what to pull

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normbac

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Have a 400A main breaker feeding residence 315 ft away single 3inch pvc conduit has one hand hole in middle of run. Is it easier to pull parallel 4/0 AL or single 600 400A breaker at house and at street have ability to accept parallel or single cu the neutral lug is for a single cu but I guess I can always change it to dual lug? Not familiar with 600 so curious about difficulty pulling it. TIA
 
SiddMartin said:
if you only have a single 3 pvc, how do you plan on pulling parallel?
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Cant you pull a parallel feed in a single conduit? 4 4/0 al and 2 4/0 for neutral?
 
William1978 said:
500 mcm copper is good for 400a per 240.4 (b)
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Be careful how you put that.

Yes, you can use 500 copper with a 400 amp breaker but that calculated load must still remain under the conductors ampacity of 380 amps.

So if you are contracted to provide a 400 amp feeder or service and you use 500 copper you will not have meet your obligation. You will have only provided a 380 amp feeder or service.
 
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normbac said:
Cant you pull a parallel feed in a single conduit? 4 4/0 al and 2 4/0 for neutral?
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For a full 400 amps in a single conduit with four current carrying condutors per 310.16 you will need.


Two sets of 4/0 copper

OR

Two sets of 300 aluminum.
 
kevdogg329 said:
Actually If you are pulling 90C wire, according to 310.16, 500 MCM is good for 430A.
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True. But according to 110.14(C), you cannot use the 90C ampacity unless you are using terminals that are also rated 90C. But since nobody appears to make such things, it is commonly just said that the 75C column of 310.16 is the limit for any conductor's ampacity.

Welcome to the forum.
 
charlie b said:

True. But according to 110.14(C), you cannot use the 90C ampacity unless you are using terminals that are also rated 90C. But since nobody appears to make such things, it is commonly just said that the 75C column of 310.16 is the limit for any conductor's ampacity.
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Then why does the NEC have a 90C chart if the lugs on the equipment can only give us 75C limits?

charlie b said:
Welcome to the forum.
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Thank You.

-Kevin
 
kevdogg329 said:
Then why does the NEC have a 90C chart if the lugs on the equipment can only give us 75C limits?



Thank You.

-Kevin
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For derating purposes! And I believe the wording allows "Under Engineering Supervision"!

I went to a different company a year ago and since than put a stop to numerous no no's as they never used the proper derating and used the 90 column to size feeders before my time. They are now commercial based and applied the dwelling unit acceptions to commercial applications before my time.
 
kevdogg329 said:
Then why does the NEC have a 90C chart if the lugs on the equipment can only give us 75C limits?
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You can use it for derating purposes, in accordance with the second sentence of 110.14(C).

For example, if you have nine current-carrying conductors in a single conduit, Table 310.15(B)(2)(a) says we must derate the conductor's ampacity to 70% of its tabulated value. Suppose you are using #12 conductors. If you used the 75C column of Table 310.16, you would start at 25 amps, multiply by 70%, and end up with an ampacity of 17.5 amps. This would not suffice, if your circuit needed an ampacity of 20 amps.

However, if you are using a type of conductor that has a 90C insulation system, such as type THHN, then you can start your calculation with the 90C column value. Thus, you get 30 amps times 70%, or an ampacity of 21 amps. In this way, you can still use #12 wire for circuits needing a 20 amp ampacity conductor, even if you need to derate the wire by 70%.
 
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