SRML wire and termination temp limits

[haze10]

I have an industrial oven to wire and need some advice. The oven consists of 6 zones of electric heaters, 3 on each side, 262 amps @ 63VAC each zone. Power is supplied via a 50A fuse to a SCR to a 21KVA 480-63 single phase transformer, 20ft of wireway to the heater connection. Since there is no secondary overload protection, I have to use the primary overload times the voltage ratio (480/63) or 50 x 7.62 =382 amps. Ambient temp along the oven wall where the heater connections are is 50C. Manufacturer suggests SRML cable in 2/0 but I don't believe this is sufficient. I know how to use the derating tables, but have a couple of questions.

Silicon Rubber insulation is listed in 310.13 as type SA with a operating temp of 90 or 200C. Table 310.17 lists SA only under the 90C rating. Table 310.18 lists 200C conductors but does not include the SA insulation. What is the correct table to use for 200C SRML.

Also, how does any of the 200C rating ampacity tables actually help anyway. Are we not limited also by the temperature rating of the termination. Checking the Burndy catalog for compression fittings all I see is 90C rated lugs. Yet Burndy says that their tin plated comrpession fittings can be used up to 375F (190C), but they are not UL listed for this temp.
The heater connection is from the manufacturer and is Dossert Ground clamp with no temp listing. Althought it is all copper bronze and is very beefy. Local cable guys says he sees them all the time on industrial furnaces.

Any comments regarding ampacities, and terminations.

I can't reduce the 50A primary as on cold startup current can reach 320 amps for brief periods.

 

[rbalex]

Re: SRML wire and termination temp limits

SRML is a UL recognized (rather than listed) conductor designed for motor leads. None of the 310 Tables apply.

With regard to type SA, the 200C rating is for special applications where design conditions require maximum conductor operating temperatures above 90C. In other words, the ambient is already so high that the conductor cannot avoid exceeding 90C. This leaves a bit of a problem, of course, since the rest of Table 310.16 gives no direction for ambients above 80C.

This would call for a supervised engineering calculation per 310.15(C) to determine the ampacity "under condition of use."

In theory, type SA can be used in ambients where the additional heat generated by the current would cause an operating temperature up to 200C. The major problem is that the overload (as opposed to the short-circuit) withstand time is very limited. As a thermoset, SA only has a tested emergency rating to 250C which can be reached very quickly under even modest overload. However, since the application is for heaters the chance of overloading is rather limited.

[Edit Add:] As you have described it though, startup may be a problem. With regard to the terminations, consult with your AHJ. "Listing" is simply third-party testing against a minimum specified Standard. A product may actually perform better than listing requires. For special applications as you have described, it is reasonable to use manufactures data. (See FedOSHA definition iii of "acceptable" in 29CFR1910, Section 399)

[ September 13, 2004, 04:49 PM: Message edited by: rbalex ]

 

[derwith]

Re: SRML wire and termination temp limits

I had an industrial heater that was installed in a crude oil tank about 60 ft in diameter and 30 ft high. The heaters were 50 ft. long and operated at 480. I don't remember the feeder size but it was probably 600-800 amps.

The heater terminals were outside the tank. I don't remember the wire insulation but I know I had to change the wire from copper to nickel because the copper was melting. I believe the insulation was some type of silicone/glass.

The heaters were supplied by others but I thought at the time that the heat should not have been transferred to the electrical terminals.