Wire Ampacities

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pride electrical

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I Have A 400a Single Phase Service On A Building With Paralell Conductors. I Pulled Paralell 4/0 Xhhw-2 Alum. Which Is Good For A Total Of 410a. The Inspector Is Telling Me That The Paralell 4/0 Xhhw-2 Does Not Meet The Apmacity Of 400a, Is This True? Becuase The Way I Read In The Nec Is That 4/0 Xhhw-2 Is Good For 205a. The Length Of The Wire Is Around 75 Feet And Each Set Is In A 2 1/2" Pvc Sch 4o Conduit.
 
pride electrical said:
The Way I Read In The Nec Is That 4/0 Xhhw-2 Is Good For 205a.
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That is the value in the 90C column. You can't use that value unless all terminations are rated for 90C, and I seriously doubt you will have that advantage. The Inspector is right. Using the 75C column, under the presumption that the terminations will be rated for no more than 75C, your conductors are only good for 180 amps each.

Reference: 110.14(C).
 
Pride,

You probally have 75 deg terminations on the equipment, so you have to use

the 75 deg part of the table and that is 180a. What is your load calc. for this

building? If this is a single dwelling then table 310.15(B)(6) will allow the

400a service 120/240vac 3w.
 
If your connected load is less than 360 amps you could still go up to the next standard size of 400 amps.
 
Here in Arizona, you even have to consider multiplying your ampacity by the "correction" factors (see table below NEC 2002 Table 310.16). I live in the SW area of the State of Arizona, it's extremelly hot (avg of 110F and above during the whole summer) and it is very important to upgrade the wire even if it meets the NEC ampacity.
Just my 2-cents
regards:smile:
 
If it's parallel 4/0 AL in a single raceway the ampacity will only be 328 amps although you could use a 350 amp overcurrent device.
 
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