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#70746 - 10/12/06 10:50 AM 2 15amp breakers = 30amps?
GETELECTRIC Offline
Member

Registered: 10/22/03
Posts: 178
Loc: toronto canada
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)

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#70747 - 10/12/06 10:57 AM Re: 2 15amp breakers = 30amps?
Roger Offline
Member

Registered: 05/18/02
Posts: 1779
Loc: N.C.
Yes, it would work. It is a violation of the NEC though, 240.8

Roger

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#70748 - 10/12/06 11:06 AM Re: 2 15amp breakers = 30amps?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
A 30 amp breaker is not available?

Oh, I missed the theory part.



[This message has been edited by iwire (edited 10-12-2006).]
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#70749 - 10/12/06 11:20 AM Re: 2 15amp breakers = 30amps?
earlydean Offline
Member

Registered: 12/22/03
Posts: 749
Loc: Griswold, CT, USA
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.
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Earl

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#70750 - 10/12/06 11:42 AM Re: 2 15amp breakers = 30amps?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
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.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#70751 - 10/12/06 11:55 AM Re: 2 15amp breakers = 30amps?
SteveFehr Offline
Member

Registered: 03/19/05
Posts: 1192
Loc: Chesapeake, VA
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).]

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#70752 - 10/12/06 12:12 PM Re: 2 15amp breakers = 30amps?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
 Quote:
240.8 is pretty clear, but I'm unable to find the exceptions that I know exist.


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

 Quote:
450.7 Parallel Operation.
Transformers shall be permitted to be operated in parallel and switched as a unit provided the overcurrent protection for each transformer meets the requirements of 450.3(A) for primary and secondary protective devices over 600 volts or 450.3(B) for primary and secondary protective devices 600 volts or less.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#70753 - 10/12/06 12:14 PM Re: 2 15amp breakers = 30amps?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
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.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#70754 - 10/12/06 01:01 PM Re: 2 15amp breakers = 30amps?
Roger Offline
Member

Registered: 05/18/02
Posts: 1779
Loc: N.C.
Steve,
 Quote:
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.
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

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#70755 - 10/12/06 09:24 PM Re: 2 15amp breakers = 30amps?
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
http://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

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