What size wire...

[flightline]

During my regular journeyman continuing education class last evening, we came upon a calculation problem that was a combination problem. By that I mean that you would have to take into account a number of different code rules and apply them. The instructors were at odds as to what the real answer was, definitively. So was the class of 50 electricians. I brought in one of MH?s books I keep in the car, but it addressed each aspect of the calculation individually, but did not answer the question as a whole. They suggested that if we visit these code discussion groups, we should pose the question. It seems like a no-brainer, and maybe it is. I know the answer I?d use, but I don?t wish to taint your responses.

By the way, this will be my first question or post for a while, and the first to the new board. I have looked for certain people like Andre, but haven?t seen him post up. I have been e-mailed by other collogues suggesting I visit a different board because they re-buff the new format. Well, here goes.

You have a continuous, non-motor load of 47 amps, served by 3 copper conductors in a raceway. A portion of this raceway runs through a boiler room with an ambient of 116 degrees Fahrenheit. Not knowing what type of terminations are in the circuit, what would be the minimum gauge THHN conductor would be allowed by code? [2002 NEC].

I?ll take all your answers back next week. If you visit the other boards, you may see the question worded differently; posted by one the other participants

 

[robbie]

Re: What size wire...

From the 1996 NEC Table 310-16 in the 90 Degree Celsius Column for THHN insulation. Also using the correction factor of 0.82 for ambient temperatures other than 30 degrees Celsius:

Minimum wire size: #6 (maybe?)

 

[aphares]

Re: What size wire...

47 amps would first have to be increased 125% before any adjustments apply.

47 x 125% = 58.75 amps (need conductor larger then 58.75amps.)

110.14(C)(1) covers what to do with the termination of the conductors.

less then 100 amps use 60'C, but adjustments can be calculated at 90'C as long as the final Calculation does not exceed 60.C column.

#4 at 90'C is 95amps. Adjustment of .82 for ambient temp.

95 X .82 = 77.9amps at 90.C
At 60'C #4 is good for 70amps.

 

[don_resqcapt19]

Re: What size wire...

Assuming that the terminations are suitable for 60?C conductors, I think that you need #4. The load is continuous so 210.20(A) requires that the ampacity of the branch circuit OCPD be 125% of the continuous load. 240.4 requires that the conductors be protected at their ampacity. 125% of 47 is 58.75. The minimum OCPD and conductor size is 60 amps. The correction factor for the high ambient is 0.82. If we divide the required circuit ampacity of 58.75 by the 0.82 correction factor we get a required conductor ampacity in the 90?C column of 71.65 amps. #6 THHN fits this requirement, but the 60?C ampacity of number 6 is only 55 amps, and the required circuit ampacity is 58.75 amps, so we have to use the next larger wire size.
Don

 

[russ]

Re: What size wire...

#6 at 60degree= 55amps fused at 60amps 240.4(B)

#6 at 90degree= 75amps x .82 = 61.5amps

#6 is my answer.

Russ

 

[russ]

Re: What size wire...

210.19 blows my first answer out of the water.
your right about the number 4.

Russ

 

[dana1028]

Re: What size wire...

I have a problem with "a portion of this raceway runs through a boiler room - 116 degrees F)....what portion? At least 10 ft. of the conduit (or 10%) must run through the boiler room, whichever is less. The question is incomplete.

Other than that argument, I agree with Don and aphares - i.e. #4

 

[iwire]

Re: What size wire...

Originally posted by dana1028:
At least 10 ft. of the conduit (or 10%) must run through the boiler room, whichever is less.

Dana1028, could you cite the code article for this?

 

[stamcon]

Re: What size wire...

Iwire, I think Dana1028 was referring to 310-15(a)
(2)exception(2001 California based on the 99 NEC).

 

[iwire]

Re: What size wire...

Thanks.