ECN Forum Forums Code Related Discussion NEC & other Code issues Why is NM restricted to 60 deg ampacity?

Bob,

Consider this as an example from a different direction: Suppose that you have three #8 THHN CCCs in a pipe. Further suppose that they are connected to 90C terminations so that they can run using the 90C table values. By table 310.16, we can use these at 55 amps.

Can we agree that the purpose of the 55 amp limit is to keep the wires below the 90C temperature limit?

Then, suppose that we add more conductors. Say, bring the total number to 20 CCCs. Because of the thermal resistance of the conduit and surrounding materials, the temperature inside the pipe would heat up to more than the 90C limit if we ran these 20 #8's at 55 amps. So, we must derate to keep the temperature below 90C. Table 310.15(B)(2)(a) says that we have to derate by 50%, to 27.5 amps. By doing so, we keep the temperature in the pipe below the 90C limit.

The same logic applies to NM. When we derate using the 90C table values, we are derating to keep the temperature below the 90C temperature limit.


Let's start out with a case that's a bit artificial. Suppose you have your cables starting at the panel, running for 20 feet with adequate separation. Then, for some reason, you have to bundle them for 20 feet, and then they run for another 20 feet with separation before they reach the load.

In this case, it's easy to see that the 20-foot bundled section can get hot, but the terminations will be cooler. So the bundle might be at the 90C limit, but you'd be fine with 75C terminations.

More realistically, the bundling or well-filled conduits normally occur right at the panel. (And I'm talking here about all wire types. This case is particularly true for THHN in pipe.) It appears that normal practice is to assume that the length of wire inside the panel, before it gets to the breaker, is sufficient to allow the wire to cool enough for a 75C termination.

I'm not sure that's really what happens, but it appears that this assumption doesn't cause problems in practice, so no one has tried to make the code more restrictive in this area.

I suspect that the reason it works out OK is the derating tables are worst-case, but the worst case just doesn't happen in practice. For example, you can run 20 #10 THHN derated to 20 amps in one pipe. In theory, if there were 20 amps on all 20 conductors, you could get up into the 90C range. In reality, you just don't have that much load on all of them. If they are, say, office receptacles, the average load might be a third to half of the 20 amps. So, in practice, the wires don't get hot enough that they're toasting the 75C terminations on the breakers. Thus, no one has bothered to change the code to fix a theoretical problem that isn't being seen in practice.


[This message has been edited by SolarPowered (edited 02-21-2007).]

I disagree with your assumption. In some cases the terminations might only be rated 60 C.

So we would have to drop 30 C or 86 F in a short distance.

Even with 75 C terms we would have to drop 43 F in a short distance inside an enclosure that would certainly be heating up.

In your other example with the NM I would say that obviously the bundled section will be warmer than the unbundled section but no section will exceed 60C.

I am still willing to admit I am wrong here but sure would like to here some other views or explanations.




[This message has been edited by iwire (edited 02-21-2007).]

Sorry that I've not been following this thread, but the original question happens to be one that really bugs me. It seems to me that the CMP did one of those 'back handed' code changes where one part of the code is adjusted to correct an error that really should have been dealt with someplace else. An example is 310.15(B)(6), where a better load calculation would probably make more sense. In this case, the CMP is correcting an error in ampacity calculation; but rather than saying that NM cables with 90C temperature ratings have _different_ ampacity than THHN conductors in conduit with 90C temperature ratings, they simply say that NM cables must be used at their 60C ampacity.

The physics of ampacity calculation is given by the Neher-McGrath equation, http://www.calcware.com/cwnmcalc2.htm . This equation is explicitly permitted to be used for ampacity calculations, under 310.15(C) if you have suitable 'engineering supervision'.

What the Neher-McGrath equation gives us is a relationship between the conductor temperature, the surrounding ambient temperature, the electrical resistance of the conductor, the thermal resistance between conductor and ambient, and the current being carried by the conductor. The conductor temperature is not set only by the current and ambient conditions, but is also set by the thermal resistance to ambient. Presumably any other sources of heat (other conductors) in the vicinity would be considered somehow as part of the ambient.

The tables permitted under 310.15(B) are based upon the Neher-McGrath equation with certain assumed parameters of thermal resistance and insulation characteristics. These parameters are never stated explicitly, however it is quite clear from the way the values of table 310.16 change with ambient temperature and permitted insulation temperature that the Neher-McGrath equation is being used.

This tells me that if a conductor has an ampacity of X amps, and I push X amps through that conductor, then if the presumed conditions of thermal resistance hold, then the conductor will heat up to its maximum temperature ( the temperature value at the top of the column). If I take three #8 THHN conductors, and bundle them together into a conduit, and run 55A through each of them, and if the ambient temperature is 30C, and <b>if the thermal resistance of my experimental setup matches that assumed when table 310.16 was generated</b> then the temperature of these conductors would hit 90C.

In particular, if the _derated_ ampacity of a conductor is based upon the 90C rating, then I expect the conductor to heat up to 90C when used at its ampacity. Going back to the example above, three #8 THHN conductors, in a conduit, in a 50C ambient condition. I run 45A through these conductors. I expect a conductor temperature of 90C, even though the conductors are being used at less than their normal 75C ampacity.

Unfortunately, I've not worked 310.15(B)(2)(a) into this thought process Take 3 conductors at full capacity and call the heat output 1. 6 conductors at 0.8 capacity would have a total heat output of 1.28, 9 conductors at 0.7 would have a total of 1.47, 20 conductors at 0.5 would have a total of 1.67, 30 conductors at 0.45 would have a total of 2.03 and 40 conductors at 0.4 would have a total heat production of 2.13. Clearly the assumptions about thermal dissipation capability have to change as the number of conductors in a raceway goes up.

I am forced to assume that 310.15(B)(2)(a) was derived in a fashion similar to table 310.16, and that if I have a set of conductors with derated and _adjusted_ ampacity of X, and I run X amps through these conductors, that the temperature of the conductors would rise to the 'temperature rating' of the conductor as used in table 310.16 prior to derating and adjustment.

This problem with this whole approach, however, is that the thermal resistance in the real world is very unlikely to match that used to derive table 310.16. The thermal resistance numbers are presumably relatively conservative. However if, for whatever reason, the thermal resistance in a given situation were _higher_ than that assumed for table 310.16, then the temperature of the conductors would end up higher than expected.

My first guess is that the CMP decided to deal with the issue of 'NM cable buried in thermal insulation' by simply saying 'use the 60C ampacity'. This is probably a much simpler approach than having to develop an entire different set of tables for 'NM cable buried in thermal insulation' and trying to deal with the enforcement nightmares. But it hides the reality that a 90C conductor is a 90C conductor, designed to be used at up the point where the wires are actually at a temperature of 90C, near to the boiling of water.

The above leads me to a different guess. NM cable is generally permitted in flammable construction where the fuel load of the plastic cable is nothing compared to that of the rest of the building. I guess the thought is that 'wood is already flammable, why worry about a bit of plastic'. The kindling temperature of wood is very much dependant upon the water content of the wood. Heat wood up to 90C for an extended period of time, and you boil the water off and make it easier to ignite. Perhaps the 60C restriction is used to protect the building materials that NM is ordinarily used with.

-Jon

But you just explained that using the derating rules the temperature will reach 90 C.

I appreciate you jumping in here, I am trying to soak this in but I am not convinced that a conductor loaded to its rated capacity 60 C under the conditions given will normally reach 60 C.

I am willing to bet that there is some headroom left there for safety.

Bob

Bob- I'm watching to see what the answer is to the question you've asked. I've asked the very question of UL and they don't come back with any answers. After I'd asked a coupla different times from a coupla different UL reps at our IAEI meetings I quit asking - figured they thought it wasn't an important enough to justify an answer. I like you, suspect that there is some fudge room and the actual tempetature reached is probably 10° or so below the insulation rating. What is clear to me is it's the insulation on the wire that we are concerned about not the copper wire. A #12 copper wire will actually carry about 100a. in the uninsulated state. Don't know where I read that information but it stuck with me.

As I said, I was only guessing as to the why of the 60C rating.

I am certain that the ampacity values are based upon the temperatures used at the top of table 310.16.

I agree that there must be serious 'fudge factors' built in to the tables, however IMHO these fudge factors are in the thermal resistance values assumed.

If the fudge factor were in the temperature (meaning that they would say 90C but really target 80C, for example) then the correction factors at the bottom of the table would be different.

Say you have 10 12/2 cables bundled together, carrying 15A per circuit (so that you have 20 current carrying conductors, 30A derated to 15A by 310.15(B)(2)(a). I believe that in the thermal resistance circumstances built in to 310.16 and 310.15(B)(2)(a) that the temperature in the core of the bundle would hit 90C, and that if the real temperature is lower this is because the thermal resistance is lower.

-Jon

CMP7 says the answer is in a report to the 84 code (ROP to the 2002 code 7-178). Maybe someone with the ROP from that cycle can see what they are talking about.

Don't know if this helps but the following is from the 1981 NEC Handbook commentary on Section 336-2:
Types NM and NMC may have conductors rated 60'C (140'F), 75'C (167'F), or 90'C (194'F) for use in different ambient temperatures. Cables with conductors rated at 75'C (167'F) are designated type NMH or NMCH, and thoase with conductors rated 90'C (194'F) are designated Type NMHH or NMCHH. The ampacities of nonmetallic-sheated cable types, regardless of the conductor tempurature rating, are those of 60'C (140'F) conductors.

The commentary from the same section in the 1984 Handbook:
Prior to the 1984 Code, Types NM and NMC could have conductors rated 60'C (140'F), 75'C (167'F), or 90'C (194'F) for use in different ambient tempuratures. Cables with conductors rated at 75'C (167'F) were designated Type NMA or NMC-A and those with conductors rated 90'C (194'F) were designated Type NMB or NMC-B. The ampacities of nonmetallic-sheathed cable types regardless of the conductor tempurature rating, are those of 60'C (140'F) conductors.

I believe all the NM cable on the market now is Type NMB. The ROP and ROC from 1981 and 1984 would seem to be the place where the answer to the 60' ampacities mystery can be found.
Alan--

When there was a mix of ampacities out there this probably made sense (like in 84) but now it is hard to find any 60c NM unless you are digging in an old wall. I am not sure why this still remains, hence my proposal.
I was never happy with the answer but it is my fault that I didn't ask for more from them in the comment phase. My bad ... sorry.