Could someone help clarify code requirements for parallel conductor ampacities? If I have two 300MCM cables per phase, is the ampacity equivalent to 2x the rating of the 300MCM cable, or is it equivalent to a 600MCM cable ampacity? The handbook's description of 310.4 makes it sound like it would be the ampacity of a 600MCM cable, but to me it seems like it would be 2x the 300MCM (assuming no multi conductor in a conduit derating). Thank you!
Parallel conductors
- Thread starter psa807
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ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Parallel conductors
You get (2)times the ampacity of the conductor.
Example: 800 amp service: parrallel 500's=760 amps, 240.4 lets you round up.
You get (2)times the ampacity of the conductor.
Example: 800 amp service: parrallel 500's=760 amps, 240.4 lets you round up.
george t. everett
Senior Member
- Location
- New York
Re: Parallel conductors
The International Residential code, table E3503.1 for service conductors, leads me to believe that parallel conductors need to equal the kcmil not the ampacity. Its a question I also would like to know the answer to.
The International Residential code, table E3503.1 for service conductors, leads me to believe that parallel conductors need to equal the kcmil not the ampacity. Its a question I also would like to know the answer to.
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: Parallel conductors
Hi George,
In that table, one example is;
400Kcmil or 2 sets of 4/0 = 400A
The cmil of 4/0 is 211600, times two = 423,200 cmil or 423.2Kcmil.
4/0 not in parallel are good for 230A@75d.
Parallel 4/0 are good for 208A X 2= 416A.
Conversely, 400Kcmil is good for 335A with no adjustments but they are rated at 400A for residential services only.
The cmil total area of several conductors will always carry more amps because of better heat dissipation and the "skin" effect of conductors.
[ February 04, 2004, 09:04 PM: Message edited by: websparky ]
Hi George,
In that table, one example is;
400Kcmil or 2 sets of 4/0 = 400A
The cmil of 4/0 is 211600, times two = 423,200 cmil or 423.2Kcmil.
4/0 not in parallel are good for 230A@75d.
Parallel 4/0 are good for 208A X 2= 416A.
Conversely, 400Kcmil is good for 335A with no adjustments but they are rated at 400A for residential services only.
The cmil total area of several conductors will always carry more amps because of better heat dissipation and the "skin" effect of conductors.
[ February 04, 2004, 09:04 PM: Message edited by: websparky ]
george t. everett
Senior Member
- Location
- New York
Re: Parallel conductors
Thanks Dave for your answer.
Thanks Dave for your answer.
bill addiss
Senior Member
Re: Parallel conductors
George,
I understand your confusion. You're in NY, right?
If our Residential Code is the same as The 2000 IRC Table E3503.1 says that 2/0 Copper is good for 200 Amps, but two sets of 4/0 are needed for a 400 Amp Residential Service.
I have no explanation though. Does anyone?
Bill
[ February 05, 2004, 07:40 PM: Message edited by: bill addiss ]
George,
I understand your confusion. You're in NY, right?
If our Residential Code is the same as The 2000 IRC Table E3503.1 says that 2/0 Copper is good for 200 Amps, but two sets of 4/0 are needed for a 400 Amp Residential Service.
I have no explanation though. Does anyone?
Bill
[ February 05, 2004, 07:40 PM: Message edited by: bill addiss ]
pierre
Senior Member
- Location
- Westchester County, New York
Re: Parallel conductors
Table 3503.1 in the NY Residential code has different parameters to follow than the NEC. If you follow Table 310.15(B)(6), it does not mention parallel conductors, it is restricted to 3 wire service entrance conductors. As for the table in the Residential code, it is more restrictive than the NEC.
Most contractors I know use two separate 200 amp main breaker panels each fed with 4/0 aluminum.
They call it a 400 amp service, but it is actually two 200 amp services.
Pierre
Table 3503.1 in the NY Residential code has different parameters to follow than the NEC. If you follow Table 310.15(B)(6), it does not mention parallel conductors, it is restricted to 3 wire service entrance conductors. As for the table in the Residential code, it is more restrictive than the NEC.
Most contractors I know use two separate 200 amp main breaker panels each fed with 4/0 aluminum.
They call it a 400 amp service, but it is actually two 200 amp services.
Pierre
Re: Parallel conductors
Current tends to travel on the outside surface of a wire instead of near the middle. This makes wire less efficent as it gets larger. The ampacity of a 800KCM wire is 490 Amps, while 400KCM is 335 amps. The reason the ampacity of the 800KCM isn't 2 x 335 Amps or 670 Amps is because of the above inefficency.
So, paralleling 2 sets of 400's can handle a lot more current than one set of 800's. In fact, thin rectangular cross sections are best at carrying current due to its large surface area (think bus bars or bus ducts).
So if you are paralling two sets of 400's for example, multiply the ampacity of one set by two.
STeve
Current tends to travel on the outside surface of a wire instead of near the middle. This makes wire less efficent as it gets larger. The ampacity of a 800KCM wire is 490 Amps, while 400KCM is 335 amps. The reason the ampacity of the 800KCM isn't 2 x 335 Amps or 670 Amps is because of the above inefficency.
So, paralleling 2 sets of 400's can handle a lot more current than one set of 800's. In fact, thin rectangular cross sections are best at carrying current due to its large surface area (think bus bars or bus ducts).
So if you are paralling two sets of 400's for example, multiply the ampacity of one set by two.
STeve
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