Parrallel Conductors

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madbob

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I have two 400A panel boards with 400A main breakers that I am feeding from an 800A breaker using parrallel 600 MCM copper conductors. The panelboards are side by side and the conductors meet the requirements of 310-4. My question is, can I terminate one set of conductors on each 400A breaker or must I join them together in a gutter and run taps to each panelboard?
 

templdl

Senior Member
Location
Wisconsin
Re: Parrallel Conductors

Have you considered "subfeed" lugs in one of the panels? These would be main lugs that would allow you to directly feed a second panel from them. It may make a simply and cleaner installation.
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
Re: Parrallel Conductors

You can separate them at the load end, by connecting one set to each panelboard. But you would have to also separate them at the source, by connecting each set to a separate 400 amp breaker. Why do you want to use a single 800 amp breaker? Also, are these two runs in the same raceway (i.e., do you have to look into derating)?
 

madbob

Member
Re: Parrallel Conductors

If I use subfeed lugs, I believe that panel would have to be rated for the full 800A. I'm using an 800A feed because the breaker is already existing in the switchboard and don't want to buy two 400's and the associated buss kits. I need 800A of distribution at the load end, but can't get enough circuit spaces in a 800A panel, and two 400's are more economical and compact. I know the definition of parrallel conductors is "electrically connected at both ends to form one conductor". I am just trying to fathom the electrical necessity of tailing these out to taps of the same size.
 

bdarnell

Senior Member
Location
Indianapolis, IN
Occupation
Retired Engineer
Re: Parrallel Conductors

I don't see a way around this other than joining them at the load end and tailing to the panels using the tap rules. A junction box and some Polaris connectors should make this pretty easy and painless.
 

infinity

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Staff member
Location
New Jersey
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Journeyman Electrician
Re: Parrallel Conductors

Wouldn't the 25' tap rule apply if the distance of the 600 kcmil conductors between the 800 amp CB and the 400 main CB is 25' or less?
 

charlie b

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Location
Lockport, IL
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Re: Parrallel Conductors

Originally posted by madbob: I am just trying to fathom the electrical necessity of tailing these out to taps of the same size.
It comes down to protecting every conductor at its ampacity. The ampacity of a 600 MCM copper is 420 amps. The ampacity of a pair of parallel 600 MCM copper is 840 amps. If you separate the conductors at the load end, then you will be attempting to protect a 420 amp conductor with an 800 amp breaker.

Consider a fault that takes place at the point of termination at the first panel. Suppose that there is an insulation failure that allows 600 amps to flow from the termination point to ground. That will not trip the 800 amp breaker. If the 600 MCM cables are parallel at both ends, then the total 600 amp fault current will flow equally in the two cables. Each will get 300 amps, and neither will be damaged. But if you separated the cables at the load end, then you will get 600 amps flowing in a conductor rated for 420 amps. The panel breaker will not trip, since it is downstream of the fault point. The 800 amp breaker will not trip, since the current is below its setting. The result is a complete failure of the cable, and a possible fire.
 

bdarnell

Senior Member
Location
Indianapolis, IN
Occupation
Retired Engineer
Re: Parrallel Conductors

Originally posted by infinity:
Wouldn't the 25' tap rule apply if the distance of the 600 kcmil conductors between the 800 amp CB and the 400 main CB is 25' or less?
Interesting question. When does a feeder become a feeder? Are the lugs of the breaker considered as part of the feeder? If so, then your statement is valid. I've never had this come up, so I don't know the answer, but if faced with the situation, I would lean towards this not being the definition of a feeder.
 

infinity

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Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Re: Parrallel Conductors

Originally posted by bdarnell:
Originally posted by infinity:
Wouldn't the 25' tap rule apply if the distance of the 600 kcmil conductors between the 800 amp CB and the 400 main CB is 25' or less?
Interesting question. When does a feeder become a feeder? Are the lugs of the breaker considered as part of the feeder? If so, then your statement is valid. I've never had this come up, so I don't know the answer, but if faced with the situation, I would lean towards this not being the definition of a feeder.
So I could come off of the 800 amp CB with 2' of 2 sets of 600 kcmil and tap a single 600 kcmil to from these 2 sets to each 400 amp main (25' or less). Seems like eliminating the 2' of parallel 600's effectively changes nothing.
 

madbob

Member
Re: Parrallel Conductors

Thanks everyone, especially you Charlie, for the help with this. I will splice these 600's together and bring 600 Kcmil taps to these individual panels within 25'to satisfy the NEC requirements.
 
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