NEC 310.10(H)(1) Feeding a Busway at Multiple Points

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DetroitEE

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We are designing power feeds to a busway, which powers a moving machine that is on electrified rails. It has been requested that we feed the busway at multiple points with the same circuit; the reasoning is that the total length of the busway is long, and the electrified bus itself is small due to physical constraints, and therefore voltage drop would be an issue if the machine were at the far end of the track and it were only fed at one location. The busway is electrically continuous for the whole length, there are no breaks. The theory is that if we use larger conductors, run along side the busway and then connected to the busway every 10-20', this would solve the voltage drop issue.

I'm trying to determine if this type of installation is even permissible per the NEC. My first thought was that this would be a violation of 310.10(H)(1) as we would have paralleled conductors of different lengths (definition of paralleled is "electrically joined at both ends"). The current would divide itself among all of the different busway taps, proportional to the impedance. This unequal division of current among the circuit conductors seems to be just what the paralleling rules seem be prohibiting.

Any feedback or thoughts would be appreciated.
 
jusme123 said:
...what about terminating only once in the middle of the rail system, if possible?
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Yeah, we did look at that option. Apparently the length is still a concern with regard to voltage drop, but I don't know if there's really another way to do it that's code compliant.
 
The reason to avoid unequal current distribution in paralleled conductors is that any one of the conductors by itself is only rated to carry a fraction of the load current. That is not the case in your setup (or should not be).
Also the bus rails are not part of the building wiring system but rather part of utilization equipment.
The argument that the rails will have significant voltage drop under load leads me to argue that you do not have the ends of the feed wires *joined* but rather have a load which is designed for multiple feeds (not feeders in the NEC sense) to different terminals in different locations, with each feeder capable of carrying up to the entire load current.
The one thing I would definitely avoid is putting disconnects or OCPD on the individual feed points, since any one of them will energize the entire system.
Note that if this were bus duct rather than part of a moving machine I would be more likely to call it part of building wiring and reject the multiple feeds.
 
GoldDigger said:
The reason to avoid unequal current distribution in paralleled conductors is that any one of the conductors by itself is only rated to carry a fraction of the load current. That is not the case in your setup (or should not be).
Also the bus rails are not part of the building wiring system but rather part of utilization equipment.
The argument that the rails will have significant voltage drop under load leads me to argue that you do not have the ends of the feed wires *joined* but rather have a load which is designed for multiple feeds (not feeders in the NEC sense) to different terminals in different locations, with each feeder capable of carrying up to the entire load current.
The one thing I would definitely avoid is putting disconnects or OCPD on the individual feed points, since any one of them will energize the entire system.
Note that if this were bus duct rather than part of a moving machine I would be more likely to call it part of building wiring and reject the multiple feeds.
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Good feedback, thank you. You're correct that in this setup, the wires would be sizes to carry the entire load. Agreed that disconnects at each feed should be avoided.
 
It is a common practice to feed the power rails for overhead cranes in multiple locations because of the voltage drop issue on the power rails. I though it would be addressed in Article 610, but I didn't see anything.
 
don_resqcapt19 said:
It is a common practice to feed the power rails for overhead cranes in multiple locations because of the voltage drop issue on the power rails. I though it would be addressed in Article 610, but I didn't see anything.
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The discussion in this thread implies that "double feeding" is indeed a violation. You say it is common practice, but do we actually know if this is code compliant?
 
DetroitEE said:
The discussion in this thread implies that "double feeding" is indeed a violation. You say it is common practice, but do we actually know if this is code compliant?
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I see that discussion as saying you can't have more than one disconnect, not that you can't have more than one connection to the rails.

I have seen manufacturer's instructions for crane rails requiring more than one connection where the rail was over "x" feet long.

I am not sure that the number of connections to the rail is even within the scope of the NEC.
 
don_resqcapt19 said:
I see that discussion as saying you can't have more than one disconnect, not that you can't have more than one connection to the rails.

I have seen manufacturer's instructions for crane rails requiring more than one connection where the rail was over "x" feet long.

I am not sure that the number of connections to the rail is even within the scope of the NEC.
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Thanks Don. Good point about the number of disconnects, I must have been reading too fast.
 
Although I'm not familiar with a Multi Feed scenario on an unbroken bussway , a feed tap every 10-20' seems like a lot.
Is this an AC or DC buss?

JAP>
 
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