ECN Forum Forums General Topics of the Trade General Discussion Area Old 500 mcm cable

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.