1 when referring to service sizes or other wiring the amp rating is the currenty capacity of each conductor. The voltage on those conductors is an independent matter. Don't confuse the two.
2 So if you have a three wire, 120/240V 400A service, you nominally have three conductors each with a capacity of 400A. Since it is 120/240V, you have two hot conductors 240V apart, and one neutral conductor which is grounded and 120V away from either hot.
3 With that you could power two 120V, 400A loads ... that would draw less than 400A on any of the three conductors.
4 So, for example, with 800A of 120V loads split evenly between the two hot legs of the service, the only way to get 400A on the neutral is if you turn on all the loads ...
5 The power company is in charge of the wiring before the service point,
6 they will use a smaller transformer than the nominal service. A 400A 120V/240V service will not have a 100 kVA transformer feeding it,
7 POCO transformers are sized to be occasionally overloaded for short periods of time, they can handle it.
Howdy Wayne, thanks for jumping in. I must not be expressing myself correctly- we seem to be agreeing here, mostly.
When you say "POCO in charge up to service point", you mean the LV side of the xfmr, correct? Just checking.
I'm really thinking that two 400A loads is counted as 800A.
3 "you could power a single 240V, 400A load, or
two 120V, 400A loads" ...
2 "you nominally have three conductors
each with a capacity of 400A"
Don't you mean 400A capacity *between* the 3 wires, as in a limit?
So if you have 400A + 400A of 120V sharing the same neutral...that's either 800A on the neutral, or zero capacity left on the neutral, if the 400A is a "limit".
1.
the currenty capacity of each conductor
So... a "400A service", if you count each conductor, I see as 200+200. Because with two 400A 120V conductors (48kVA each), it would be 96kVA coming from the xfmr. It would also be 400A of 240V, because that's also 96kVA.
But a 400A service isn't 100kVA, like you said, it's a 50kVA xfmr! The 50kVA coming from the xfmr comes from two points- each is supplying 120V and 200A, and you can have the two 200A loads (also called 400A) at 120V or a single load of 200A at 240V.
A 400A service supplied through a 400A rated meter main (like zman's) can't supply 120V twice, as in two 400A legs of 120V- there would be 800A going through the 400A meter socket and main breaker.
Or....the 400A main breaker will allow 48kW through, so that's 200A at 240, same as 200+200 at 120V!
If you have 180A of 240V motor loads, or 43.2kW, you only have 4.8kW, or 40 amps at 120V left on that 48kW 400A breaker.
If it was a 96kW breaker, it would allow 800A of 120V through and would have to be labeled as an 800A breaker.
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2 I've got that- the same thing holds for a 3ph wye system, except 3 Ls and an N, the capacity is "shared" between the 4 wires instead of the 3. Correct?
3 Aren't the two Ls of 120V going to the loads "returning" on the same N? So you can have 200A on each L and therefore there would be very little amperage on the N, BUT 400A "available" capacity on N if L1 and L2 at 200A each short to each other?
4 That's where I'm losing you- if you turn on both 400A 120V loads, don't you have 800A on the neutral then?
5 The service point...can be the point where overhead wires are attached to a house, or if underground it would be the....wires coming from the LV side of pad xfmr, or is it the first switch after that for UG?
6 Right.
400A service = 50kVA service xfmr.
200A serv.= 25kVA.
100A serv. = 10kVA.
Right?
7 That has to do with the 3 hour/continuous thing, right? If someone is drawing 100A of 120V, so 12kVA all day long from a 10kVA xfmr, that might be just about the limit.
If someone with 100A service and 10kVA xfmr puts in a 200A panel and starts pulling 150A or 18kVA all day, not so good for efficiency or life of a 10kVA xfmr.
