sizing a three phase breaker for a single phase transformer

[BrkfldEE]

I have a single phase 10kVA transformer that is fed from two phases of a 480V 3 phase supply. If I want to protect the transformer with a 3 phase circuit breaker, how should I size it? The full load amps of the single phase 10kVA transformer is 20. So do I use 20A as my basis for choosing the 3 phase circuit breaker? Or do I size the 3 phase circuit breaker as if it were feeding a balanced load of 30kVA (10kVA on each phase)?

 

[davidbeach]

You use a 2-pole breaker sized for the load, not a 3-pole breaker.

 

[BrkfldEE]

So I should use a 30A two pole breaker. What if I were to use a 3 pole 30A breaker, would it accomplish the same thing?

 

[xnuke]

See Article 450 of the NEC edition in use in your jurisdiction.

xnuke
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[dpc]

The breaker only "knows" the current in each phase. It has no idea about what is connected. And the current in each pole is monitored individually, even though all three poles are tripped for any fault.

So you use the actual current that will be flowing through the poles of the breaker.

You can generally use only one or two poles of a 3-pole breaker - this is fairly common. In the US, the UL listing for the breaker must indicate suitability for this use. Simplest thing is to check with the breaker manufacturer and get a copy of the instruction sheet for the breaker being considered. It should be covered there.

David Castor

http://www.cvoes.com

 

[itsmoked]

Something not noted here yet is that - of course - the wire size must be sized correctly for the breaker you choose.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rbulsara]

If you do use a 3 pole breaker, I would wire two of the poles in series in one of the legs. The amp rating would not be any different as dpc indicated.

Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

wire two of the poles in series in one of the legs

In general, not a bad idea. In panelboards though it is essentially impossible.

 

[rbulsara]

David, yes.

Rafiq Bulsara

http://www.srengineersct.com

 

[jraef]

The series wiring issue is not universally necessary, in particular it was never an issue for North American breaker designs. The reason it is sometimes necessary is if the breaker has a differential trip mechanism, as is typically used in IEC thermal overload devices to protect motors against single phasing. In places other than North America, breakers are often used as a sort of "manual motor starter" with the thermal trips being adjustable to be set at the motor FLA. That is not allowed on a UL489 breaker, so was never a criteria for circuit breakers in North America. Using only 1 or 2 of the 3 poles is not only permissible, it's necessary in some situations. For example, if a breaker is used as a main disconnect for a control panel and you want a through-the-door rotary operator, in many cases the mechanism that operates the breaker toggle will only mount onto 3 pole breakers, because 1 and 2 pole breakers are too narrow.

If you are not in North America however, the safest bet is to wire 2 poles in series or just do it so as to not have to think about it.

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