NEC User
Senior Member
- Location
- Marlboro, NJ, USA
I have an existing panel operating on 120/208 3 phase which will feed a new 25KVA single phase transformer with a 208V single phase primary and a 480V single phase secondary.
Square D confirmed they will manufacture a custom transformer to meet the above conditions.
However, I'm performing construction support for the project which is now under construction. I believe there is an error on the primary side of the transformer. Our Contract plans were showing a 3 pole 100A breaker and 3#2 wires going into the primary side of the transformer. Now I realized the intention is to use it as a single phase transformer on the primary side.
So my calculations per NEC is as follows:
P=IV (without radical 3 since the primary side is operating on 208 single phase) - should the radical 3 be used?
25000 VA = I 208V
I= 120A
Per NEC Table 450.3 B Primary protection only: the maximum primary protection is 120 * 1.25 = 150A.
However, the existing panel can only use 100 A circuit breakers maximum.
My questions are:
Are my correct to not use the radical 3 in the calculation above?
Will the 25KVA transformer always draw 25KVA and thus 120Amps?
Can I use a 2-pole 100Amp breaker within the panel to protect the transformer?
What calculations do I need to do to determin the 100A breaker is still acceptable to feed the trasnformer and the load on the secondary side?
Square D confirmed they will manufacture a custom transformer to meet the above conditions.
However, I'm performing construction support for the project which is now under construction. I believe there is an error on the primary side of the transformer. Our Contract plans were showing a 3 pole 100A breaker and 3#2 wires going into the primary side of the transformer. Now I realized the intention is to use it as a single phase transformer on the primary side.
So my calculations per NEC is as follows:
P=IV (without radical 3 since the primary side is operating on 208 single phase) - should the radical 3 be used?
25000 VA = I 208V
I= 120A
Per NEC Table 450.3 B Primary protection only: the maximum primary protection is 120 * 1.25 = 150A.
However, the existing panel can only use 100 A circuit breakers maximum.
My questions are:
Are my correct to not use the radical 3 in the calculation above?
Will the 25KVA transformer always draw 25KVA and thus 120Amps?
Can I use a 2-pole 100Amp breaker within the panel to protect the transformer?
What calculations do I need to do to determin the 100A breaker is still acceptable to feed the trasnformer and the load on the secondary side?
