Rating of Disconnects (NEC 225.39(D))

[ohmti787]

Came across this article, which i think might be an exception to the 125% rule for wire and OCPD sizing.....maybe?
To give an example: i have this 36A PV system broken down into three separate strings. each string will have it's own 20a breaker inside the AC combiner box. if i go off of the 125% (36*1.25 = 45), i can put a 50A breaker/fuse between the AC combiner and the grid and call it a day. but this section will require me to upsize the ocpd to 60a as well as the wires. i guess i'm not following the logic behind this article. can somebody shed some light?

 

[wwhitney]

I believe 225.39 is generally understand to refer to the rating of the disconnecting means, not of the OCPD therein (if any). So if you use a 60A rated disconnect box, and put 50A fuses or breaker in it, it would comply.

Cheers, Wayne

 

[jaggedben]

A 60A rated fused disconnect with 50A fuses would be considered a 60A rated disconnecting means. A 50A circuit breaker would not.

I have to say in the more than a decade I've been doing this I've never had an AHJ bring this up. And thank goodness. It's out of date and stupid, in my opinion. Probably goes all the way back to when people would wire a whole residence or outbuilding with two-to-four circuits with large lightning loads and too many outlets on one circuit. Probably also goes back to when disconnecting means were mostly fused disconnects and the language may never have been intended to apply to circuit breakers in higher rated panelboards, although I'm speculating there. I also can't tell you how many 3 circuit subpanels I've seen on 40 and 50A breakers.

As far as a solar system output, arguably chapter 7 supersedes chapter 2 here. One could argue the other way, contending that 705 doesn't explicitly exempt you from the requirements of 225 here. But the positive argument would be that 705 tells you how big your overcurrent device needs to be and chapter 2 doesn't.

 

[ggunn]

In a related story...

I have run into situations where the utility made us run conductors with ampacity matched to the rating of the switch rather than the rating of the fuses in the switch. I don't remember the context but it happened.

 

[wwhitney]

A 60A rated fused disconnect with 50A fuses would be considered a 60A rated disconnecting means. A 50A circuit breaker would not.

Sorry, how's that? Suppose I take a QO2TR, which is a 60A rated 2 position panelboard, and I put a QO250 2 pole 50A breaker in it. How is that any different with respect to 225.39 from taking a 60A fusible disconnect and putting 50A fuses in it?

Cheers, Wayne

 

[jaggedben]

Sorry, how's that? Suppose I take a QO2TR, which is a 60A rated 2 position panelboard, and I put a QO250 2 pole 50A breaker in it. How is that any different with respect to 225.39 from taking a 60A fusible disconnect and putting 50A fuses in it?

Cheers, Wayne

Well, for one thing, the fuses aren't the device that disconnects the circuit, but the circuit breaker is.

 

[Fred B]

Sorry, how's that? Suppose I take a QO2TR, which is a 60A rated 2 position panelboard, and I put a QO250 2 pole 50A breaker in it. How is that any different with respect to 225.39 from taking a 60A fusible disconnect and putting 50A fuses in it?

Cheers, Wayne

I agree with the concept here, I would see a bigger issue with a 30A fuse switch with a 50A fuse. I've seen it and the resulting melt down. The 60 A rating on the switch is simply the maximum rating of the switch, no different than putting a extra duty 20a switch on a 15a circuit with 14awg wire, not a violation unless you put a 20a breaker on the 14 wire (generally accepted sizing limits not absolute).
Also why on the solar installation field making of the amperages on the disconnect are required and not relying on the disconnect's rating.

 

[ohmti787]

Well, for one thing, the fuses aren't the device that disconnects the circuit, but the circuit breaker is.

In this particular case we had to make room for our PV breaker, so we installed a125A rated subpanel and put our 50A breaker there. Would that be considered a violation then?

 

[jaggedben]

In this particular case we had to make room for our PV breaker, so we installed a125A rated subpanel and put our 50A breaker there. Would that be considered a violation then?

Like I said, someone could make that argument. Thankfully from my point of view, no one ever has made that argument on any installation I've worked on. This seems to be something people either just don't know about or don't interpret that way or don't care about. Is there a real problem with having three 15A circuits on a subpanel that's on a 50A breaker? No. You'd have to meet someone with a real stick up their you-know-what if they want to issue that as a correction, in my opinion.