Sub panel

[SAP]

We are back feeding 40 amps into a existing Subpanel 125 Amp rated bus bar protected by 100 Amp OCPD, the existing feeders are # 4 . My. Question is is that number 4 too small using the 120 percent of the bus bar .would I have to use # 2. This is a residents thanks

 

[Smart $]

What's the rating of the ocpd supplying the #4? Hopefully not more than 90A. :blink:

 

[jaggedben]

Of the couple dozen AHJs I work in, only one enforces the 120 percent rule on the feeder conductor in that situation. However the 2013 California code still includes the 'Busbar or Conductor' 120 percent rule, so if your AHJ says it's too small then yes, you'd have to upgrade the conductor size. Or, alternatively, if that 100A breaker isn't the lone service disconnect, you could downsize it; you might have to show your AHJ a compliant load calculation on the sub to get them to allow that.

In 3 months California will go on the 2014 NEC equivalent and then you won't have this conundrum as long as the main and PV breaker at at opposite ends of the feeder.

 

[jaggedben]

What's the rating of the ocpd supplying the #4? Hopefully not more than 90A. :blink:

It may qualify under 310.15(B)(7) [2011 code reference], in which case 100A is allowed. However in that case the 100A breaker would have to be the lone service disconnect, meaning he would not be allowed to downsize it below 100A per article 230. Moreover, the AHJ could still decide to enforce the 120percent rule on the 95A rating in which case he'd be screwed out of installing to the sub, for the next three months. Which is kind of a shame because it'd be perfectly safe.

My strategy, if I didn't know their policy, would be to submit a very accurate line diagram to plan check and see what they say.

 

[Sierrasparky]

Of the couple dozen AHJs I work in, only one enforces the 120 percent rule on the feeder conductor in that situation. However the 2013 California code still includes the 'Busbar or Conductor' 120 percent rule, so if your AHJ says it's too small then yes, you'd have to upgrade the conductor size. Or, alternatively, if that 100A breaker isn't the lone service disconnect, you could downsize it; you might have to show your AHJ a compliant load calculation on the sub to get them to allow that.

In 3 months California will go on the 2014 NEC equivalent and then you won't have this conundrum as long as the main and PV breaker at at opposite ends of the feeder.

I don't see what the feeder size has to do with it. Unless the 100 amp sub is fed from a larger panel. The feeder will only see the max supplied by the utility. The bus will see the total consumed by the back-feed and the utility. I would argue if the AHJ calls you on this ask him what his thinking is.

 

[jaggedben]

I don't see what the feeder size has to do with it. Unless the 100 amp sub is fed from a larger panel. The feeder will only see the max supplied by the utility. The bus will see the total consumed by the back-feed and the utility. I would argue if the AHJ calls you on this ask him what his thinking is.

You're absolutely right about physics, but try to argue with the letter of 705.12(D)(2), 2011 code. It's not so easy. Been there, done that, with that AHJ. Can't wait to get back at 'em with the next code.

 

[Smart $]

It may qualify under 310.15(B)(7) [2011 code reference]...

...if he simply has a service disconnecting means ahead of the subpanel, i.e. no other loads.

I don't think in those terms when I hear subpanel. To me, that's a feeder supplied main panel.

 

[jaggedben]

...if he simply has a service disconnecting means ahead of the subpanel, i.e. no other loads.

That's quite common around here. However, often the service equipment has one or two more disconnecting means, e.g. for the air conditioner, which would nullify 310.15(B)(7).

I don't think in those terms when I hear subpanel. To me, that's a feeder supplied main panel.

I refer to any panelboard that's not part of the service equipment as a subpanel. But 'subpanel' isn't an NEC term so anything is fair.

 

[Smart $]

T...
I refer to any panelboard that's not part of the service equipment as a subpanel. But 'subpanel' isn't an NEC term so anything is fair.

310.15(b)(7) refers to a qualifying feeder as serving as the main power feeder. Seems only appropriate to call the panel it supplies the main power panel... and obviously shorten it to just main panel.