Breaker installation with 75kva transformer

[kwired]

What about it?

I see nothing in it that changes my view.

What exactly do you disagree with, overcurrent protection for the secondary conductors, overcurrent protection for the panelboard supplied by the secondary, both, secondary with/without line to neutral loads, something else?

 

[topgone]

Maybe not the exact intent of the section, but 408.36 (2008) seems to not disallow it.

"... This overcurrent protection shall be located within or at any point on the supply side of the panel board." Several exceptions follow.

So, allow as if there was a 75 kVA, single-phase transformer, 480:240 volt, two-wire primary, two-wire secondary. We could then have a:

156 amp primary, protected at 175 amps. Or 200.

Secondary rating of 312, Let's say a panel board with a 300 amp bus. Or 200. No OCPD.

Illegal?

Strictly speaking, for most two-wire step-down situations it seems acceptable to this section, and 240.4(F) seems ok with it too.

If you protect both primary and secondary sides, your primary OCPD will be no more than 250% of the primary full load and your secondary OCPD will be no more than 125% of the secondary FLA per 450.3.

You could also provide sufficient protection on the primary only by selecting a primary OCPD no more than 125% of the primary FLA, in this case = 156 x 1.25 = 195A = 200A

 

[Volta]

If you protect both primary and secondary sides, your primary OCPD will be no more than 250% of the primary full load and your secondary OCPD will be no more than 125% of the secondary FLA per 450.3.

You could also provide sufficient protection on the primary only by selecting a primary OCPD no more than 125% of the primary FLA, in this case = 156 x 1.25 = 195A = 200A

Yes but my post was about exploring the possible legality of protecting a panelboard from the primary side of a two-wire transformer feeding it. What section forbids that?

 

[david luchini]

Can you cite a code section to back that position?

408.36(B) prohibits the primary circuit breaker to protect the secondary panelboard, except for specific conditions (I didn't read all the way thru to the exception earlier.) But the exception doesn't apply to all single phase transformers, nor does it apply to only single phase transformers.

 

[david luchini]

Maybe not the exact intent of the section, but 408.36 (2008) seems to not disallow it.

"... This overcurrent protection shall be located within or at any point on the supply side of the panel board." Several exceptions follow.

So, allow as if there was a 75 kVA, single-phase transformer, 480:240 volt, two-wire primary, two-wire secondary. We could then have a:

156 amp primary, protected at 175 amps. Or 200.

Secondary rating of 312, Let's say a panel board with a 300 amp bus. Or 200. No OCPD.

Illegal?

Strictly speaking, for most two-wire step-down situations it seems acceptable to this section, and 240.4(F) seems ok with it too.

Yes, this would be illegal. A 300A or 200A panelboard on the secondary could not be protected by a 175A or 200A primary circuit breaker. A 400A panelboard would be OK.

 

[Volta]

408.36(B) prohibits the primary circuit breaker to protect the secondary panelboard, except for specific conditions (I didn't read all the way thru to the exception earlier.)

Well, no, it doesn't. The text of that section demands that the overcurrent protection be located within or at any point on the supply side of the panelboard. No prohibitions in the paragraph.

But the exception doesn't apply to all single phase transformers, nor does it apply to only single phase transformers.

Transformers are not mentioned at all.

Yes, this would be illegal. A 300A or 200A panelboard on the secondary could not be protected by a 175A or 200A primary circuit breaker. A 400A panelboard would be OK.

Absolutely true electrically. That is just what 240.4(F) says. And that ratio is allowed to protect the secondary conductors. No mention of the panelboard those conductors feed.

So I'll give you the ratio, though not specifically applicable, and we can call it a 400 amp panel. Do you allow as the Section 408.36 allows primary protection of both the two-wire windings and the panelboard?

 

[iwire]

408.36(B) prohibits the primary circuit breaker to protect the secondary panelboard,

You will have to show me where 408.36 does any such thing.

 

[david luchini]

You will have to show me where 408.36 does any such thing.

Has the text in 2011 changed? I'm reading the same thing in 2002 (408.16(D)), 2005 (408.36(D)), 2008 (408.36(B)), and a draft copy of 2011 (408.36(B)). (Connecticut is so far behind the times, we are still on 2005.)

"where a panelboard is supplied through a transformer, the overcurrent protection required by 408.36 shall be located on the secondary side of the transformer."

 

[iwire]

2011 below, 2008 the same, I don't have the 2005 handy.

408.36(B) Supplied Through a Transformer. Where a panelboard
is supplied through a transformer, the overcurrent protection
required by 408.36 shall be located on the secondary side of
the transformer.

Exception: A panelboard supplied by the secondary side
of a transformer shall be considered as protected by the
overcurrent protection provided on the primary side of the
transformer where that protection is in accordance with
240.21(C)(1).

 

[david luchini]

2011 below, 2008 the same, I don't have the 2005 handy.

Isn't that exactly what I said in post #24?

 

[david luchini]

Well, no, it doesn't. The text of that section demands that the overcurrent protection be located within or at any point on the supply side of the panelboard. No prohibitions in the paragraph.

"...the overcurrent protection...shall be located on the secondary side of the transformer."

That seems like a specific location on the supply side of the transformer, rather than "at any point."

Transformers are not mentioned at all.

Section 408.36(B) is titled "Supplied through a Transformer" :?

Absolutely true electrically. That is just what 240.4(F) says. And that ratio is allowed to protect the secondary conductors. No mention of the panelboard those conductors feed.

So I'll give you the ratio, though not specifically applicable, and we can call it a 400 amp panel. Do you allow as the Section 408.36 allows primary protection of both the two-wire windings and the panelboard?

240.4(F) doesn't apply to the transformer, 408.36(B) does. The ratio is specifically applicable to the panelboard via 408.36(B), exception, through reference to 240.21(C)(1).

 

[tkb]

2005 NEC says,

2005 NEC
408.36 Overcurrent Protection

(D) Supplied Through a Transformer. Where a panelboard is supplied through a transformer, the overcurrent protection required by 408.36(A), (B), and (C) shall be located on the secondary side of the transformer.

Exception: A panelboard supplied by the secondary side of a transformer shall be considered as protected by the overcurrent protection provided on the primary side of the transformer where that protection is in accordance with 240.21(C)(1).

 

[Volta]

"...the overcurrent protection...shall be located on the secondary side of the transformer."

That seems like a specific location on the supply side of the transformer, rather than "at any point."



Section 408.36(B) is titled "Supplied through a Transformer" :?





240.4(F) doesn't apply to the transformer, 408.36(B) does. The ratio is specifically applicable to the panelboard via 408.36(B), exception, through reference to 240.21(C)(1).

Well sure, if I read the whole thing. :ashamed1:

I didn't see that the section continued after the exceptions, whoops.

 

[david luchini]

Well sure, if I read the whole thing. :ashamed1:

I didn't see that the section continued after the exceptions, whoops.

Glad to know I'm not the only one that does that.:lol: