There is an old (1960's) unfused 600 amp disconnect fed with parrallel 500's.
We are looking to replace the switch. I was wondering what type of insulation was used back then. I should send a picture of the wire.
For an amperage rating of these wires, can I use the left coloumb (310.15(B)(16) in the NEC (60c-TW), to play it safe?
I know that table will give it a lower rating, plus I have to derate 70%. It's 6" gal, with 6-500's, and 2-250's for the nuetral.
Thanks, SJT

TW is a good guess.

Most of the larger sizes that I ever saw from that era were either THW or RHHW. I don't remember seeing type TW in anything above #10, but I'm not saying that it didn't exist.

I guess I was really just trying to establish a column in 310.16
I agree the insulation did not look like TW. I used to have a piece of 750 MCM under the seat of my car as a persuader in serious social situations.

I still have a piece of 600THHN for the persuader; hopefully it will just stay where it is.

I remember an industrial demo job that had 500 RHHW, and some 'newer' THW. Both had the same values at the scrap yard!!

I don't know where that piece of 750 is now. I gave it to my old buddy who was a cable splicer for Ma Bell. He planned on stripping it, dipping the fluxed strands in a lead pot and making a big lead head mace like you make a candle.

There is a building locally, that has 350 MCM TW (260A) for the main w/ 400A fuses in the main switch, the load regularly exceeded the rating of those conductors in the summer.