Parallel Feeders

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dillon0122

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I have an existing 800A underground service fed from a transformer and the owner wants to upgrade to a 1200A. There are two 4 " conduits from the transformer to the building, due to existing conditions it is almost impossible to add any underground conduits. Can paralleled feeders be pulled in the existing conduits? This would be two sets of 4-400MCM in each conduit.
Thanks in advance.
 
400 MCM copper has a 90C rating of 380 amps. Four of them have an ampacity of 1520 amps. Derating 80% for 6 current-carrying conductors (presuming the neutral does not have to be counted) gives you 1216 amps. So that works.

But can you even get 2 sets of 4-400 MCM conductors (i.e., 8 conductors total) in a 4 inch conduit? I don't think so. What type of conduit is it?
 
Evening Charlie.

As you know it will depend on insulation.

I believe THWN is the smallest wet location insulation available.

Eight - 400 THHNs will 'fit' in 4" EMT, RMC, IMC, PVC 40 but not PVC 80.

However that is pretty much maxed out and depending on length or route may not be easy or even possible.
 
This might be a job for the application of 310.15(C).

Has anyone had experience getting conductor ampacity calculated based upon soil conditions and ambient temperatures?

-Jon
 
I have performed several calculations for underground installations. I used a commercially available software package. In every case, the calculated ampacity was lower than the values posted in Table 310.16.
 
I have also performed the calculation and generally found lower calculated values. It is very dependent on the material the surrounds the raceway and the spacing of the raceway. For example most concrete will do a better job dissipating heat as compared to stone or fill.
 
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