Elsewhere you'll find similar logic. Dryer, stove, welding, truck outlets are usually 240V, 2
phase or
bi-phase, 2 hot legs of opposite waves with a
neutral. When you look at the breakers they are joined so one
tripping takes the whole
circuit down. Therefore the size of a hot
leg is far more important than the grand total, which relies on perfectly balancing the legs.
"
Load Diversity" is an important concept here too. It's the idea that you'll never fill those dimmers and run them all up to max. Again the idea is common. Standard duplex outlets have 2 15A rated receptacles and there are often many of them on a single 15 or 20A
breaker. Don't use too many
power tools at once!
@brucek mentioned
headroom which is also critical to
power calculations. 80% of the rated supply is a common
safety factor. Before going over that you should know a lot more than the nominal load.
Bi-phase? Two
Phase? There was such a thing as 2
phase power (there was also DC service on Broadway) but I'm pretty sure there are only a couple of plants to produce that service.
In small commercial and residential occupancies we have "240/120 Volt
split-phase" service, The
neutral is taken from the center tap of the
transformer's 240V secondary (and bonded to grounding electrode
system). The
neutral carries the *imbalance* of load between the L1 and L2.
Three
phase Wye service has the same
neutral behavior - the imbalance of
current between L1, L2, and L3 flows on the
neutral. {optional story} As
arena electrician 20+ years ago I was hooking up services for a
holiday tour's LX. The show ME mentioned that the 100% moving light design was load balanced and we'd see only a couple of Amps on the
neutral, caused by the load of
FOH console and misc on L1. Bored and challenged, I put the clamp
ammeter around the
neutral and measured more than 2 amps (the exact number now escapes me, but ~8 amps comes to mind). This is both curious and alarming, because if the show loading is genuinely balanced, either a load is mis-patched or there is a compromised
conductor/connection. I measure L1, L2, and L3, and L2 is pulling more
current than the others. I go find the show ME and tell him what I've found. He's not calling or operating, so we follow the
feeder from
switch to dimmers/
distro. We find a hot CamLok in the
feeder run that's noticeably warmer than its black, blue, white and green friends. The ME tags it "reterminate" with white
gaff tape. Had he not mentioned his balanced loading and my
bit of boredom, it would likely have gone undetected until it became a really hot, smelly mess. {/optional story}