Parallel Grounds

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jonny1982

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Can someone please explain to me why when parallel feeds are run, the ground in each must be big enough to protect the entire parallel set rather than just the conductors in each given raceway?

Thanks
 
tom baker said:
A line to ground fault can be fed from each of the circuit. The EGC must be full size to carry the fault.
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If the EGC's are all connected on both ends why wouldn't several smaller conductors act like on large one of the fault was in the equipment at the end?

I guess that if an ungrounded conductor within a raceway got connected to a smaller EGC within that raceway there could be an issue, like if someone cut into the raceway with a saw.
 
tom baker said:
A line to ground fault can be fed from each of the circuit. The EGC must be full size to carry the fault.
Click to expand...
Leave out part of what you wanted to say there?


If we have four parallel conductors but in separate raceways, individual conductors have 1/4 ampacity of the overall set.

However they have determined what is minimum EGC size for any given OCPD about has to consider how much current the conductor can handle without being severely damaged, you don't want the conductor to fail before the OCPD can respond.

Part of OP's question probably is considering same for the ungrounded conductor which will be smaller than the OCPD rating, but most the time still is just as large or larger than the min required EGC.
 
The withstand rating of the EGC has to be able to accommodate all of the fault current that the upstream fuse / breaker can let through until it opens.

It it was not full sized in each raceway, then a fault within an individual raceway would have a reduced sized EGC trying to carry the fault current by itself and burn-up until the OCPD clears the fault, which might be never at that point if the EGC can't maintain continuity.
 
ron said:
The withstand rating of the EGC has to be able to accommodate all of the fault current that the upstream fuse / breaker can let through until it opens.

It it was not full sized in each raceway, then a fault within an individual raceway would have a reduced sized EGC trying to carry the fault current by itself and burn-up until the OCPD clears the fault, which might be never at that point if the EGC can't maintain continuity.
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The ungrounded conductor that becomes faulted must also withstand the current, if you want the OCPD to trip before it burns out the conductor. But the ungrounded conductor often is larger than the EGC anyway. I won't say you can't run into needing a larger EGC than the ungrounded conductors, but would be pretty rare I would think.
 
ron said:
The withstand rating of the EGC has to be able to accommodate all of the fault current that the upstream fuse / breaker can let through until it opens.

It it was not full sized in each raceway, then a fault within an individual raceway would have a reduced sized EGC trying to carry the fault current by itself and burn-up until the OCPD clears the fault, which might be never at that point if the EGC can't maintain continuity.
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I can agree for raceways that do not qualify as an EGC like PVC but to use full size EGC's in each parallel run of EMT or RMC is just a waste of money.
 
kwired said:
The ungrounded conductor that becomes faulted must also withstand the current, if you want the OCPD to trip before it burns out the conductor. But the ungrounded conductor often is larger than the EGC anyway. I won't say you can't run into needing a larger EGC than the ungrounded conductors, but would be pretty rare I would think.
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Section 250.122(A) indicates that EGCs never have to be larger than the ungrounded conductors of the circuit.
 
ron said:
Section 250.122(A) indicates that EGCs never have to be larger than the ungrounded conductors of the circuit.
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I agree except for parallel runs where the ungrounded conductor is the sum of all the parallel sets so you could end up with the EGC's in each parallel raceway that are required to be larger than the condcutors within each raceway.
 
The equivalent impedance of parallel feeds is lower than single feed. So fault current is more and so EGC to be big enough to carry the increased fault current without burning up.
 
infinity said:
I agree except for parallel runs where the ungrounded conductor is the sum of all the parallel sets so you could end up with the EGC's in each parallel raceway that are required to be larger than the condcutors within each raceway.
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My thought also.

Can't say I have ever run into it, maybe if you decided to never use larger than the minimum requirement of 1/0 for your parallel sets and were running a 4000 amp circuit:blink:
 
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