Parallel Feeders - NEC 310.4(B)

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cdcengineer

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Question

Can a parallel set of 500kcmil CU be changed to a comparable sized set of AL conductors mid stream? Both sets would terminate in a set of lugs mounted in a junction box and the EMT w/ CU feeders would continue on as AL MC cable.

Thx
 

roger

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Yes you can. As long as each conductor of the particular leg, phase, or neutral has the same characteristics through out the run, even if the run is made up of breaks and multiple materials, all is fine.

You basically have an individual parallel run at each change.

Roger
 

Dennis Alwon

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I agree with chris if the requirements are met for parallel runs. Each part of the run must follow those requirements- same length, characteristics, etc. I see Roger beat me to it-- I agree with him also.
 
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G._S._Ohm

Senior Member
Location
DC area
CU be changed to a comparable sized set of AL conductors mid stream?
AL has 1.7x the resistance of CU, so this wire section will be hotter because it will dissipate more milliwatts/foot, other things being equal. Maybe the AL wire insulation can stand this higher heat without degrading unusually fast.

The example at the bottom of this link
http://www.electrician2.com/articles/ampacity.htm
shows that the thermal resistance of wire [Ri] depends on three things: the conductor diameter and the insulation thickness and thermal resistivity [400 to 450 for PE, 650 for PVC].
 
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tkb

Senior Member
Location
MA
MC is usually provided with an equipment ground sized for an individual feeder.
The equipment ground has to be sized for the total parallel size as previously noted as per 250.122(F).

To change from 500kcm CU to AL would most likely be 750kcm AL w 3/0g per cable. (Assuming 800 amp feeder)
Can you get 750kcm MC with a 3/0 ground?

250.122(F) Conductors in Parallel. Where conductors are run in parallel in multiple raceways or cables as permitted in 310.4, the equipment grounding conductors, where used, shall be run in parallel in each raceway or cable.
Each parallel equipment grounding conductor shall be sized on the basis of the ampere rating of the overcurrent device protecting the circuit conductors in the raceway or cable in accordance with Table 250.122.
 
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Question

Can a parallel set of 500kcmil CU be changed to a comparable sized set of AL conductors mid stream? Both sets would terminate in a set of lugs mounted in a junction box and the EMT w/ CU feeders would continue on as AL MC cable.

Thx

It really wouldn't be any different than splicing from aluminum to copper cause you need to fit under a lug. You are just changing where the splice occurs.
 

tkb

Senior Member
Location
MA
MC is usually provided with an equipment ground sized for an individual feeder.
The equipment ground has to be sized for the total parallel size as previously noted as per 250.122(F).

To change from 500kcm CU to AL would most likely be 750kcm AL w 3/0g per cable. (Assuming 800 amp feeder)
Can you get 750kcm MC with a 3/0 ground?

I think I may have been incorrect with my AL sizing for a parallel 800 amp feeder.
I believe it should be sized at 800kcm and not 750kcm.
 
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