transformer primary wiring

[jzups]

I am working on a project which has a 225kva /480v primary- 208v/120v/3phase
secondary transformer.The elec.engineer has speced the primary conductors to be 3#3/0 fed from a 200amp breaker. Secondary is speced to be paralel 500mcm conductors feeding a 800amp distribution panel. Is the primary undersized to supply 800amps? Should the primary not be sized at single 500mcm fed from a 400amp breaker to produce 800amps on the secondary?

 

[iwire]

Welcome to the forum.:smile:

I am working on a project which has a 225kva /480v primary- 208v/120v/3phase

That is better described as a 480-208Y/120 225 kVA transformer.

The transformer ratings are

Primary Current 270 amps.

Secondary Current 625 amps.

The elec.engineer has speced the primary conductors to be 3#3/0 fed from a 200amp breaker.

That limits the kva to about 166 kVA so now the secondary side is limited to about 461 amps.

Secondary is speced to be paralel 500mcm conductors feeding a 800amp distribution panel.

Two sets of 500 (Copper?) will provide a capacity of 760 amps and 240.4(B) allows the next standard size breaker to be used so the secondary side is OK. (EDIT, this is incorrect, see below)

Is the primary undersized to supply 800amps?

Yes, with a 200 amp primary the secondary is limited to about 461 amps.

Should the primary not be sized at single 500mcm fed from a 400amp breaker to produce 800amps on the secondary?

The 225 kVA transformer could not supply 800 amps regardless of the primary side overcurrent protection. At max loading that 225 can provide 625 amps on the secondary.

BUT, there are no code violations here assuming the calculated load is under 461 amps.

The engineer can design as conservatively as they want. The 800 amp panel is not required to be supplied with 800 amps.

You may want to politely ask the engineer to verify that was the intended design, but there are no violations.

 

[infinity]

Secondary is speced to be parallel MGM conductors feeding a map distribution panel.

Two sets of 500 (Copper?) will provide a capacity of 760 amps and 240.4(B) allows the next standard size breaker to be used so the secondary side is OK.

Are secondary conductors ever permitted to use 240.4(B)?

 

[charlie]

Are secondary conductors ever permitted to use 240.4(B)?

(C) Transformer Secondary Conductors. A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.

​

No.

 

[iwire]

I am sorry for for missing that.

OK there is a violation here.

 

[nakulak]

this is just guesswork, but I'm guessing the engineer meant to spec 3 sets of 3/0 to give him over six hundred amp, and upsized the gear to 800.

 

[jzups]

Thanks again guys,I will talk to the E.E. on monday. As I 've said before ,this site and its members are an invaluable tool for our industry!

Fraternally, J Zups