240(B)

[Zsmith40]

Hi everyone i have a quetion dealing with 240(B) on how to use it. Can you put a #8 THHN conductor on a 50 amp breaker if the equipment terminals are listed at 60 degrees? I know that #8 in the 60 degree column is only 40 amps and #6 is 55 amps but could you use a # 8 and refer to 240(B)?

 

[tortuga]

You probably mean 240.4(B)

Can you put a #8 THHN conductor on a 50 amp breaker if the equipment terminals are listed at 60 degrees?

No I don't read 240.4(B) as modifying 110.14(C), if something is marked 60C terminations its probably pretty old and was made for TW wire. New devices in the 50 amp class that I see are all marked 75C.

 

[LarryFine]

Welcome to the forum.

It depends on the equipment, the type of load.

Motors and HVAC loads have higher OCP allowances for starting current.

Also, most of those loads contain overload protection.

 

[retirede]

To answer your literal question, yes, you can attach #8 to a 50A breaker, then transition to #6 before attaching to the 60C equipment. I’m presuming the breaker is rated 75C.

It’s not clear if “equipment” is the load, or the breaker?

 

[tortuga]

Welcome to the forum.

It depends on the equipment, the type of load.

Motors and HVAC loads have higher OCP allowances for starting current.

Also, most of those loads contain overload protection.

Don't confuse the allowed OCPD with the temperature limitation of terminations.
The fine print note in 430.21 states

Informational Note: See 110.14(C) and 430.9(B) for equipment
device terminal requirements.

If say you have some odd duck motor or breaker rated for 60C terminations you would be stuck in the 60C column when doing the adjustments you can do in article 430.

 

[tortuga]

Just to add a little more I'll use a old motor that matches the OP's numbers:
A 460V 25HP three phase motor, Design B (480/277V system)
and say for fun everything is marked '60C terminations' with big warning stickers.

Per the table 430.250 the amps would be 34.
Per 430.22 430.23(A) Minimum conductor Ampacity would be 34 * 1.25 = 42.5A
Overloads would be about the same.

Next we want to reuse an exsiting breaker (60C terminations)
, per Table 430.52 we go for 250%:
34A * 2.5 = 85A
breaker can be 85 amps.

Now Per the Exception 1 in 430.52(C)(1) we can do the same as 240.4(B):
round up to next standard size in 240.6
Next standard size abouve 85 = 90A breaker.

However we still need a conductor with a 60C ampacity of 42.5A
So the #6 is the answer.

Thats my extended take on it.
Cheers