Is this conductor derating because of higher temperatures in the transformer?

No, the 125% is to accomidate the inrush current that happens when a transformer is energized.

Inrush from a transformer? I assumed only motors have an inrush current when energized. I'm going to test that tonite. I have to shut down some power after hours.

#184551 - 02/13/0902:42 AMRe: why the 125% rule for transformer conductors?
[Re: Eddy Current]

AC squirrel cage induction motor and transformer are essentially the same, with the motor having the secondary (or rotor) shorted out. I dont think you will see large inrush on a transformer as in a motor unless the transformer is loaded up to its rated KVA.

#184567 - 02/13/0901:35 PMRe: why the 125% rule for transformer conductors?
[Re: jay8]

I will disagree on that statement. I have heard the conductors SLAP in the conduit when energizing an unloaded transformer. I believe it is called magnetizing current.

In essence, the inductive reactance of the transformer is zero until the current starts flowing, therefore the transformer looks like a short circuit upon energization. A large current starts to flow, which produces strong magnetic fields, which pushes the conductors away from each other, resulting in the slapping of the conductors against the conduit.

Larry C

#184573 - 02/13/0901:59 PMRe: why the 125% rule for transformer conductors?
[Re: LarryC]

I will disagree on that statement. I have heard the conductors SLAP in the conduit when energizing an unloaded transformer. I believe it is called magnetizing current.

In essence, the inductive reactance of the transformer is zero until the current starts flowing, therefore the transformer looks like a short circuit upon energization. A large current starts to flow, which produces strong magnetic fields, which pushes the conductors away from each other, resulting in the slapping of the conductors against the conduit.

Larry C

And that is why we size the conductors 125%. Because of the inrush, not ambient temperature.

26-258 (3) contradicts any inrush theory. We are allowed to use a 75 kva transformer for example where you would need 60 kva and size the conductors as if you had a 60 kva transformer. Seems that inrush is not considered as long as the overcurrent device does not operate. Not a good design practice sometimes. Stating that conductors are required to be sized at 125% because it says so in 26-256 is not really answering the question of what exactly are we derating for. I'm sure there is a reason somewhere.

#186130 - 04/20/0902:34 PMRe: why the 125% rule for transformer conductors?
[Re: bigpapa]

It is all to do with ampacity of the load. Transformers are continuous loads and conductors are sized per all other continuous loads. Our electrical equipment is sized this way. It deals with the heating from the conductor on the breaker and the transformer terminals. Conductors are sized to the rating of the transformer and not the O/C device. You size the conductors for a hot water heater or a lighting load the same way. It sometimes appears a little differently when we say you can only load a 15 amp breaker to 80% but we could think that is the inverse of 125%. In the case of a plug circuit we wire with #14 and the 15 amp wire is only loaded to 80% or 12 amps. I hope I am making sense. A 75 KVA transformer is 208 amps at 208 volts the wire must be 260 amps. (75000/208*1.73)1.25 The other way to look at it is what is the largest load than can be connected to 260 amp wire? 260*.8 or 208 amps @ 208 volts 3 phase(75 KVA)