sum of all breakers rule, 2017 705.12(B)(2)(3)(c)

[ggunn]

I guess I am still not understanding why its not "sum off all load devices". IT seems very simple and more consistent with electrical theory. IS there a situation I am missing when it fails?

One thing that absolutely makes sense is that the sum of 1.25X the maximum output currents from multiple inverters should in no case be more than the rating of the busbars in an AC combiner panel. Modules are so inexpensive these days that inverters are frequently loaded to the point where the maximum AC output current is likely to be reached and maintained for hours from time to time.

 

[electrofelon]

One thing that absolutely makes sense is that the sum of 1.25X the maximum output currents from multiple inverters should in no case be more than the rating of the busbars in an AC combiner panel. Modules are so inexpensive these days that inverters are frequently loaded to the point where the maximum AC output current is likely to be reached and maintained for hours from time to time.

Actually I disagree. I think it should be 100%, there is no 25% adder for continuous loads for panel boards in other instances so why should there be here?

 

[jaggedben]

I guess I am still not understanding why its not "sum off all load devices". IT seems very simple and more consistent with electrical theory. IS there a situation I am missing when it fails? Also that would have covered the common meter main with panelboard with feed thru lugs scenario.

Okay that's a good point. I'm guessing that the inclusion of supply is in part because the original impetus for the rule was to legitimize combiner panels. But the minimum ampacity for inverter output is actually covered by other rules. Also we'd still arguably need clarification on how to treat devices that could be both; and an advantage of the current rule is you should never fail an inspection based on any misunderstanding of whether a source breaker could also be a load.

 

[ggunn]

Actually I disagree. I think it should be 100%, there is no 25% adder for continuous loads for panel boards in other instances so why should there be here?

The main terminals of the panel are likely rated at 75 degrees, so 1.25X.

 

[electrofelon]

The main terminals of the panel are likely rated at 75 degrees, so 1.25X.

I don't follow, what is termination temperature have to do with continuous load or not on a bus bar?

 

[wwhitney]

The main terminals of the panel are likely rated at 75 degrees, so 1.25X.

That's a non-sequitur.

If you use, say, a 200A 100% rated breaker to protect a 200A MLO panel, you may load that configuration to 200A continuously. Busbar ratings are continuous ratings.

Cheers, Wayne

 

[ggunn]

That's a non-sequitur.

Why? Most AC combiners are MLO,; the terminal lugs are usually rated at 75 degrees, are they not? If the lugs are rated the same as the busbars, the continuous use heating should be taken into account. But of course, the code is the code, so from that perspective the sum of all breakers rule makes it irrelevant. Also of course, if the lugs are rated for higher current or at 90 degrees, then never mind.

 

[electrofelon]

Why? Most AC combiners are MLO,; the terminal lugs are usually rated at 75 degrees, are they not? If the lugs are rated the same as the busbars, the continuous use heating should be taken into account. But of course, the code is the code, so from that perspective the sum of all breakers rule makes it irrelevant. Also of course, if the lugs are rated for higher current or at 90 degrees, then never mind.

I am still not following you. Lugs and panel boards are not required to have the continuous derating. That is for conductors and OCPDs

 

[wwhitney]

Why? Most AC combiners are MLO,; the terminal lugs are usually rated at 75 degrees, are they not?

3/0 Cu has a 75C ampacity of 200A. That means it can carry 200A continuously in 30C ambient and not exceed an insulation temperature of 75C.

The 200A MLO panelboard should have lugs that take at least 3/0 Cu with a 75C rating. The lugs and busbar will likewise have a 200A ampacity, meaning they can operate at 200A continuously without the lugs exceeding 75C. So all is copacetic, assuming you can get around the OCPD issue by using a 200A 100% rated breaker in a separate enclosure.

Cheers, Wayne

 

[wwhitney]

I am still not following you. Lugs and panel boards are not required to have the continuous derating. That is for conductors and OCPDs

Fixed it for you. : - )

Cheers, Wayne

 

[ggunn]

Well, duh. Sorry it took so long for me to get it through my thick skull; it must be all that L-tryptophan I have been consuming lately. When you connect a conductor that is too small to carry a particular continuous current to a 75 degree terminal, it is the conductor, not the terminal, that gets too hot, and the conductor heats the terminal. Otherwise, upsizing the conductor would not fix anything. Duh.

 

[wwhitney]

When you connect a conductor that is too small to carry a particular continuous current to a 75 degree terminal, it is the conductor, not the terminal, that gets too hot, and the conductor heats the terminal. Otherwise, upsizing the conductor would not fix anything.

You could, in theory, have a lug/connector that is rated for say 200A non-continuous and 160A continuous. But I'm pretty sure that the MLO panelboard testing regime under UL 67 involves instrumenting the panelboard with temperature sensors in numerous regulated locations and running the full rated current through the panelboard until the temperatures reach steady state. Which means that if those measured temperatures pass (within the temperature rise limits), the assembly could carry its rated current indefinitely without overheating.

Cheers, Wayne

 

[electrofelon]

Fixed it for you. : - )

Cheers, Wayne

hmmm, what about 210.19(A)(1), 215.2(A)(1), 230.42, and 690.8(B)

 

[wwhitney]

hmmm, what about 210.19(A)(1), 215.2(A)(1), 230.42, and 690.8(B)

Note that those rules all have exceptions that say when connected to a 100% rated OCPD, the conductors may be used at their ampacity, no derating required for the conductors. So we can paraphrase those rules as saying "the conductors need to be rated at 100% of the load, but hey, if you have continuous loads, you're going to need to upsize the breaker if it's not 100% rated, so you'll need to upsize the conductors to match."

Cheers, Wayne

 

[electrofelon]

Note that those rules all have exceptions that say when connected to a 100% rated OCPD, the conductors may be used at their ampacity, no derating required for the conductors. So we can paraphrase those rules as saying "the conductors need to be rated at 100% of the load, but hey, if you have continuous loads, you're going to need to upsize the breaker if it's not 100% rated, so you'll need to upsize the conductors to match."

Cheers, Wayne

Certainly the NEC could be arranged and worded in such a way that only the OCPD need the extra 25%, and conductors would just naturally follow

 

[ggunn]

Certainly the NEC could be arranged and worded in such a way that only the OCPD need the extra 25%, and conductors would just naturally follow

But until (and if) that happens, complying with the existing code is not that hard.