Wire that is running out to a machine in plant that requires 24 amps but no 30 amp breaker available,what about paralleling up to 15amp breakers? will this work (theory only)

Yes, it would work. It is a violation of the NEC though, 240.8

Roger

A 30 amp breaker is not available?

Oh, I missed the theory part.



[This message has been edited by iwire (edited 10-12-2006).]

The problem with paralleling two 15 amp CBs is there is no way to fine tune the breakers to trip at the same time. Each CB will not trip at the same level, and each will have differing times for that trip. Also, how can we make sure that the length of the conductors and the termination devices are all matched perfectly? There is a reason why parallel feeders have a minimum size requirement. Those paralleled main breakers we find in some panels are a factory matched set.

Earl why would we need to calibrate the breakers?

Why do they need to trip at the same time?

One will trip first followed shortly by the other.

IMO the reason for not paralleling breakers has more to do with the safety issues of people turning one breaker off and assuming there is not another breaker still supplying the circuit.

It's permissible, and not uncommon, to parallel transformers, UPS, and other components to achieve a desired capacity. These are usually on separate breakers to allow for maintenance. I don't see any fundamental difference between this and using two 15A breakers in parallel. I don't really like the idea, but I don't see that it would be unsafe, or even illegal. 240.8 is pretty clear, but I'm unable to find the exceptions that I know exist. On a circuit this simple, they probably wouldn't apply, though.

Theoretical- during a fault, one breaker will trip before the other due to minute differences in manufacture or installation of the cables, cosmic rays, etc. The other breaker will then be faced with twice the load, and will quickly trip. Provided both breakers each have sufficient kAIC to clear a fault, it really doesn't matter that two breakers tripped before the circuit lost power.

Worst-case scenario is that there is a fault or overload that trips one breaker, but is someone reduced prior to the other tripping. The entire 24A circuit will now be fed from a single 15A breaker and is likely to trip soon thereafter just during normal non-fault operation.

[This message has been edited by SteveFehr (edited 10-12-2006).]

[This message has been edited by SteveFehr (edited 10-12-2006).]

Steve you can parallel the transformers but they must be switched as a unit.

Actually now I am not sure if i have to switch them as a unit or if I am just permitted to.

Sorry to waste space here.

Steve, there are no exceptions to 240.8 and if you go to 404.17 (fused switches) it sends you back to 240.8

Earl, I agree with Bob, when one of the breakers tripped the whole load or fault would shift to the other tripping it pretty quickly. (almost simutaneously)

Roger

https://www.electrical-contractor.net/ubb/Forum1/HTML/007347.html

We went round and round with this on 2, 200A breakers. Just divide down the discussion by 13.3.

I don't know why a 30A isn't available either. What about going bigger for less voltage drop and dropping back down with the OCP and OLs at the machine end.
Joe

I could always send y'all a box of 30A single-pole British breakers.......

Just a thought: If you were going to parallel two 15A breakers, NEC notwithstanding, why not use a handle tie?

(I say "if" because even without code issues I don't much like the idea.....)

It was not as if you coudn,t buy a 30 amp it
was a maitenance issue where one wasn,t readily available. I would use the British breakers but don,t you run on some old Victorian voltage and frequency?

The way most panels are designed, you wouldn't be able to use a 2-pole 30A breaker, as each pole would be on a different phase. (or phase shift if it was 240/120 1-phase). Now, a yoked double 1/2" breaker MIGHT be able to fit on a single phase, but I've only ever seen them designed such that it's impossible to do that.

Back to the paralleling transformers thing- I know I've seen paralleled transformers on separate breakers, but only in regards to UPS and generator power, and only ever in redundant configuration- EG, the two transformers were there in case of one failure, and they're never required to run parallel. Although when hooked up through a UPS, they *do* run in parallel, and from different breakers, plus typically an additional bypass run that is JUST the transformer. When switching to maint bypass when the UPS has failed into static bypass mode, there is a short period of time when two or more transformers are in parallel. But I suppose that's a completely different animal once you throw a UPS into the picture...

[This message has been edited by SteveFehr (edited 10-13-2006).]

Oh, maybe use a 3 phase 15A breaker in a 2 phase panel. The two outer breakers would be on the same phase. But that's likey not code. besides, you'd waste less panel space if you just used a 240V 2 space 30A breaker and left the extra section unconnected. Assuming there are no single 30A breakers.

I would do the same like wa2ise. Use a 3Ø braker, these are essentially 3 single phase brakers with a bar across the handles.

Use only 2 x 15 Amps segments of it and it should work OK.

It will avoid the danger someone switching it off while the other braker is still on.

Of course label the braker accordingly too.