Secondary Conductor Sizing

[kevinware]

I have spent the last few days reading a lot of the threads here on the forum concerning the sizing and protection of the secondary conductors of a transformer, and now I would like to compare what I think I know, to a transformer installation job we just had done that I am having issues with.

“We just had a 480/208Y/120V 45KVA transformer installed, and it is being supplied with a 60 amp circuit breaker from a distribution cabinet on the primary (3 wire) side wired with 8 AWG THHN, and coming off the secondary (4 wire) side we have AWG 2 THHN (ABC:N) and this is feeding a single 225 amp rated 30 pole panel not over 25 feet away with a 125 amp main.”

My concerns are these. I feel the wire on both the primary side and the secondary side are undersized.

When I work the math for the transformer I get: 45000 / (480x1.732) = 54.128 amps of primary current.
45000 / (208x1.732) = 124.911 amps of secondary current.

Looking at the 2008 NEC Handbook, Article 450 shows a nice picture of Primary Overcurrent Portection Only which is what the electrical contractor tried to setup, in my opinion.

When I work the math for the Primary Overcurrent Portection Only according to Table 450.3(B) I get: 54.128 amps x 1.25 = 67.660 amps as a maximum CB size. I don’t want to go above 125% and have to add secondary overcurrent protection so to stay with Primary Overcurrent Portection Only the CB size according to 240.6 is rolled back to 60 amp and according to Table 310.16 the size of the wire should be a 6 AWG THHN, @ 75 degrees C. So, in my opinion feeding this 45KVA transformer with a 60 amp CB on the primary side is fine, but the 8 AWG THHN should be removed, and replaced 6 AWG THHN. So, because there is Primary Overcurrent Portection Only, table 450.3(B) does not require the 45KVA transformer to have secondary overcurrent protection. But the secondary is feeding a single panel with a 125 amp main, so according to 240.4, 240.21(C) that wire needs to be sized based on the 125 amp main in the panel, which means the 8 AWG, THHN should be removed and replaced with size 1 AWG THHN, @ 75 degrees. All opinions are welcome.

Thanks,
Kevin

 

[augie47]

First, keep in mind Art 450 is concerned with protection of the transformer and has little to do with the conductors.
Since you elect to depend on primary only protection rules of 450.3. This would allow you a primary OCP device of 70 amps or less. If a 60 amp is selected, then the conductors must be so rated. in your case, normally a #6 THHN.
Since you are allowing the primary to protect the transformer, no secondary protection is required as far as the transformer is concerned.
As far as the secondary conductors, they must be protected per 240.21(C).
Note in 240.21(C) {General} the rules of 240.4(B) shall not apply.
If your secondary OCP device is 125 amp then this would normally require a #1 THHN, as you state.
From there just make sure you installation meets the applicable provisions of the appropriate 240.21 rule.
You appear to have a good grasp of the applicable rules.

 

[dahualin]

Are you sure you can use the allowable amp rating of #6 at 75 degree without knowing the terminal temperature rating? I normally do is to use 60 degree column for any feeder under 100A as per 110.14(C)(1). In your case, you should use #4 wires.

 

[david luchini]

Are you sure you can use the allowable amp rating of #6 at 75 degree without knowing the terminal temperature rating? I normally do is to use 60 degree column for any feeder under 100A as per 110.14(C)(1). In your case, you should use #4 wires.

David, you make a good point about the 60 vs. 75 degree column, but the #6 awg would be OK for the 60A c/b, even at the 60 degree rating, per 240.4(B).