# Easy Question: wire gauge



## Shawncfer (Nov 2, 2010)

So Im a theatre major. Tech theatre major.. and we got this question on homework. And it looks really simple and I feel really stupid for asking but I just wanted a little help.

*"3. Given a dimmer rated for 1200 Watts, how many 100 Watt light bulbs can be connected to the dimmer using one 16 gauge cable?"*

Now 1200w would already tell me it's gonna be 12 lamps. And 16 gauge cable I think is rated for 10 amps. Right? So 10 amps, 120v then it would still be 12 lamps. Am I right on this? It seems too easy...


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## MNicolai (Nov 2, 2010)

And what if the lamps are rated for 130v and get 115v after a few volts drop across dimmer and the distance from the dimmer to the stage?

For that matter, if you're using 220V, current is half what it would be if you're using 120v.

So what broad assumptions are you allowed to make for the purpose of this problem? That you are getting an ideal 120v out of your dimmer (which is unlikely) and that your cable is infinitely short with zero resistance?

(says the electrical engineering student on his way to a circuit analysis class)


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## derekleffew (Nov 2, 2010)

Shawncfer said:


> *"3. Given a dimmer rated for 1200 Watts, how many 100 Watt light bulbs can be connected to the dimmer using one 16 gauge cable?"*...


Moving the question to QotD. Standard rules apply.

Followup:
What are the applicable rules, and what assumptions need to be made, to answer the question?


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## shiben (Nov 2, 2010)

Well, you need to assume you can twofer some. Otherwise the answer is one, as you only have one cable and one dimmer.


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## headcrab (Nov 2, 2010)

Assuming the lamps are halogen, rated 100W at 120V, that the output of the dimmer is 115V RMS, and the 16AWG cable carries power to all the lamps. I ignored the resistance of the cable, since we don't know its length. The lamps will only draw 92W at full power due to the lower supply voltage. However, 92W at 115V is 0.8A, so you can only run 12 lamps on this dimmer.

This dimmer is capable of dimming 10A, any more could trip the breaker or cause damage.


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## mstaylor (Nov 2, 2010)

shiben said:


> Well, you need to assume you can twofer some. Otherwise the answer is one, as you only have one cable and one dimmer.


And the smart alec in me would require me to answer exactly that. One cable= one light.


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## sk8rsdad (Nov 2, 2010)

The smart alec in me would ask if it's an union house. There are so many ways to answer this question depending on the assumptions. For example, I note the word "safely" does not appear in the criteria. However, Derek considers me a _professional _ so I will refrain from further comment.


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## shiben (Nov 3, 2010)

mstaylor said:


> And the smart alec in me would require me to answer exactly that. One cable= one light.


 
I resemble that remark. However, Im not just being a smart ass. The question specifically says that we could also list assumptions that would need to be made. Now if I was to be a smart ass, I would say you dont even know if the cable and dimmer could be connected (the cable being 2P&G and the dimmer being edison), if they are legal in the jurisdiction, or if local electrical code permits 16 ga. cable to be used to connect such things. At any case, those are also assumptions you need to make (however, ones that would probably get you docked points on the question for bringing up)


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## erosing (Nov 3, 2010)

shiben said:


> Well, you need to assume you can twofer some. Otherwise the answer is one, as you only have one cable and one dimmer.


 
I had a similar question to this in a class once and that was the answer; I felt a tad dumbfounded when I saw that was the answer being sought.


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