Parallel conductors

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electricmason86

Member
Location
rochester, mn
Question, in the attached picture, you will see that the parallel conductors have a fuse in each conductor, the other end terminates with both wires of the phase on one terminal, is this a good practice? I'm thinking that if a fuse would blow than the other conductor would have to carry the entire load? I have never seen parallel conductors installed in this manor before! What do you all think?
 

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I don't see the issue. Think of a large water heater where the loads are sub divided. It really isn't different. Those wires on the load side are not parallel conductors but they are using the tap rule.
 
My concern is if you blew 1 fuse on the same phase what then? Huge phase imbalance? Loss of 50% of the conductor? Weird voltage imbalances? It looks OEM but I'm just wondering why would OEM wire it that way? Seems odd.
 
electricmason86 said:
i know it doesn't look that way but they terminate on the same point on the other end
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Here is the definition of parallel
conductors shall be permitted to be connected in parallel (electrically joined at both ends) only in sizes 1/0 AWG and larger
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The load side of the breaker shown is not a parallel connection. Sure the have 2 wires coming off each phase but they split at the other end. They are not electrically joined at both ends.
 
IMHO we have to distinguish the NEC definition of 'parallel conductors' from other forms of parallel.

The conductors are not 'parallel' per the NEC strict definition of 'parallel conductors' specifically because they don't go from one solid electrical connection to another. The NEC definition is pretty much about a 'wire' formed from smaller wires, and if some device interrupts the smaller wires than you don't have a 'parallel conductor'.

But they are clearly electrically in parallel. Current flows from the breaker lug, splits to take one of 2 paths, goes through the fuses and then the wire, and joins back up at the next lug. It then goes through the load, splits because you have 3 phases, hits the next load lug, splits to take one of 2 paths, goes through the fuses, and then ends up on a breaker lug.

Is this a good practice? I don't know. It looks very much akin to 'cable limiters' or 'limiting lugs' that are intended to isolate parallel segments in the event of short circuit; but those are ordinary fuses that would blow on overload or short circuit. The breaker is an 800A breaker, the fuses are 150A fuses.... If you had a 400A overload on that setup I'd expect one fuse to blow and the second one to follow quickly.
 
winnie said:
The conductors are not 'parallel' per the NEC strict definition of 'parallel conductors' specifically because they don't go from one solid electrical connection to another.
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Is that really part of the NEC definition or more of an interpretation?
 
infinity said:
Is that really part of the NEC definition or more of an interpretation?
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My interpretation of 310.4.

310.4 is specifically about using multiple separate conductors as a single conductor.

It isn't about parallel circuits in general, just the specific requirements for placing two (or more) pieces of wire in parallel and using them as a single piece of wire.
 
winnie said:
My interpretation of 310.4.

310.4 is specifically about using multiple separate conductors as a single conductor.

It isn't about parallel circuits in general, just the specific requirements for placing two (or more) pieces of wire in parallel and using them as a single piece of wire.
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I agree, but how does the insertion of a fuse physically change that in this case?
 
electricmason86 said:
i know it doesn't look that way but they terminate on the same point on the other end
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I think I see what's going on now. The conductors are joined together again after the fuses. I missed that possibility. Those fuses are JT 150 which are 150 amp time delay fuses that are, as @ron stated, current limiting.


1762522662976.png
 
The install, as pictured and described by the OP, is not per the NEC.
It is likely not a UL508A control panel based on the apparent age of the modified back panel versus the conductors and fusing.

My guess is someone was not using the tap rule provisions of 240.21(B).
 
infinity said:
Is that really part of the NEC definition or more of an interpretation?
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I submitted a PI a while ago to add the word "directly" (IIRC) to the parenthetical definition "electrically joined at both ends" of parallel conductors found in 310.10(G), and the CMP said it was unnecessary because it already meant that.

So yes, the word "joined" in the parenthetical definition is intended to mean a direct connection, rather than a connection through other circuit elements. That means, for example, that certain light switching configurations do not violate 310.10(G) when the switches are in a particular configuration.

Cheers, Wayne
 
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