Small parallel conductors

[wireman]

Will someone please explain these exceptions for control wiring to NEC section 310.4 Conductors in parallel?

The way I read it, it is OK to run (2) #12's in parallel but the max overcurrent protection is 25Amps (ignoring derating factors for this discussion).

What's the point of running small parallel conductors if you still have to meet Exceptions (b) and (c)shown below:
=========================
(b) The ampacity of each individual conductor is suffıcient to carry the entire load current shared by the parallel conductors, and

(c) The overcurrent protection is such that the ampacity of each individual conductor will not be exceeded if one or more of the parallel conductors become inadvertently
disconnected.
=========================

 

[iwire]

Re: Small parallel conductors

Could it be for voltage drop on long runs?

 

[bphgravity]

Re: Small parallel conductors

I agree, voltage drop.

 

[mikeames]

Re: Small parallel conductors

I think it answeres it here.

For example, in control wiring and circuits that operate at frequencies greater than 360 Hz, it may be necessary to reduce cable capacitance or voltage drop in long lengths of wire. A 14 AWG conductor might have more than sufficient capacity to carry the load, but by installing two conductors in parallel, the voltage drop can be reduced to acceptable limits. This method is permissible, provided the safeguards listed in Exception No. 2 are followed.

 

[roger]

Re: Small parallel conductors

In many control situations it may be simply for redundancy. In otherwords if a single conductor were to fail, a whole production line or safety valve could be compromised.

Roger

 

[roger]

Re: Small parallel conductors

As far as the voltage drop angle, this would not be the case due to the fact ( in essence) we would be paralleling conductors to increase capacity. Which would violate 310.4 exception #2 (b).

Roger

 

[iwire]

Re: Small parallel conductors

As an example, you want to operate a contactor with a coil that needs 120 volts 4 amps, 500 feet from the source.

Using the voltage drop calculator from Mike Holt

Free Windows Voltage Drop Calculator

you find with a single #12CUthe voltage drop would be 6.666% or 8 volts

Parallel 2 #12CU and voltage drop is 3.333% or 4 volts

If you had run a multiconductor cable, doubling up would be easer then running another heaver cable.

 

[iwire]

Re: Small parallel conductors

Originally posted by roger:
As far as the voltage drop angle, this would not be the case due to the fact ( in essence) we would be paralleling conductors to increase capacity. Which would violate 310.4 exception #2 (b).

Roger

310.4(B) references ampacity, I do not think that is the same as increasing conductor size for voltage drop

 

[roger]

Re: Small parallel conductors

Bob, if the voltage drop presented a problem with the operation of the control device, it would be in proportion with the current, and in this case would mean that the single conductor would not have the ampacity (or abillity to deliver) at this voltage to carry the entire load shared by both conductors.

Now, if we are paralleling the conductors to keep the voltage up, and the device would work even with the voltage drop, we are only providing redundancy.

Roger

[ March 02, 2003, 05:35 PM: Message edited by: roger ]

 

[alex winkler]

Re: Small parallel conductors

I have a different take on this. I have seen in many control layouts where a single device such as a solenoid is feed by more than one control switch. It is possible that both switches would call for the solenoid to be on at the same time. Exception #2 would allow this without pulling 1/0 for a dinky little solenoid.

 

[roger]

Re: Small parallel conductors

Alex, Are you saying that it would take a larger wire than the single conductor if say a #12 were used in parallel? If this is the case, it is definately a violation of 310.4 ex 2 (b)

Roger

 

[iwire]

Re: Small parallel conductors

I do a lot of fire alarm work and when we build redundancy in fire alarm systems it would never be in the same raceway or cable , which is what 310.4(a) calls for. I do not believe this article has anything to do with redundancy, there is not much to be gained from redundant conductors in the same raceway or cable.

Alex may have the most accurate answer yet, but I still think OCP and VD are two separate issues, and yes I know the load on the circuit changes the voltage drop.

 

[alex winkler]

Re: Small parallel conductors

Roger, I may not of said that very well. What I was trying to say is that a device, such as a solenoid, could be fed by the same control circuit, say 15 amp #12 wire, through many control switches. Several of the control switches may call for the solenoid to be on at the same time thus paralleling the returns from each switch. If it was not (310.4 ex 2) I would have to use 1/0 instead of #12 for the control wire to the solenoid from the switches.

 

[roger]

Re: Small parallel conductors

Bob, a class A loop is not parallel. Alex, if it took a 1/0 wire to supply the devices, and you supplied the load with parallel conductors of a smaller size, which on their own as single conductors could not service the load, it is in violation.

Roger

 

[alex winkler]

Re: Small parallel conductors

Roger, I agree the situation you described would be a violation. In the one I laid out 310.4 would require a minimum of 1/0 for the switch returns, even if the solenoid were only 50 VA, if not for exception No. 2.

[ March 02, 2003, 07:31 PM: Message edited by: alex winkler ]

 

[iwire]

Re: Small parallel conductors

Originally posted by roger:
Bob, a class A loop is not parallel.
Roger

No kidding :roll: The point is you do not build redundancy with more conductors in the in the same raceway or cable if something happens to that one raceway or cable you have NO redundancy.

 

[roger]

Re: Small parallel conductors

Bob, if something happened to the raceway I agree.
Let's say for the sake of conversation only, if the starting point and device point had parallel lugs, either conductor could fail and the operation would still continue. With that said, I ask you to give your oppinion of the part of 310.4 ex2 (b) The ampacity of each conductor is sufficient to carry the entire load current shared by the parallel conductors.

Roger

 

[iwire]

Re: Small parallel conductors

Roger, For my opinion I would say look at the example I gave earler,

you want to operate a contactor with a coil that needs 120 volts 4 amps, 500 feet from the source.

Using the voltage drop calculator from Mike Holt
you find with a single #12CUthe voltage drop would be 6.666% or 8 volts. Parallel 2 #12CU and voltage drop is 3.333% or 4 volts

If one of the parallel conductors fails or breaks the load current will still be well within the ampacity of a single #12, the voltage at the end of the circuit will fall but the load current is still 4 amps, even less if it's a resitive load, due to reduced voltage.
Bob

 

[roger]

Re: Small parallel conductors

OK Bob, If the voltage fell as you say, yet could still handle the load, go back to my earlier post of "redundancy", (all of them, but specifically at 5:30) the single conductor obviously could carry the full load current.



I'm done here

Roger

[ March 02, 2003, 08:34 PM: Message edited by: roger ]

 

[don_resqcapt19]

Re: Small parallel conductors

Alex,
Your example of multiple switches controlling the same device is a parallel circuit, but is not an example of parallel conductors in the meaning used in 310.4. For the application of 310.4, the conductors must be joined together at each end to form a single conductor.
Don