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Running large load split between 2 breakers

1932 Views 37 Replies 13 Participants Last post by  mpoulton
Here's my question: Is this against code, and if so, specifically why (what section/article)? I've done some research on it, asked around a couple electricians, and haven't found a solid yay or nay.

Scenario: Running a large load on 2x 2-pole breakers. 200amp 240v main panel, say I had 2x 60amp or 70amp 2-pole breakers, and plan to run 2 conductors per phase, 1 conductor of each phase to each breaker (obviously). It's almost the same as having 2 large separate loads running, but instead it's 1 load spread across 2 breakers (because I can't easily find a breaker large enough for this plug-in type panel 125amp or 150amp, or running 1 large conductor per phase is just not a feasible in this space). I've also read that some panels have a max circuit sized, but that this was due to conductor size vs bend area. This would also be alleviated by running 2 smaller conductors per phase rather tan 1 large conductor per phase.

The 2 conductors would then be bolted to a terminal in the load control box, and the rest downstream doesn't matter for this question.

I thought of "stacking" the breakers to place a 4-pole tie bar, but realized it didn't matter, as long as each 2-pole breaker has it's own tie bar, I won't be single-phasing anything should 1 2-pole breaker trip. Just that the remaining breaker will see a sudden load increase until it as well trips.

My non-professional thought is as long as I don't exceed 80% panel rating overall with other loads, I should be good. But, A couple electricians I have talked to haven't been able to point out a specific violation of code doing this, but agree that it sorta makes sense, and very unusual. So long as the loads are reasonably balanced between the dual conductors on each phase.

What I'm after is really if it's a violation or not, or a "gray area".
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no you cant parallel breaker output as the load will not be shared equally (internal resistance of each breaker is never the same) so one breaker may have 80 amp and the other one 40.
In that case, wouldn't the breaker just trip if it was overloaded due to imbalance in load?
Let's say it's two 60amp breakers. If one is allowing 80amps to pass due to low resistance, then it should trip within a few moments, right? If and when that happens, breaker #2 now sees full load of say 120amps, meaning it'll trip fairly quickly. At least, my rough understanding that's how I assume it'd play out.
yes one will trip and then the other one also, but remember that codes says your calculated load can't be over 80% of the breaker capacity
You would want them all bridges so the trip together and turning them all off at the same time. Just because you can't find it, doesn't mean it would pass.
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Yes I'm aware of not exceeding 80% for continuous load. I just hadn't accounted for that in this hypothetical situation with the numbers....

Going back to the question, does this violate code? Is there an article in the code that explicitly prohibits this configuration?
Having all 4 bridged is fine with me.
What panel are you trying to put a large breaker in? Make and model number (photos of the label work wonders for us).

There might be the option of a feeder kit to a fusible disconnect switch.

Be aware, that a 125 or 150 amp breaker might be more than you're thinking it will cost.

Sent from my new phone. Autocorrect may have changed stuff.
Yes, the cost of the 125a or 150a breaker is the reason I was considering going with 2 separate ones. There's plenty of space in the panel for them, I'll have to see what exactly the panel is when I get home.

Side note, I was reading up on other scenarios of panels only being rated for a max circuit branch of 90 or 100amps, but the reason being due to bend space of that required size conductor, not necessarily that the panel itself couldn't handle it. This would appear to make sense, unless it's due to bus bar material, aluminum vs copper or plated copper, and the ability to draw that much over the area of 2 lugs.
After a slight change in search terms, and instead search for parallel breaker setup, it appears that NEC does not allow this unless it is a factory-manufactured device, and also that 2 separate circuits cannot be tied together downstream. For the 2nd half of that, if the 2 breakers are bridged, would that be considered the same circuit? I guess it wouldn't matter, since the 4-pole overall "assembly" I was envisioning isn't factory supplied/balanced, and really isn't an option period for a 200amp panel.

If only a 4-pole main breaker would fit on the lugs... Then it'd be a factory-supplied unit.
You cannot put breakers in parallel like that. NEC 240.15(B) does not allow this, regardless of the load configuration or handle tie situation. One of the essential reasons for prohibiting this is that this requires BOTH breakers to trip in order to de-energize the circuit, this multiplying the probability of a dangerous failure and increasing the fault-clearing time.
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Maybe I'll try a single, 2-pole 125amp breaker, if the panel max branch circuit allows it. Otherwise, I don't feel I have a whole lot of options short of having a 400amp service upgrade and feeding this load separate from the panel altogether.
Side note, I was reading up on other scenarios of panels only being rated for a max circuit branch of 90 or 100amps, but the reason being due to bend space of that required size conductor, not necessarily that the panel itself couldn't handle it.
The bend radius is limited by the gutter space, it has nothing to do with stab rating.

The bus stab rating is an electrical rating based upon the connection between the bus bar and the bus stab.

Here an explanation from Sq D

Most Square D QO or HOMELINE panels are supplied with "double row" bus
bar construction. This means that the bus bars will have branch circuit
breakers plugged onto the bus bars from the left and from the right.
The "bus stab", "bus finger" or even "connector finger" is the point on
the bus bar that the branch circuit breakers electrically connect to the
bus bar. For instance, a 40 space panel will have 20 "bus stabs". The
bus stabs accept circuit breakers from both sides, providing the
capability to plug on 40 1-pole circuit breakers.

If your panel is restricted to 125 ampere per bus stab, that means that
you cannot put two breakers onto that bus stab (one from each side)
that exceed 125 amperes between them. For instance, you could place two
60 ampere breakers (60A+60A=120A) across from one another on the same
bus stab(s), but you could not place two 70 ampere breakers
(70A+70A=140A) across from one another on the same bus stab(s).

This restriction is not applied to all QO or HOMELINE panels. The
restrictions are applied only when we must limit the total amperage on a
stab to pass applicable UL heat rise tests.

I hope that this has satisfactorily answered your questions. Please let
me know if you need anything further on this subject.

Rick Snapp
Product Technical Support Group
Square D Company - Lexington Plant
( bold highlight by me)
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Thanks for that reference about the stab rating! Makes perfect sense. If I did put in a 125a breaker, I'd just to be sure nothing was on the other side of it.

FWIW, possibly later this year or next yr, I am planning on a 400amp service upgrade anyway. Whether or not I go with 2 separate 200amp panels, or a single 400amp panel is yet to be determined. But, I would think going with a 400amp panel would allow me a larger range of breaker size on the higher end. Again, haven't looked into it much yet to verify that assumption.
Inquiring minds need to know, what is this large load?

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What brand of panel is this anyway?

Make and model would help too, but I may have a breaker that would work (I disconnect and remove a lot of electric furnaces).

We would first have to determine if the panel could even support it first though.

Sent from my new phone. Autocorrect may have changed stuff.
Beware, wall of text:

It's for a large rotary phase converter for my garage shop. No 3ph available here. So it's a fluctuating load, not really continuous. Hardest part (peak draw) is startup obviously, after that it's not that bad. Continuous load is limited to ballpark 120amp, but even that isn't that continuous. It's loaded in that amp range for maybe 10-40 minutes at a time, depending on operation, and then idle (1-2 dozen or so amps?) for another 10-15 minutes.

Reluctant to do so, but I'll admit, it's already wired in, in the configuration I've described, with a 4-pole tie bar/bridge, and has performed flawlessly for the last 4-5 months (I know, shame on me). I did look this evening, and there are no other breakers opposite these on their stabs/lugs. I must have already considered that without remembering, several months ago when placing them in panel. Further down and opposite are the other larger branches, 50amp welding outlet, etc. I am running 4awg copper THHN in tandem/parallel, which accounts for motor FLA * 125%. I have verified loads are within reason balance-wise, while under load, via ammeter. I have several panel-mount ammeters and voltmeters to monitor the loads and voltages of each phase, both at single phase panel, and 3 phase panel. Last I checked, the load-sharing balance of the conductors that I'm running in tandem/parallel was just under 10%. I found the largest influencer of this balance error to be in how, and in what "shape" (contact area) each conductor was terminated at each end. As in, was one slightly more "fanned out" by clamp screw than the other. After a couple attempts, they were reasonably balanced. Initially, it was off by quite a bit more than 10%. But after closely matching how each end was clamped, the balance was much closer. Hopefully this makes sense, and doesn't sound overly unsafe. I'm now aware it does in fact violate code (thanks mpoulton for NEC ref), due to the non-factory breaker setup and merging of conductors/branches downstream. Although, one won't backfeed the other due to tie bar/bridge (at least not for more than a few ten thousandths of a second...). One 2-pole breaker will indeed bring the other with it via tie bar. I did verify this several times, with each breaker, upon initial setup. I do believe I got a 125amp 2-pole off ebay as a backup in case this 4-pole arrangement didn't physically work out, might even still be on my bench out there. I haven't taken the cover off the panel yet to see what make/model it is, likely won't happen this evening.

Alllllll this being said, I know I do need more power and a more legal setup. I'm really at the limit of my panel and transformer (again), with the phase converts running. (yes, more than 1 converter). I was upgraded to a 50kva xformer a couple yrs ago, because the 10kva was not enough for the 30hp total phase converters (10 and 20hp) I had at the time. Blew fuse at pole. Interestingly, the fuse was about 2x the xformer rating (3amp, 7200v line). Lineman said they'd get me a bigger xformer for free because I have 200amp service on garage, and 100amp service on house, but not enough power to draw even close. They upped the fuse size to handle inrush current temporarily from starting phase converters, and couple months later put up a 50kva. Don't know what fuse is size now, but I haven't blown it yet. The power company has already approved and prepped site for 100kva xformer, now waiting on 400amp service upgrade. Have gotten a couple quotes on installs, just a matter of deciding what I want to do. I'd prefer a single 400amp panel. Do it once the 1st time, rather than cheap out like some have suggested (not here) and go with 2 200amp panels.

Hopefully that upgrade will happen this summer, along with shop expansion.
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This forum is littered with posts that go along these lines. ...”I have it wired and it works”...

The post is usually attempting to justify a code non compliant hack job as kosher since it works. The NEC is primarily about safety not minimum requirements to get it “to work”.

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NEC 240.8 prohibits parallel fuses/breakers. 310.10 prohibits conductors in parallel smaller than 1/0.
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