thank you all for the responses,
i took a couple hours going
circuit box hunting and found the upstream box
it contains a large 400 amp
breaker, and its a labeled three
phase 4
wire box (more modern than my
dimmer system though).
I know that i have drawn over 500 amps previously, so i suspect 400 amps refers to the capacity of each
leg. The lower
rating on the actual
dimmer rack leads me to believe that the
breaker has been replaced and was replaced with one of a higher wattage upon later inspection
You most likely did not actually draw 500A continuously for a long time. Even at an overload condition of 500A, a 400A
breaker will not trip instantaneously (only a
short circuit will cause that). Perhaps you had 500A connected load, but never ran everything at full for a significant time.
Unless really old, the actual
breaker trip
rating should be indicated on the
breaker handle (often dust covered) or on a sticker, which you might not see w/o your electrician taking the enclosure
cover off. Or you may see a model number. Ratings on the enclosure may be less reliable. Again, use your electrician to investigate this.
The 400A means each
phase is rated to handle max. 400A. If any
phase draws over 400A (+/- tolerances), the
breaker will eventually trip. It is really not useful to think you have 1200A (400 x 3) here to work with, even though you sort of do... You have a 3-phase
system designed to carry a max of 400A on each
leg. You can't put 600A on one
phase and 300A on the other two...
Further, if anyone replaced a
breaker with a higher rated
breaker, this should not have been done w/o also increasing the
wire size commensurately.
I'm still not
clear if your DR is rated 900A or 300A per
phase. The way equip. is rated now days, it would almost always mean 900A per
phase. If it is only good for 300A per
phase, then protecting it with a 400A
breaker or anything more than a 300A
breaker is a code violation and unsafe condition.
on further thought, i still have a remaining question
the
breaker had a 400 written on the lever, and the box was labeled 4
wire, 3
phase. Additionally, the labeling paper said
Phase 3 followed by a greek letter (i believe it was phi). Does 400 refer to the total capacity of all phases at 120 Volts, the total capacity of all phases at 208 volts, or the capacity of each individual
phase at 120 volts?
note that this entire box was wired in 3
phase.
Greek "phi" (circle w/ slash) means "
phase". 4-wire means you have a
neutral in addition to the three
phase wires, so you can connect things (like dimmers) up 120V or 208V. (A three-phase, 3-wire
system could only be used to connect at 208V, and might be used for mechanical equipment or an elevator). No need to carry the
neutral for them.
For me at least, capacity is best thought of in terms of KW or
KVA, because KW and
KVA can be directly translated across different
system voltages and phases, whereas amps cannot.
You seem to have 400A at 120/208 volts, 3-phase, 4-wire. Total capacity per previous post calc. is 400 x .36 x .80 = 115
KVA (80% de-rating required to allow for possible continuous loads). To get to KW (so we can talk dimmers), assume .90
power factor, which gives total 104KW or 35KW per
phase. You have 104KW to work with, regardless of
voltage (e.g. 120V dimmers on 208V, 3-ph.
system)..
If you have 50A dimmers connected at 120V, 50 x .120 (remember single-phase now, no sqrt 3!)) = 6.0. These are 6KW dimmers. The
system could handle 5 x 6 = 30KW, or five 6 KW dimmers on each
phase.
With five 6KW dimmers on each
phase, you have total 90KW used of available 104KW on your DR
feeder. This allows for running all dimmers at full, or the connected load. Typical theater will not run everything at full, so you may be able to increase the connected load, but now we're getting into a
bit more serious engineering...
Ok, to get back where we started, how many amps does the 90kW draw? Two ways to figure- As a 3-phase system- 90 / .36 = 250A per
phase, out of max. 320A (400 x .8). Or look at it as three 1-phase systems- each 30 / .120 = 250A (again per
phase). Three wires, with each carrying 250A. But don't think of it as 750A, or that you need an 800A
breaker.
Better quit now, I think I may be starting to go in circles... Hope this helps.
The above being hypothetical, as always, seek the services of your local engineer and electrician for the specifics of your
system.