400 amp residential service question

[jzadroga]

Hi All. I am using the new 2017 code and have a question of the size conductors I can use for a 400 amp residential service. Here is what I have

90 degree C Copper conductors
400 amp Single Family Service
Ambient Soil Temp of 70 degrees
Conductors have no more than 10' above grade.
Conductors in conduit
Paralleled conductors

Conductors supply entire dwelling load

As I understand using chapter 3,

Temp correction multiplier is 1.04
6 conductors in a raceway multiplier is .8
Single Family Service multiplier is .83
75 degree C terminations

using this info I come up with:

Applying the Service demand factor I only need an ampacity of 332 amps (400x.83) or parallel conductors with an ampacity of 166 amps.

Using 3/0 THWN from the 90 degree C column. it is good for 225 amps. Multiply that by 1.04 (225x1.04) for Ambient Temp Correction I get 234 amps

Then correcting for 4 to 6 conductors in a conduit I get (234x.8) which is 187.2 amps per 3/0 conductor

Parallel that, I get 374.4 amps.

and finally because I have 75 degree C terminations I have to use the 75 degree C column to find that I'm OK with 3/0 for the termination (75 degree C is 200 amps per termination or 400 amps). 2/0 would be OK if it had the ampacity.

Does anyone else agree or disagree?

Taking that further: If I terminate the underground portion using 90 degree C connections before the enclosures containing the 75 degree C breakers, and I use 2 conduits instead of 1. I think that allows me to use 1/0.

 

[Julius Right]

In my opinion it is correct -According to NEC 2014 Table 310.15(B)(16) for 75oC 3/0# copper conductor [200 A] in earth [of 90 RHO] at 70oF[21.1oC] k1=1.04[90oC insulation rated]. For 6 loaded conductors k2=0.8 then total multiplying factor =1.04*.8=0.832
You need 332/2/.832=166/0.832=199.5 A permissible.
You may neglect 10' above the ground run.
You have to check the voltage drop-branch and feeder-no more than 5% as recommended by NEC 2014 art.215.2 Minimum Rating and Size. Information note no.2
3/0 # copper Zeff [See ch.9 Table 9] =0.095[single phase to neutral]
If the total length is not more than 890 ft. for 480 V DV=166A*SQRT(3)*0.094/1000*890=24.05 V=5.011%.

 

[mwm1752]

sounds like you already have conduit in the ground -- 1st things 1st-what is the service load calc for the residence? wanting & requiring are 2 different issues

 

[petersonra]

you don't start by assuming a 400 Amp load. You have to do the calculations to see what the load actually is and go from there.

 

[jzadroga]

Thanks for the replies. I am speaking hypothetically for now. I want to know the smallest size for this given situation.

What are your opinions about the second part?

 

[electrofelon]

Thanks for the replies. I am speaking hypothetically for now. I want to know the smallest size for this given situation.

What are your opinions about the second part?

Yes you can use the 90 degree column ampacity if you transition back to and terminate with a size based on the 75 degree column ( assuming your conductors and splicing devices are appropriate). What is not entirely clear is how long the transition conductors must be or if the splice can be in the same enclosure. This has been debated before and there are some theories based on UL test procedures, but you wont find and specific guidance in the NEC.