1. Is sizing a #10 egc with a #4 phase wire for a 60A circuit from the 60 degree column acceptable? I say yes, but I seem to have run into the next question.
2. Let's say I size #4 for a 60A circuit from the 60 degree column. At the job site, the terminations of the equipment and the conductor installed are 75 degree rated. Would my #4 wires technically be considered upsized and I would need to upsize the ground as well? My #10 egc now appears to be undersized.
3. Does anyone know of a device, not made for residential, that has terminations that are only 60 degree rated?
1. The temperature rating of the wire does not apply to the equipment grounding conductor. Whether you use #10 TW, #10 THWN, or #10 THWN-2 for your EGC, it will all have the same grounding performance. This does bring up the question of whether or not it complies to use 90C rated current-carrying wires with a 60C rated EGC, and completely ignore the 60C rating of the EGC. I would think it is, due to the fact that the EGC can be bare and still do its job, while the main reason for the insulation on the EGC is to curtail wire abrasion and wire/conduit corrosion. This is more of an an academic question, because 60C rared wire is rare.
2. Good question. The intent of requiring upsizing the EGC proportionally to the phase conductors is for applications where you have to upsize to curtail voltage drop. The idea is that if your circuit is long enough for this to matter, your grounding conductor needs extra size to reduce the resistance for the length. If you have to upsize for ampacity reasons, NEC2014 does not require you to do this, since your starting point is "the minimum size that has sufficient ampacity for the intended installation".
Per the letter of the NEC, it doesn't state that voltage drop is the only reason why you would have to upsize the EGC. Even if you use larger wire than needed due to it being left over from the previous job, it still requires you to proportionally upsize the EGC. Even if you use larger wire than needed due to ignorance about whether you have 60C or 75C terminations, and you end up with 75C terminations, you'd still need to upsize the EGC.
The loophole on this, is that you can sneak in a 60C rated splice, and make your EGC now correctly sized. First of all...good luck finding one. Second of all, while that makes the system comply, it actually makes it slightly less safe. One more component to add resistance to the main circuit, and potentially fail.
3. Remember: The rule is that the burden of proof is on the one who claims it is compliant at 75C for 100A and less. The default rule is that it is rated for 60C for 100A and less, and 75C for over 100A. Unless listed and marked otherwise. Today, most terminations and equipment are listed and marked otherwise. Even though it is a mostly academic rule, it still requires your due dilligence
Here's an example of a component that has a 60C rated termination. Notice that it is rated for #8 wire, and only indicates 40A per circuit.
http://www.mcmaster.com/#7618k658/=zlxwj1
Also, any time you use ROMEX, even 90C rated ROMEX, you have to comply with 60C rated terminations.
334.80