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NEC Wire Ampacity

When wiring two devices for instance a transformer to over current protection device of say 300A, I relaize would require 350kcmil THHW or 300kcmil THHN. Somewhere I thought I remember reading that you have to size wire with the ampacity of the LOWEST temp rated device in the circuit. That is if a panel board is rated at 75 degrees than you must use the 75 degree column in table 310.16 even if you are using THHN wire. Is this correct, and aren't most panel boards and breakers rated at 75 degrees unless specifically designed for 90 degrees? Thanks for the help.

Reply to
Brad
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The problem isn't always the panel board, or other device. One area where you'll experience limitations regarding which temperature ampacity column to use is the conductor termination. Very few lugs/connectors have a 90 degree rating. I sure you are aware of other conditions requiring deductions (multiple conductors in conduit, ambient temperature, etc.) in the allowable current capacity of conductors as well.

Louis--

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Reply to
Louis Bybee

Ok, so I am right in using 70 degree ampacity for THHN wire when some termination along the way is only rated for 70 degrees? Also, aren't most circuit breaker termination lugs rated for 70 degrees? The ultimate question is when designing how will I know which devices are rated for 70 degrees or 90. Subs often call me and say you sent 1/0 THHN wire for this 150A feed we can use #1 if we use THHN....I say yeah but the breaker is rated at 70 degrees..yada yada, they just look at me like I'm dumb and say, "but #1 THHN is good for 150A"...am I right or wrong on this one? TIA Brad

Reply to
Brad

.>? Also, aren't

Read the breaker label, it is on there. If nothing is on the label it is 60c Generally speaking, if it has a regular screw and plate it is 60c. If it is a hole with a set screw it is 75c. (typically over 30a since 240.4(D) restricts "small" wire)

Reply to
Greg

Actually, 75C (according to Table 310.16). But the answer is 'yes'. The conductor temperature rise should not exceed the rating of the equipment it terminates at. Likewise, the equipment temp. rise shouldn't exceed the wire rating. So, even if you use THHN (rated for 90C max), if it terminates with something rated at 75C, the maximum ampacity should be that from the 75C column in Table 310.16.

That's right. You are sizing the wire to restrict its temperature rise to 75C to maintain compatibility with its terminations.

Reply to
Paul Hovnanian P.E.

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