Why does 705.12(B) always use a 125% factor on inverter maximum current?

[wwhitney]

Continuous loads (or supplies) require a 125% factor in conductor sizing solely due to the requirement of a 125% factor in breaker sizing for non-100% rated breakers. This is reflected throughout the NEC in numerous exceptions for 100% rated equipment, including in Article 705. Namely explicitly in the new in the 2023 NEC exceptions to 705.28(B) (although arguably the 2020 NEC implicitly allowed the same thing via the "where not elsewhere required or permitted in this Code" language).

So given that, why does the 125% factor occur in 705.12(B) without any corresponding exceptions? For example, take (2020) 705.12(B)(3)(1). An MLO panelboard with a 200A busbar (a continuous rating) should be allowed to be supplied by both a 150A utility-side breaker and up to 50A of inverter output current, rather than 40A.

Cheers, Wayne

 

[jaggedben]

... For example, take (2020) 705.12(B)(3)(1). An MLO panelboard with a 200A busbar (a continuous rating) should be allowed to be supplied by both a 150A utility-side breaker and up to 50A of inverter output current, rather than 40A.

First of all I don't think it's correct that's a continuous rating. A 200A panelboard would be rated 160A continuous. (Unless you mean "listed assembly together with it's overcurrent device" that's so rated, but I don't think that exists at least in standard fare residential panelboards).

As far as the rest of your question, can you elaborate? I don't understand if you are giving an example using the 120% rule or the sum of all breakers rule, etc. I believe your trying to make a general point but the rule used would still affect the numbers, no?

 

[wwhitney]

First of all I don't think it's correct that's a continuous rating. A 200A panelboard would be rated 160A continuous.

Well, I was under the impression that a panel busbar rating is a continuous rating; if it's not, my question is moot. I guess I need to chase down the listing standards to see. But my impression seems to be the consensus in this old thread:

MLO service panel rating with continuous loads

Say I have 1000 amps of continuous load and the service equipment is a MLO service rated panel with 4 breakers serving as the service disconnects, and the buss has a 1000 amp rating. Compliant?

forums.mikeholt.com

I don't understand if you are giving an example using the 120% rule or the sum of all breakers rule, etc.

I specified 705.12(B)(3)(1) in the OP. That's the "sum of all sources" rule, which has no limit on other breakers.

The case in the OP might be the only applicable case. Because the "sum of all breakers" rule is based on breaker ratings, not inverter ratings. And the 120% rule if reformulated would just become a 116% rule, given that 120% was an arbitrary number in the first place.

So I guess I'm just arguing that if the busbar rating is continuous as per my understanding (to be confirmed), then the "sum of all sources" rule has no reason to be use a 125% continuous factor on the inverter output current.

Cheers, Wayne

 

[wwhitney]

I guess I need to chase down the listing standards to see.

Well, I didn't go to UL 67 directly, but this UL "Panelboard Marking and Application Guide" says that the rating is a continuous rating. Section 7 "Current Rating" starts off "The current rating of a panelboard is the maximum continuous current that can be supplied through the main terminals."

Panelboards - Marking and Application Guide | UL Solutions Code Authorities

December 2021

code-authorities.ul.com

So if you have a 200A 100% rated disconnect supplying a 200A MLO panelboard, that panelboard can supply 200A of continuous loads.

Cheers, Wayne

 

[jaggedben]

Okay that's interesting. Consider me surprised.

Still trying to wrap myself around the weirdnesses here though. Like, if the 200A breaker isn't 100% continuous rated together with the panelboard, since none are, then we are back to 160A continuous. So, like, does 705 need to consider whether the calculated load through the main is continuous or not? I certainly hope not.

 

[wwhitney]

So, like, does 705 need to consider whether the calculated load through the main is continuous or not? I certainly hope not.

Since the panelboard busbar ratings are continuous, then 705 as written now is conservative. The only upshot I see is that 705.12(B)(3)(1) (sum of all sources) is overly conservative by requiring a 125% factor for the inverter current.

Cheers, Wayne

 

[jaggedben]

Since the panelboard busbar ratings are continuous, then 705 as written now is conservative. The only upshot I see is that 705.12(B)(3)(1) (sum of all sources) is overly conservative by requiring a 125% factor for the inverter current.

Cheers, Wayne

Except that if all the inverters output at full for 3 hours (which they probably will, at some point) then if the panel and main breaker aren't "100% listed" then the main breaker will trip. Further, the breakers used for the inverters would have to be '100% listed' to not trip.

IOW, I think you're point is only relevant if all the breakers are listed together with the panelboard as an assembly. Which probably involves testing in every arrangement one might want to use, i.e. not worth anyone's time or effort.

 

[wwhitney]

Except that if all the inverters output at full for 3 hours (which they probably will, at some point) then if the panel and main breaker aren't "100% listed" then the main breaker will trip. Further, the breakers used for the inverters would have to be '100% listed' to not trip.

I'm not following your scenario. If you have a regular 200A MB panelboard, then 705.30(B) limits you to connecting 160A of "continuous output current" of power production equipment. [And that section has the usual exception for 100% rated equipment.]

Anyway, 705.12(B) is about protecting the busbar, not about avoiding nuisance tripping on OCPD from continuous currents. So such considerations don't belong in 705.12(B).

Cheers, Wayne

P.S. But if I'm missing something and there's a situation where the 125% factor in 705.12(B)(3)(1) prevents a problem that a 100% factor and the rest of 705 would allow, I'd love for you to spell it out for me. Thanks!