SO Cord Ampacity and Termination Temp per 110.14(C)

JuliaTruchsess

JuliaTruchsess

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With THHN etc and Type W portable cord, the NEC tables provide ampacities at various insulation temperatures, so they can be used to ensure compliance with 110.14(C), e.g. if you have 75C terminations you use the 75C column of the table.

Table 400.5(A)(1) for SO, SOW, SOO, SOOW etc cord, however, does not have temperature columns - I guess it assumes that all SO is 90C? So how is one supposed to verify compliance with 110.14(C)? Measure the temperature under load? Makes it difficult to plan in advance.
 
Portable cords are a different use case than the usual building wire, if nothing else they're usually unconfined rather than in a conduit or wall (ref- prohibitions on the use of flexible cords).

Pretty much assume 90C as most or all manufacturers rate their type SO/etc at 90C, check their spec sheets to be sure (some single conductors are rated for 105C). Since the conductors are either 90 or 105, fall back to the ratings of the terminals - if the cable is rated for 60 amps at 90C, it's also rated for 60a at 75C and at 60C.
 
zbang said:
if the cable is rated for 60 amps at 90C, it's also rated for 60a at 75C
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You miss my point. 400.5(A)(1) tells me the cord's ampacity, but because it doesn't have temperature columns it doesn't help me select a gauge that ensures I won't exceed the temperature rating of my terminations. When 310.15(B)(16) tells you that 75C 8AWG THHN can handle 50A, it's also telling you that if you put 50A through 8 AWG THHN, the conductor will be at 75C (less some margin). That's how you comply with 110.14(C) - you use the column corresponding to your terminations' temperature limit.

If 400.5(A)(1) is for 90C insulation, then it means that for 8AWG SO at 35A, the copper is close to 90C. But it doesn't provide a way for me to know what gauge I need to use in order to keep the temperature under 75C for my terminations at a given current.
 
JuliaTruchsess said:
If 400.5(A)(1) is for 90C insulation, then it means that for 8AWG SO at 35A, the copper is close to 90C. But it doesn't provide a way for me to know what gauge I need to use in order to keep the temperature under 75C for my terminations at a given current.
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Would using 310.15(B)(16) be prudent enough?
 
JuliaTruchsess said:
If 400.5(A)(1) is for 90C insulation, then it means that for 8AWG SO at 35A, the copper is close to 90C.
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I don't think so--I don't think the jacket on SO cord is so much thicker than on a cable-type wiring method that its ability to disperse heat will be so much less, causing the copper temperature to be higher at a given current level. Rather, I think the values in Table 400.5(A)(1) are lower than the values in Table 310.16 as an extra safety factor because of the qualities of the cord and its usage environment. So I would say for purposes of 110.14(C) termination temperature, you can use the values in Table 310.16 as usual even with cord.

Now if I were wrong, and if you knew that with #8 AWG in certain conditions 35A would cause a 90C copper temperature in 30C ambient, you could answer your original question via Equation 310.15(B). Namely, if 35A causes a 60C temperature rise, then for 45C temperature rise (so 75C in 30C ambient), the allowable current would be sqrt(45/60)*35 = 30A, and for 30C temperature rise (so 60C in 30C ambient), the allowable current would be sqrt(30/60)*35 = 25A. [Which, BTW, is precisely the pattern in Table 310.16 for #10 Al. Up to rounding, any set of 3 entries in Table 310.16 will be in the ratio 0.707 : 0.866 : 1.]

Cheers, Wayne
 
wwhitney said:
I don't think so--I don't think the jacket on SO cord is so much thicker than on a cable-type wiring method that its ability to disperse heat will be so much less, causing the copper temperature to be higher at a given current level. Rather, I think the values in Table 400.5(A)(1) are lower than the vales in Table 310.16 as an extra safety factor because of the qualities of the cord and its usage environment. So I would say for purposes of 110.14(C) termination temperature, you can use the values in Table 310.16 as usual even with cord.
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That's an interesting take and food for thought, thanks. 400.5(A)(1) tells me I can put 35A max through 90C 8AWG SO (ignoring all derating). Following your logic, 310.15(B) would then tell me that an 8AWG conductor in an SO cord carrying 35A will be < 60C. I'm going to measure it and will let you know :)
 
It is strange because the next table down for type G and W has the much higher ampacities that they offer in the same AWG, and at the usual 60/75/90C ratings.

If you hold 8 AWG type SO and 8 AWG type W in your hand, you won't tell the difference unless you read the jacket.

I feel that G, G-GC, and W are a higher quality cable, and offer much higher ampacity, so I rarely use SO for any reason.

To the OP, why not use one of these instead of SO?
 
Practical application note:

Using SO cord in free air or lying on the ground, for practical use of NEC tables is only academic for non OSHA application.

I regularly use my 50 foot 10 AWG SO extension cord for a 208V 60A pressure washer and also use it when I need to get a 200A arc elder closer to the workpiece. Warm to the touch After a few hours continuous use but have never had any problems.

Of course, as already mentioned, if buried in wall insulation it is an entirely different story.

In a pinch, have even use 12 AWG NM as non osha 'extension cord' in open air for over 50 amps.
 
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