Three Phase 480VAC -> 208/120 VAC

[Jerry Albertelli]

I have a three phase 480 VAC -> 208/120 (Delta wye) xformer protected on the primary by a 60 amp breaker w/o the instantanious trip. I am suppling a 125 breaker panel. Can someone please tell me what size wire I need to run for the secondary side and how do you calculate the wire size concerning this determination.

 

[iceworm]

I have a three phase 480 VAC -> 208/120 (Delta wye) xformer protected on the primary by a 60 amp breaker w/o the instantanious trip. I am suppling a 125 breaker panel. Can some one please tell me what size wire I need to run for the secondary side and how do you calculate the wire size concerning this determination.

With no more information that you have given, and if the 125A panel has a 125A main CB, consider sizing secondary conductors for 125A. You will need to look up the size and type up in T310.15.

You may also want to read Art 450 on transformers, and maybe look for a class as well.

And you probably should get and post the transformer nameplate information. That will help.

ice

 

[infinity]

The secondary conductors must be sized according to the OCPD where they terminate. For a 125 amp OCPD you would need a minimum of #1 copper conductors. System bonding jumper, SSBJ and GEC are sized according to 250.66 and the size of the secondary conductors.

 

[iceworm]

Just curious - Is that Vidor, TX?

 

[Jerry Albertelli]

Yes.....Vidor Texas. I have an Engineer that says to take 45,000 (kva) and divide 208, then use the 1.73 multiplier for three phase as well as 1.25% according to the NEC which = 156 amps so a 1/0 would be the correct size. I'm not sure, but I'm to size according to the Secondary side of the Xformer and not the 125 amp Panel, right?

 

[Jerry Albertelli]

I believe the #1 would suffice; however, #1 is extremly hard to get

The secondary conductors must be sized according to the OCPD where they terminate. For a 125 amp OCPD you would need a minimum of #1 copper conductors. System bonding jumper, SSBJ and GEC are sized according to 250.66 and the size of the secondary conductors.

 

[jim dungar]

Yes.....Vidor Texas. I have an Engineer that says to take 45,000 (kva) and divide 208, then use the 1.73 multiplier for three phase as well as 1.25% according to the NEC which = 156 amps so a 1/0 would be the correct size. I'm not sure, but I'm to size according to the Secondary side of the Xformer and not the 125 amp Panel, right?

Basically transformer secondary side conductors are sized per 240.21(C), which pretty much says they need to be sized, at minimum, to the protective device they feed.
Article 450.3 is about providing protection to the transformers, not the conductors.
Article 408 has requirements for protecting panel boards.
These three sections work in harmony.

 

[Jerry Albertelli]

Thank you very much.....

 

[infinity]

Yes.....Vidor Texas. I have an Engineer that says to take 45,000 (kva) and divide 208, then use the 1.73 multiplier for three phase as well as 1.25% according to the NEC which = 156 amps so a 1/0 would be the correct size. I'm not sure, but I'm to size according to the Secondary side of the Xformer and not the 125 amp Panel, right?

125 Amp conductors would be the minimum based on the OCPD. This is a design issue and I'm thinking that you could feed a 20 amp OCPD with #12's on the secondary.

 

[Besoeker]

I have an Engineer that says to take 45,000 (kva)

Wonder where he got the 45,000 kVA from?

 

[infinity]

Wonder where he got the 45,000 kVA from?

I think that it's fair to guess that he meant 45,000 VA which is the size of the transformer.

 

[rlundsrud]

Here is a helpful link, it is an excerpt from EC&M magazine about transformer sizing and was written by Mike Holt. It has step by step instructions that even include all applicable code citations, it makes for a very useful rubric.

http://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/TransformerInstallation~20020516.htm

 

[kwired]

Yes.....Vidor Texas. I have an Engineer that says to take 45,000 (kva) and divide 208, then use the 1.73 multiplier for three phase as well as 1.25% according to the NEC which = 156 amps so a 1/0 would be the correct size. I'm not sure, but I'm to size according to the Secondary side of the Xformer and not the 125 amp Panel, right?

Assuming this means you have a 45kVA transformer, what you have there is 45,000/208/1.73=124.9 amps close enough to 125 to call it 125.

If you are supplying a 125 amp overcurrent device you need minimum of 125 amp conductor.

If you have 125 amps of continuous load (and not using a 100% rated breaker)- then your minimum conductor ampacity and minimum overcurrent protection both will need to be increased by another 25%. So that would leave you with needing a 156 amp conductor and next standard overcurrent protection device of 175 amps.

AFAIK the 45 kVA transformer would be rated for 125 amps continuous loading.

 

[Jerry Albertelli]

There's what I was looking for, and that was "Continuous Load". Oh...and yes....I meant 45 kva. and also, I forgot to establish the footage since that is very important according to the NEC. The footage is 75 feet from secondary side of Xformer to 125 amp line-side breaker panel. Thanks guy's you've been very helpful. Enjoy your holidays....

Assuming this means you have a 45kVA transformer, what you have there is 45,000/208/1.73=124.9 amps close enough to 125 to call it 125.

If you are supplying a 125 amp overcurrent device you need minimum of 125 amp conductor.

If you have 125 amps of continuous load (and not using a 100% rated breaker)- then your minimum conductor ampacity and minimum overcurrent protection both will need to be increased by another 25%. So that would leave you with needing a 156 amp conductor and next standard overcurrent protection device of 175 amps.

AFAIK the 45 kVA transformer would be rated for 125 amps continuous loading.

 

[templdl]

I have a three phase 480 VAC -> 208/120 (Delta wye) xformer protected on the primary by a 60 amp breaker w/o the instantanious trip. I am suppling a 125 breaker panel. Can someone please tell me what size wire I need to run for the secondary side and how do you calculate the wire size concerning this determination.

Mmmmm, a breaker W/O instantaneous trip? Whose breaker is it and what is the part number? Since the secondary breaker for this transformer protects the transformer from being overloaded the primary breaker's purpose is to trip should there be a transformer short circuit failure removing the transformer from the distribution system.