the more headroom you'll want to plan for.


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## JD

Proper dimmer protection is quite a design challenge. For example, on a 20 amp circuit, we want a breaker to trip if there is a sustained draw above 20 amps. That is the easy part! The hard part is getting a breaker that will not trip during the inrush over-current of a 2000 watt load starting up, but will quickly trip if an unrestricted surge (such as a shorted cord) occurs. The ultimate breaker would have a hard kickoff curve the was somewhat under the device failure rating, but would somehow quickly detect a sustained flow of 21 amps. We don't have that yet, but they are getting closer!

In the "old" days, when SCR dimmers first came out, a 2.4kw dimmer would have a 20 amp slow response breaker as well as a fast response 35 amp rectifier fuse. (Sometimes called "silver sand" fuse.) These would be wired in series. (The old TTI packs come to mind.) The breaker would trip if the circuit was loaded with more than 2.4kw (after awhile) but the fuse would pop if a dead short was attached. 

Advances in SCR, Triac, SCR and IGBT design have allowed these components to handle more abuse in modern dimmers. The secondary fuse has disappeared into memory, and modern dimmer breakers have a trip curve that is closer to the fantasy curve in the first paragraph. All this can be said about modern professional dimmers (such as ETC) but does not apply to the "shoebox" dimmer. 

Shoebox dimmers find their way into stage work as a handy way to grab an extra channel in an emergency or temporary situation. Very few have a UL rating, and in many venues are banned from stage use. Most all use a "GMA" type fuse. As I stated in an earlier post, this fuse may be effective in preventing a fire, but are poor at preventing internal component failure. Often, these dimmers are listed at "10 amps per channel." This is more of a current limiting rating and should not be read or factored to a working wattage rating.


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## chausman

Just want to say thanks to everyone who has posted in the thread!


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## MNicolai

> Sorry but practical experience has proven the whole running dimmers only at 90% still uses the full amount as being false. I would also like to point out running all the fixtures at 80% is going to also help you with not blowing the internal fuses on those packs as well as possibly helping with making sure you don't blow the breakers.




> Sorry, but that's just not true. Assuming a perfect world, if a lamp is rated for 500w at 120V, and you feed it 120V exactly, it will consume exactly 500w.
> 
> However, Les is absolutely right in that you should not keep your lights dimmed to avoid exceeding the breaker capacity. Even if you were able to do it correctly, there's always the potential for someone else to run the lamps at full, and besides the safety factors associated with intentionally overloading a circuit breaker, it's just a bad habit to get in to.



Les is more correct than you give him credit for. The relationship between power consumed and the dimmer level is not at all linear. Derek ran an experiment on a shoe box dimmer awhile ago and showed that 60% dimmer intensity was still 91% voltage getting sent to a 575w HPL lamp.

At one point, I took an entire rig of 50 S4's on a 48-dimmer ETC Unison rack and put an ammeter on the feed going into the dimmer rack and received pretty much the same results as Derek, showing that the last big chunk of intensity represents only a very small portion of difference in power.


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## LXPlot

MNicolai said:


> Les is more correct than you give him credit for. The relationship between power consumed and the dimmer level is not at all linear. Derek ran an experiment on a shoe box dimmer awhile ago and showed that 60% dimmer intensity was still 91% voltage getting sent to a 575w HPL lamp.
> 
> At one point, I took an entire rig of 50 S4's on a 48-dimmer ETC Unison rack and put an ammeter on the feed going into the dimmer rack and received pretty much the same results as Derek, showing that the last big chunk of intensity represents only a very small portion of difference in power.


 
That would make sense. I was at a venue once where a guy who was using shoebox dimmers with an Express who had written dimmer profiles to try and counteract this (or I assume that's what he meant, I didn't get it precisely at the time). This seems like a little bit of overkill because the actual output seems more closely related to the level than the electrical output. Although that could be self fulfilling prophecy.


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## JD

Yes, dimmer curves are designed to give you the "perceived" effect that the light is increasing brightness in a liner fashion. Since lamps are anything but liner, the dimmer curve needs to be an inverted version of the lamp gamma. As a result, dimming to 80% has nothing directly to do with what power is being delivered to the lamp. (there are some conversion charts on another thread.) Lamps gain in brightness quickly as they near the rated line voltage, therefore there is less of a voltage output change than one would expect coming from the dimmer at higher settings. It is also why there is voltage regulation built into better dimmers, as sag is more noticeable at higher settings. (Not to be confused with the hysteresis sag that occurs in "wall" dimmers at lower settings.)


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