200A underground feeders

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Hfalz1

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Houston, Tx.
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For a 200 amp underground service 3/0 thhn is rated for 225A and 2/0 is rated for 195. Could i use the 2/0 or do i have to use the 3/0.
 
1st keep in mind, those ampacities (90°) are useless in most cases except for derating. Taking 110.14 in mind the more useful amapcities are 200 for 3/0 and 175 for 2/0.
2nd. When selecting conductor ampacity make sure you know the calculated load. Rule 1 dictates the conductor be able to carry the load.
3rd Do to the exception allowed by 310.15(B)(7), the answer to the question will depend on the service ( or main feeder) particulars (120/240 single phase ??) and the type structure you are supplying (residence ?)
 
If they are feeders, then I would suggest you check one of the following tables based on the kind of wire/cable you are putting in the duct. Underground has heat dissipation issues. Detail 1 is a single duct without others nearby.
TABLE B.310.15(B)(2)(6) Ampacities of Three Insulated Conductors, Rated 0 through 2000 Volts, Within an Overall Covering (Three-Conductor Cable) in Underground Electrical Ducts (One Cable per
Electrical Duct) Based on Ambient Earth Temperature of 20C (68F), Electrical Duct Arrangement in
Accordance with Figure B.310.15(B)(2)(2), Conductor Temperature 75C (167F)
and
TABLE B.310.15(B)(2)(7) Ampacities of Three Single Insulated Conductors, Rated 0 Through 2000
Volts, in Underground Electrical Ducts (Three Conductors per Electrical Duct) Based on Ambient Earth
Temperature of 20C (68F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2),
Conductor Temperature 75C (167F)
 
ron said:
If they are feeders, then I would suggest you check one of the following tables based on the kind of wire/cable you are putting in the duct. Underground has heat dissipation issues. Detail 1 is a single duct without others nearby.
TABLE B.310.15(B)(2)(6) Ampacities of Three Insulated Conductors, Rated 0 through 2000 Volts, Within an Overall Covering (Three-Conductor Cable) in Underground Electrical Ducts (One Cable per
Electrical Duct) Based on Ambient Earth Temperature of 20C (68F), Electrical Duct Arrangement in
Accordance with Figure B.310.15(B)(2)(2), Conductor Temperature 75C (167F)
and
TABLE B.310.15(B)(2)(7) Ampacities of Three Single Insulated Conductors, Rated 0 Through 2000
Volts, in Underground Electrical Ducts (Three Conductors per Electrical Duct) Based on Ambient Earth
Temperature of 20C (68F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2),
Conductor Temperature 75C (167F)
Click to expand...



OP didn't mention "ducts" he only mentioned underground which usually is just conduit, mainly PVC.
 
Hfalz1 said:
For a 200 amp underground service 3/0 thhn is rated for 225A and 2/0 is rated for 195. Could i use the 2/0 or do i have to use the 3/0.
Click to expand...

As others have mentioned you cannot use the 90° C rating of the conductors for final ampacity. So #3/0 THHN would be good for 200 amps. If you can comply with 310.15(B)(7) you may be able to use the 83% rule.
 
Daniel malack said:
If it is residential their is a chart in the nec that you need to apply.

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Daniel malack said:
Don't always go to 310-16, if it is residential you have an alternative.

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Chart disappeared in 2014 NEC. 83% of normal conductor ampacity is the rule now, but still gives you same size as the charts did before any ampacity adjustments come into play.

Also it must be a conductor that supplies the entire load of an individual dwelling unit, not just any residential related conductor.
 
THHN

THHN

Please note that THHN is not listed for wet locations as would be if installed underground in PVC.
Most THHN sold today is dual rated (THHN/THWN) that would allow installation in wet locations.
Just verify when purchasing wire.

Good Luck
:D
 
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