Feeder

[yellow77]

I will be feeding a panel with a 600 amp main breaker. The breaker is rated at 100%, so to calculate the feeder size I need to multiply 600 by 1.25 and that will tell me what amperage to size for. Is this Correct?

 

[yellow77]

Almost forgot it feeds all continuous loads

 

[nakulak]

you might save a couple dollars if you use the calculated load

 

[kwired]

read the exception

read the exception

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220. The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.
Exception: Where the assembly, including the overcurrent devices protecting the feeder(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the feeder conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.

2005 NEC

 

[yellow77]

It will be serving the bypass on 2 ups modules and the Main bypass. i cant calculate the actual load because they dont know how populated the PDUs are going to be in the future. Its 2- 225kva units and everything is 100% breakers

 

[yellow77]

so 600 amps worth of wire will be good.

 

[kwired]

may want to consider rating of device on both ends of the feeder. my understanding is the reason to size a conductor at 125% for a continuous load is because of the termination rating not the conductor rating and a 100% rated device does not need to have conductor size increased. There are not very many 100% rated devices out there in small sizes, I have not been around a lot of equipment larger than 600 amp so am not aware of how common 100% rated equipment is on larger sizes.

 

[yellow77]

I have confirmed the existing breaker is 100% rated

 

[bob]

may want to consider rating of device on both ends of the feeder. my understanding is the reason to size a conductor at 125% for a continuous load is because of the termination rating not the conductor rating and a 100% rated device does not need to have conductor size increased. There are not very many 100% rated devices out there in small sizes, I have not been around a lot of equipment larger than 600 amp so am not aware of how common 100% rated equipment is on larger sizes.

You are confusing the temperature rating of the terminals and the conductor temp rating. 75C rated conductors are allowed if the terminal ratings are 75C rated.

 

[kwired]

You are confusing the temperature rating of the terminals and the conductor temp rating. 75C rated conductors are allowed if the terminal ratings are 75C rated.

no I am not if the terminals are rated 75 deg you still must use 125% of continuous load to size your conductor even with 90 deg conductors (still need to use 75 degree column)but not on a 100% rated device. the 100 percent rated device is able to dissipate more heat at the terminals is the reason why 100 percent sized conductor is allowed.