Parallel Service Conductors

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mike12347

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120/240 single phase service with two sets of 250's in seperate conduits would have an ampacity of 580A, because 310.15 (B) 2 would not apply.
Correct?
 
You'll find differing opinions as to whether or not you can use 310.15(B)(2) for parallel service entrance conductors. One thing is for sure 2 sets of 250 KCMIL THHN copper will have an ampacity of 510 amps. (255+255)
 
mike12347 said:
Infinity, How are you getting 255A with THHN
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As Bob stated, almost all equipment has lugs rated for 75 degrees C. This means that the conductors must be sized for 75 degree C also. 250 KCMIL Cu THHN has a 75 degree ampacity of 255 amps. You can look at the Article Bob referenced, 110.14(C)(1)(b)(2) for more information.
 
mike12347 said:
My thought is with seperate conduits it wouldn't apply.
Agree?
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In reading 310.15.(B)(6) I do not see anything that prevents this installation because of seperate conduits. What is the ampacity of the main panel?
 
mike12347 said:
My thought is with seperate conduits it wouldn't apply.
Agree?
Click to expand...
IMO, because it does not explicitly state that we can. The rule is a fork in the road from "the normal way" of sizing conductors, with a boatload of restrictions on it.

The conductors have to serve an individual dwelling unit.

The system has to be 120/240V. 120/208 is not allowed, because it's not mentioned. They intend for it's exclusion, so they intentionally omit it.

When we start to rationalize it and say, "Well, this is practically the same as that..." we start to stray farther from what they intended. It's an exception from the rule, not the rule itself (as far as sizing conductors for equipment goes).
 
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