SO Cord Ampacity and Termination Temp per 110.14(C)

[JuliaTruchsess]

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.

 

[zbang]

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.

 

[JuliaTruchsess]

if the cable is rated for 60 amps at 90C, it's also rated for 60a at 75C

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.

 

[LarryFine]

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.

Would using 310.15(B)(16) be prudent enough?

 

[wwhitney]

If 400.5(A)(1) is for 90C insulation, then it means that for 8AWG SO at 35A, the copper is close to 90C.

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

 

[JuliaTruchsess]

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.

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

 

[LarryFine]

Keep in mind that current ratings are based on avoiding insulation damage.

 

[JuliaTruchsess]

Keep in mind that current ratings are based on avoiding insulation damage.

Of course. That's how we know that when we look at a specific gauge and ampacity in 310.15(B), the conductor temperature at that current will be the insulation's temperature rating, less some margin.

 

[wwhitney]

That's how we know that when we look at a specific gauge and ampacity in 310.15(B), the conductor temperature at that current will be the insulation's temperature rating, less some margin.

In the worst case thermal environment (e.g. embedded in thermal insulation) at the given ambient temperature.

Cheers, Wayne

 

[Birken Vogt]

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?

 

[junkhound]

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.

 

[JuliaTruchsess]

I feel that G, G-GC, and W are a higher quality cable, and offer much higher ampacity

I'm 100% with you there, and am trying to get my client to change over to Type W. But for now they're using SO and I have to deal with it.

 

[Birken Vogt]

I'm 100% with you there, and am trying to get my client to change over to Type W. But for now they're using SO and I have to deal with it.

In that case, it seems you are spending too much time thinking about an inferior product with an inferior specification, and maybe need to tell the customer how the cow eats the cabbage. Or they can just pay for oversized copper.

It seems obvious that the termination temp is way under 60C if using SO, since the 60C ratings of the same spec type W are much higher.

 

[zbang]

the conductor temperature at that current will be the insulation's temperature rating, less some margin.

I don't think that's necessarily a valid assumption - for instance, take a 10' piece of 12g THHN and run 20 amps through it for an hour. Will it feel warm? Maybe, will it heat up to 60C? I doubt it. That 10 feet only has a resistance of about 0.016 ohm and is going to throw off about 6 watts; not going to get very hot unless there's nowhere for the heat to go.

you are spending too much time thinking about an inferior product with an inferior specification,

Why is SO inferior to W?

One thing missing in the discussion is the actual use case and conditions.

 

[Birken Vogt]

Why is SO inferior to W?

One thing missing in the discussion is the actual use case and conditions.

Look at tables 400.5(A)(1) and (A)(2).

8 AWG SO is rated at 40-35 amps for 2-3 CCC.

8 AWG type W is rated at 55-48 amps under the same conditions at 60C, and even higher at 75 or 90C.

Besides that, there is the insulation-falls-off-the-conductors factor that frequently comes up with SO. No rhyme or reason to that one though.

 

[JuliaTruchsess]

I don't think that's necessarily a valid assumption

It's not an "assumption", it's the way the table works. Confirmed explicitly here by Mike Holt himself a while back, but I can't find the post anymore.

Why is SO inferior to W?

That's a question for the ages and the code authors. We can only speculate that it has to do with the construction and materials, but looking at both they don't seem different enough to account for W's much higher ampacity.

One thing missing in the discussion is the actual use case and conditions.

3-phase busway tap-off box drop cords in 40C ambient.

 

[wwhitney]

It's not an "assumption", it's the way the table works. Confirmed explicitly here by Mike Holt himself a while back, but I can't find the post anymore.

90C Type SO has a lower ampacity than 90C Type W. Possible reasons for this that I see are:

1) The construction of Type SO means that in identical environments it can reject heat less well than Type W can.
2) Type SO is more likely to be used in environments which inhibit heat rejection than Type W is.
3) Type SO deserves a greater margin of safety because of the relative robustness of its construction relative to the physical demands of the environment within which it is being used. That is, keeping the heat cycling to a lower magnitude reduces stresses on the materials, and/or Type SO is expected to be used in more abusive environments.

So certainly reasons (1) and (2) would mean that when used at its ampacity, the equilibrium temperatures of Type SO and Type W would be similar. But reason (3) would not mean that. The discrepancy in ampacities is so large (for #12 Cu, 35A vs 20A, or if the heat rejection rates were equal, a factor of 3 difference in temperature rise) that I figure reason (3) must be at play.

Cheers, Wayne

 

[JuliaTruchsess]

1) The construction of Type SO means that in identical environments it can reject heat less well than Type W can.

This was my thinking.

2) Type SO is more likely to be used in environments which inhibit heat rejection than Type W is.
3) Type SO deserves a greater margin of safety because of the relative robustness of its construction relative to the physical demands of the environment within which it is being used. That is, keeping the heat cycling to a lower magnitude reduces stresses on the materials, and/or Type SO is expected to be used in more abusive environments.

Interesting, although the NEC doesn't really provide any support for this other than classifying SO as

• "Hard service cord” for “Extra-hard usage”

and Type W as

• “Portable power cable” for “Portable, extra-hard usage”

Pretty ambiguous and unhelpful terminology.

reasons (1) and (2) would mean that when used at its ampacity, the equilibrium temperatures of Type SO and Type W would be similar. But reason (3) would not mean that. The discrepancy in ampacities is so large (for #12 Cu, 35A vs 20A, or if the heat rejection rates were equal, a factor of 3 difference in temperature rise) that I figure reason (3) must be at play.

Excellent point.

 

[wwhitney]

Interesting, although the NEC doesn't really provide any support for this other than classifying SO as

FWIW, Type SO cord is listed to UL 62 "Flexible cord and cables", while Type W is listed to UL 1650 "Portable power cables". Perusing those standards (which can be done online with a free account at shopulstandards.com, although the interface is inconvenient) would tell you the difference in construction in terms of insulation and jacket materials and thickness.

Cheers, Wayne

 

[JuliaTruchsess]

Type SO cord is listed to UL 62 "Flexible cord and cables", while Type W is listed to UL 1650 "Portable power cables".

Ah, now that's great info, thanks.