Parallel DC Circuit

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BWIMMER

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I've found several code articles about the requirement for paralleling an AC circuit in the same conduit. Can a DC circuit be paralleled in the same conduit. My situation is we have a UPS breaker feeding battery cabinets. Rather than install one large feeder, we want to parallel two conductors in the same conduit. Is this allowable?
 
Would the wire size be 1/0 or larger?
I believe the parallel rules would apply to DC wiring but it’s not a question that has been asked before
 
230304-1332 EDT

The title of the thread does not say anything about the source voltages. However, the first post indicates a single voltage source feeding parallel conductors. If these parallel conductors are the same size and material, connected together at both the input end of the cable and at the output end, and if termination points at each cable end are near zero impedance relative to cable impedance, then their currents will be nearly identical, and the circuit is capable of carrying twice the current of a single conductor.

If the source voltage at the input of each wire is different, and the output ends of the cables are connected, then a current will exist in the conductors with no load on the connected point of the wires. This current is the voltage difference between the sources divided by the impedance of series total of the two wires. But with sufficient load at the load end, then current will flow toward the load in both conductors, but load current on each source will not be the same.

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gar said:
230304-1332 EDT

The title of the thread does not say anything about the source voltages. However, the first post indicates a single voltage source feeding parallel conductors. If these parallel conductors are the same size and material, connected together at both the input end of the cable and at the output end, and if termination points at each cable end are near zero impedance relative to cable impedance, then their currents will be nearly identical, and the circuit is capable of carrying twice the current of a single conductor.

If the source voltage at the input of each wire is different, and the output ends of the cables are connected, then a current will exist in the conductors with no load on the connected point of the wires. This current is the voltage difference between the sources divided by the impedance of series total of the two wires. But with sufficient load at the load end, then current will flow toward the load in both conductors, but load current on each source will not be the same.

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What we have is a battery cabinet with a 600A 480v DC breaker feeding busbar in the UPS. This will both charge the batteries and feed the UPS when normal power is out. So same voltage both ways.
 
Dennis Alwon said:
I don't see anything that would not allow DC circuits from being paralleled
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I don't either. But some of the requirements in 310 for conductors in parallel would not apply to DC circuits from a physics stand point. The requirements seem to just assume that it is an AC circuit. I wonder why this was never clarified? Given that more and more we run into DC things such as battery banks, it would seem like there should be separate requirements for AC vs DC.
 
texie said:
I don't either. But some of the requirements in 310 for conductors in parallel would not apply to DC circuits from a physics stand point. The requirements seem to just assume that it is an AC circuit. I wonder why this was never clarified? Given that more and more we run into DC things such as battery banks, it would seem like there should be separate requirements for AC vs DC.
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Yes, but some sections call out specifically DC circuits or there is an exception for dc circuits. This is why I believe, everything is for ac or dc unless otherwise mentioned.
 
texie said:
But some of the requirements in 310 for conductors in parallel would not apply to DC circuits from a physics stand point
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Do you have an example in mind? I think most of those sections exempt DC circuits as appropriate from a physics stand point.

Cheers, Wayne
 
wwhitney said:
Do you have an example in mind? I think most of those sections exempt DC circuits as appropriate from a physics stand point.

Cheers, Wayne
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Yes, I agree. I was only referring to parallel conductor requirements in 310. Some of those requirements don't apply to DC. For example, you could run the + and - in separate raceways and even different lengths a paths and not be an issue.
 
I don't believe there's any general reason that we can assume DC doesn't present the same issues from a physics standpoint, just to a (usually far) lesser degree. A DC source, or load, can have a ripple or other variability that creates similar magnetic heating issues as AC. Even just switching on and off the load changes the magnetic field. Essentially, the magnetic field issues will always be less severe with DC than AC, but never entirely non-existent, and it will require detailed knowledge of the equipment and application to guarantee that there is no similar safety issue. Although in most DC applications I suspect it really isn't an issue.

In other words, I really don't know why 300.20 should only apply to ac circuits but 300.3(B)(1) makes no distinction.
 
jaggedben said:
I don't believe there's any general reason that we can assume DC doesn't present the same issues from a physics standpoint, just to a (usually far) lesser degree. A DC source, or load, can have a ripple or other variability that creates similar magnetic heating issues as AC.
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If the magnitude of the effect is an order of magnitude or two smaller for DC, seems like it becomes negligible for almost all purposes. For example, IIRC the Canadian Electrical Code only requires considering the effect of 60 Hz AC inductive heating for currents in excess of 200A.

Cheers, Wayne
 
Since the conductors in question are 1/0 or larger there is no question the install will comply with 310.12(G), the interesting part is 310.12(G) exception No. 1 for 360 hz and above. Is DC infinite Hz or zero Hz ?
 
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