Minimum size conductor for a 50A Dedicated Circuit

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Balasquide

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A plan for a modernization of a public housing dwelling unit in Puerto Rico (the NEC applies) requires # 8 THHN (90 Deg C Rated) conductor for a 50A circuit dedicated for the range. Shouldn't the conductor be #6 in accordance with Table 210.24 (2002 NEC)?

The designer's argument is that, that table applies for cicuits that have two or more outlets in accordance with paragraph 210.24 and that table 310.16 should be used to determine the conductor size.

Is the designer right in his argument?

Balasquide
 
1793 said:
2005 Table 310.16 #8 @ 75deg. is good for 50 amps
If run NM #6 @ 60deg. for 55 amps.

We'll see what others say.
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I say the same thing, as long as the wiring method is not NM and the wire insulation is rated for at least 75C operation (which THHN is).
Just be sure that the breaker and receptacle is rated for 75C.
If you're using #8 NMB, the ampacity must be selected at 60C, which is 40A.
steve
 
Table 220.55 of the 2005 NEC specifies the permitted circuit loads for ranges.

Ranges from 12 to 27 kW nameplate rating may be sized based on the following demand calculated per Table 220.55: (See footnotes 1 and 4 of the table.) (I have simplified the math a bit for you.)

Calculated Demand = 8 kW + 0.4 kW(Nameplate kW - 12 kw)

That allows a 16 kW nameplate range to be served by a 9.6 kW (40 Amp) circuit and a 22 kW nameplate range to be supplied by a 12 kW (50 Amp) circuit.

For 40 Amps, you can use #8 in NM (Romex), or #8 THHN in conduit.

For 50 Amps you can use #6 NM, or #8 THHN in conduit.

You can use #6 THHN in conduit up to 65 Amps.
 
dcspector said:
Would one not have to use the 60c column tbl.310.16 according to 110.14(C)
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Only if the circuit breaker, outlet, wire nuts, or other termination point of the THHN are not rated to at least 75C.

If the circuit breaker and the 4-prong outlet for the range are rated at 75C, then you can use the 75C rating for the THHN.
 
dcspector said:
Yep....60c / 75c labled, listed breaker or device, equipment temination provision. would one take the lower...60c?
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If it's labeled 60C / 75C then either is permissible, including 75C.

Even 90C wire is permissible, but the current rating can't exceed the HIGHEST temperature-current rating for which the lowest-rated termination of the wire is labeled.

Is this supposed to be a trick question from the inspector?
 
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Balasquide said:
A plan for a modernization of a public housing dwelling unit in Puerto Rico (the NEC applies) requires # 8 THHN (90 Deg C Rated) conductor for a 50A circuit dedicated for the range. Shouldn't the conductor be #6 in accordance with Table 210.24 (2002 NEC)?

The designer's argument is that, that table applies for cicuits that have two or more outlets in accordance with paragraph 210.24 and that table 310.16 should be used to determine the conductor size.

Is the designer right in his argument?

Balasquide
Click to expand...
Bea said:
Originally Posted by dcspector
Would one not have to use the 60c column tbl.310.16 according to 110.14(C)


as refered by 334.80 also
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If the terminations of the wire are at 75C rated terminations, then the 75C ampacity may be used.

Bea; 334.80 refers to NM, which is rated at 60 C.

The origianal posting clearly stated that the wire was THHN and 90 C. Nothing was said about NM.
 
Bob NH said:
If it's labeled 60C / 75C then either is permissible, including 75C.

Even 90C wire is permissible, but the current rating can't exceed the HIGHEST temperature-current rating for which the lowest-rated termination of the wire is labeled.

Is this supposed to be a trick question from the inspector?
Click to expand...

Yes......sorry for any inconvenience......:)
 
Thank you all for your feedback. I guess that the designer is right.

The conductor is in NM encased in concrete. In Puerto Rico 99% of housing construction is made out of concrete, however, in some instances the local power authority requires more restrictive demands on electrical systems construction.
 
Balasquide said:
Thank you all for your feedback. I guess that the designer is right.

The conductor is in NM encased in concrete. In Puerto Rico 99% of housing construction is made out of concrete, however, in some instances the local power authority requires more restrictive demands on electrical systems construction.
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If your using NM cable,you need to use the 60 c column. You said the designer is calling for a 50a circuit.# 8 is only good for 40 amps.

The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60?C (140?F) conductor temperature rating.
Rick
 
Balasquide said:
Thank you all for your feedback. I guess that the designer is right.

The conductor is in NM encased in concrete. In Puerto Rico 99% of housing construction is made out of concrete, however, in some instances the local power authority requires more restrictive demands on electrical systems construction.
Click to expand...


This sounds more like RNC with THHN conductors in counduit, not NM-B cable.

Mark
 
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