Re: Derating:Experts Only
I nominate ?iwire? for an ?attaboy? on his responses. Let me elaborate.
Originally posted by iwire:The surrounded conductors can not dissipate the heat as well as those that are exposed to ambient temps.
That is the key to understanding the derating concept. But first a digression into background information.
Considered in isolation from the rest of the universe, the ability of a
bare conductor to carry current is limited by its own cross-sectional area. Too much current will threaten the integrity of the metal structure, and will eventually lead to melting. Now you add insulation to the bare conductor. The heat generated from too much current will threaten the integrity of the insulation system long before the metal is in danger of melting. The better the insulation system, the more heat it can take (translate, ?the more current it can handle?), before there is a risk of failure. The ampacities shown in the 75C and 90C columns of Table 310.16 are higher than those of the 60C column for this very reason: the insulation system of a THW is better than that of a TW, and the insulation system of a THHW is better still.
So we limit the current to the values posted in 310.16 to protect the insulation systems. But 310.16 is based on a specific type of installation (?Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth?) and on a specific limit on ambient temperature (30C). If you start at a higher ambient temperature, then you have less of a ?heat rise? to go before you exceed the insulation system?s temperature limit. But if instead of raising the temperature of the surrounding dirt, you add more current-carrying conductors, then each conductor will have to dissipate its heat in the presence of other conductors that are trying to do the same. None of them will have an easy time of it. From the perspective of each individual conductor, the ambient temperature surrounding it has risen, and that will cause its own temperature to rise.
Here is what I think and maybe a engineer can let me know if I am on the right track. I believe that . . . a bigger conductors ability to dissipate heat is already figured in to the ampacity tables.
I am not an expert, but I am an engineer. You are right. Before taking into account any derating factors, a 3/0 copper THHN is good for 225 amps, and a #12 copper THHN is good for 30 amps. If you put 3 of the 3/0 in one conduit, and three of the #12 in another conduit, then you could safely run every bit of the 225 amps through the first circuit, and every bit of the 30 amps through the second circuit, and you will not have to worry about the insulation systems of either circuit. (ASIDE: You also have to consider terminal provisions, but that?s another story.) Table 310.16 has already taken into account the greater ability of the larger conductor (and its larger insulation system) to carry current, when it assigned the 225 amp limit to the 3/0. If you now add a fourth current-carrying 3/0 to the first conduit, then each of the 3/0 conductors will see a higher ambient temperature that was assumed in the development of Table 310.16. That is why we can no longer pass every bit of the 225 amps through that circuit.
It seems the larger the conductor the less capacity it has for a given area.
That is because the current tends to travel along the outer edges of the conductor. This is the ?Skin Effect,? as ?the other Bob? has already correctly pointed out.
