Transformer secondary tap

[bdarnell]

I should know the answer to this, but since we just got failed by the local inspector, I'll ask for some backup.

He failed us on our transformers which are 480-208Y/120 feeding panelboards. They are in a bar joist ceiling (exposed) and the wire length is probably right at 25'. He cited 240.21(C)(2) and my opinion is we are compliant with 241.21(C)(6). The wire is 500 kcmil copper from our 150 kVA transformer and the primary protection is a 90A C/B. It terminates on a 400 amp C/B in the panelboard, which he also says is a violation (no rounding up in this case ?)

So, I throw this out to the assembled masses and await your opinions.

Thanks

 

[charlie b]

Re: Transformer secondary tap

Sorry, Brad, but I agree with the Inspector. The case turns on an admittedly fine point, but I think it's a clear point.

We all know that if you have a load of 370 amps, you can supply it with a set 500 MCM copper conductors (ampacity 380 amps), and then you can protect those conductors with a 400 amp OCP. But it is 240.4(B), and not 310.15, that allows the "rounding up." The ampacity of the conductors is set by 310.15, which in turn refers you to 310.16.

But for your situation, 240.21(C)(6) says that the OCP must limit the current to the ampacity of the secondary conductors. The ampacity of a 500 is 380 amps, not 400.

 

[bob]

Re: Transformer secondary tap

He cited 240.21(C)(2) and my opinion is we are compliant with 241.21(C)(6).

I think you meant 240.21.C6. The problem is that you are not compliant with 240.21.C6.2 The rating of 500 kcm is 380 amps and the breaker is a 400 amp.

 

[bdarnell]

Re: Transformer secondary tap

Originally posted by bob:

He cited 240.21(C)(2) and my opinion is we are compliant with 241.21(C)(6).

I think you meant 240.21.C6.

Maybe my book is different from yours, but my article is 240.21(C)(6) and I understand the provisions of 240.21(C)(6)(2). My misunderstanding is in the protection of the conductor as allowed by 240.4(B). The wording indicates that the wire shall terminate on an overcurrent device that is sized to the conductor ampacity, not the size permitted by 240.4(B).

I tend to agree with Charlie after mulling it over. We've already ordered the 600 and are pulling the 500 out.

 

[sandsnow]

Re: Transformer secondary tap

Please check my math (not my strongest attribute) here:

240.21(C)(6)(1)

Is x Es/Ep => 1/3 x P-OCPD

Solve for Is:
Is => Ep/Es x 1/3 x P-OCPD

Is - secondary conductor ampacity
Ep - Primary voltage
Es - Secondary voltage
=> - greater than or equal to
P-OCPD - primary overcurrent protective device

P-OCPD = 181 x 1.25 = 226.25 or round up to 250

Is => 480/208 x 1/3 x 250
Is => 192.30767 conductor ampacity

So if my math is correct you are in compliance with that aspect of 240.21(C)(6) with 25' of conductor.
I agree with the other violation above.
I upgraded your primary OCPD device from 90 amps. I assumed that was a typo.

You need either a 350 amp OCPD in panelboard (if load will allow) or 600kCMIL conductors.

Was he OK with the location of the XFMR up in the bar joists??? Are we talking exposed rafters???

Above rafters would not be in compliant with the letter of 450.13(A) which allows elevation of XFMR only when in the open. I would take open to be clear space in front and below the XFMR. I have to see the install to say yes or no.

 

[bdarnell]

Re: Transformer secondary tap

Originally posted by sandsnow:
I agree with the other violation above.

I don't, but, oh well. You can't always get what you want.

I upgraded your primary OCPD device from 90 amps. I assumed that was a typo.

No, it wasn't. We have a 90A OCPD.

You need either a 350 amp OCPD in panelboard (if load will allow) or 600kCMIL conductors.

Pulling in 600 as we speak.

Was he OK with the location of the XFMR up in the bar joists??? Are we talking exposed rafters???

Yes and yes. Exposed bar joists. Transformers are suspended below the bar joist with plenty of access on all sides.

 

[bdarnell]

Re: Transformer secondary tap

I mispoke in my earlier posts. We are dealing with 2 different transformers - 75's and 150's. We have the 90A OCPD on the 75 and 200 on the 150.

My apologies for any confusion. Been one of those mornings.

 

[LarryFine]

Re: Transformer secondary tap

Originally posted by bdarnell:
Pulling in 600 as we speak.

Talk about multi-tasking! I'm impressed!