I already have a primary PV inverter connected up through a 40A breaker. I'm looking to expand by adding a second PV inverter. To comply with the 2014 NEC 705.12(D)(2) rule I need to know the panel bus bar rating. The existing system drawings were done based on the assumption that the main breaker and the bus bar were rated equivalently (at 200A_, but if I'm reading the label in the panel correctly, the bus bar is rated at 225A.

If the bus bar is indeed rated at 225A then my allowance under the NEC rule is 1.2*225 = 270 - 200 (main breaker) = 70A. With the existing system taking up 40A, then I could install another 30A breaker for the second inverter. If not I'd have to replace the main breaker with a 175A breaker.

I have partial picture of the sticker inside the panel, and realized it matched a sample from a data sheet, so I'm including the data sheet picture.

panel data sheet rating label

Am I reading the "Mains 225A max" as meaning that the bus bar is rated at 225A?

  • Ah. Can you find the part number for the actual interior your panel uses? – ThreePhaseEel Oct 7 '19 at 1:15
  • That’s not the BX38A? I’m out of town right now, but can look when I’m back. Would that be a separate sticker? And where is it usually Inside the panel? – jcoleman Oct 7 '19 at 2:50
  • It should be on a label on the actual interior (the black plastic thing that holds the breakers and busses). BX38A is the Square-D part number for the cabinet enclosure. – ThreePhaseEel Oct 7 '19 at 3:15
  • I’m back in town now, and pulled the panel cover off to look, but virtually the entirely bus bar is covered with breakers. I looked at Schneider’s website for pictures, and it seems the interior panel spec sticker is under the main breaker anyway, so there’s really no way to easily get a look at it. – jcoleman Oct 17 '19 at 1:09
  • That being said, for 42 space single phase interiors, Schneider only shows 225A options on their website. Given it’s a less than 2 year old house, it seems plausible to me that’s a good indication. Thoughts? – jcoleman Oct 17 '19 at 1:11

Originally I was wondering if the Mains 225A max. from the label inside the panel box was the rating of the bus bar. Since it seems there are/can be addition labels on the panel interior itself (the black plastic holding the breakers and bus bars), I wasn't sure if I'd have to pull the meter and main breaker to confirm.

Researching Schneider's website, I noted that for 42-space single phase interiors in QO plug-on neutral style there appear currently to only be 225A bus bar ratings available for purchase. While that seems to be a pretty good indicator, I still wanted to confirm a bit more if possible.

I contacted Schneider's technical support to ask if they could confirm that they've only ever sold 225A bus bar interiors for the style/size/phase of my box. The representative wasn't able to do that. But representative proceeded to say in our chat:

But if you look at the wiring diagram label (should be on the inside of the box on the right or left endwall), it will have a rating along the lines of "Mains max" or somewhere along those lines. That will give you the rating of the bus. It is most likely 225 A, but I can't say for sure without knowing the actual part number

So it appears that you can determine the bus bar rating from the panel box interior label without needing to find a label on the breaker holding interior itself.

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  • I believe "Mains max" refers to the maximum main breaker size that panel can accept. It of course follows that the bus bars must be able to support at least that (and likely exactly that, since to support more it would usually be a waste of materials). – Nate S. Oct 17 '19 at 17:36
  • From that perspective it seems like it's definitely exactly the bus bar rating, since you don't always have to have a main breaker (i.e., if the panel is being used as a subpanel). – jcoleman Oct 17 '19 at 17:42
  • I'd say it's the minimum guaranteed bus bar rating -- it'd be possible for a manufacturer to make a panel that, say, only supports 225A when used with a main breaker, but supports 250A when used in a main lug configuration. I don't know of any manufacturer that does this, but it's not prohibited. – Nate S. Oct 17 '19 at 18:01
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    @NateStrickland -- the only sane way you'd get into the situation you'd describe is if the busbars are 250A, but the panel cabinet doesn't have enough bend space for 250A wires with a main breaker fitted, which can happen (especially with chunkier MCCBs as mains) – ThreePhaseEel Oct 18 '19 at 1:16

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