ECN Forum Forums Technical Discussion Electrical Theory and Applications xfmr over-current protection

In Art 450 (b) it states for 600v nom or less 125% over current protection IF REQUIRED

Where does it state when it is required?

I have also protected the primarty side of a transformer according to the KVA rating. Ex. 75KVA 480v-208/120v I would use 90a over-current protection for primary side.

Is this necessary? I was always of the understanding that w/out primary protection, if the secondary side is protected correctly, you could burn the transformer.

Ex. using transformer in previous example, I would use 200a protection on secondary side.

Am I wrong?

I have been seeing alot of small transformers in office suites, with say 9KVA, 15KVA with fuse protection way to high. Primary fuses I have seen 50a for these. Yet transformer shows no sign of burning.

Can more amps be induced through a transformer, than the windings alow?
I cnat beleive I asked this, but I am not sure now. It has been 10 years since my electrical theory school days.

[This message has been edited by ayrton (edited 10-26-2006).]

Are you referring to 450.3(B)? In NEC 2005, it doesn't state "if required", merely references table 450.3(b) where secondary OCP is not required in some cases. Primary protection is always required.

Quote:
"Can more amps be induced through a transformer, than the windings alow?"

Specifically as your question is stated? No.

But if you are asking if a transformer can supply more current than the amount calculated from its nameplate Kva rating, then yes.
Theoretically if supplied from an unlimited bus, a transformer can supply the inverse of its rated impedance. So if a transformer has a 2% impedance (i.e., 2/100), then it theoretically can supply 50 times its rated full load current.

Quote:
"I was always of the understanding that w/out primary protection, if the secondary side is protected correctly, you could burn the transformer."

If a fault occurs between the secondary terminals of the transformer and the low side protection, then the transformer would burn up without primary protection. Is that what you were asking?

[This message has been edited by WFO (edited 10-27-2006).]

If a fault occurs between the secondary terminals of the transformer and the low side protection, then the transformer would burn up without primary protection. Is that what you were asking?

Yes. Thanks for the responses.

"If a fault occurs between the secondary terminals of the transformer and the low side protection, then the transformer would burn up without primary protection."

You have all seen the video of the transformer on the golf course exploding, this is why. The fault continued until the oil in the tank heated enough to lift the pressure relief, oil srays into the fault and BOOM.