3 phase wire sizing 200 amp

[Stevenfyeager]

My customer wants a new 200 amp 3 phase, 120/208 service for two 60 amp machines. I want to use #250 AL, but then remembered that the amperage in 310.15 B 16 (2017) 205 amps rating is for no more than 3 current carrying conductors. In my riser, I will have 4 wires. Do I have to derate these? Thank you.

 

[roger]

Read 310.15(B)5

Roger

 

[mbrooke]

If its 3 phases and a neutral, no de-rating required.

 

[Stevenfyeager]

If its 3 phases and a neutral, no de-rating required.

Thank you. I was reading 310.15(B) (5) a, b, and c and wasn't sure.

 

[infinity]

The neutral would only be considered a CCC if the majority of the load were nonlinear which is not applicable in your installation so no derating required.

 

[mbrooke]

Thank you. I was reading 310.15(B) (5) a, b, and c and wasn't sure.

Welcome

The neutral only sees the difference or imbalance. The only way for the neutral to be carrying full load current is if all the load was placed across Phase A and B but none on C. So even in that worse scenario its still 3 current carrying conductors- despite having 4 in the conduit.

 

[kwired]

My customer wants a new 200 amp 3 phase, 120/208 service for two 60 amp machines. I want to use #250 AL, but then remembered that the amperage in 310.15 B 16 (2017) 205 amps rating is for no more than 3 current carrying conductors. In my riser, I will have 4 wires. Do I have to derate these? Thank you.

Others have covered the number of current carrying conductors issue, other thing here is your load is two 60 amp machines. Which can possibly mean they call for 60 amp circuits but actual load is even less than 60. But for simplicity sake you only have 120 amps of load. If you want to design for 200 amps of load for possible other future loads that is fine. As is you can put 4/0 aluminum on 200 amp overcurrent device because it is next standard size higher, and your load is still under ampacity of 4/0. Just something to consider.

Also if this is limited load installation and there is little or no neutral load- the grounded service conductor can be as small as the minimum required grounding electrode conductor.

 

[Stevenfyeager]

Others have covered the number of current carrying conductors issue, other thing here is your load is two 60 amp machines. Which can possibly mean they call for 60 amp circuits but actual load is even less than 60. But for simplicity sake you only have 120 amps of load. If you want to design for 200 amps of load for possible other future loads that is fine. As is you can put 4/0 aluminum on 200 amp overcurrent device because it is next standard size higher, and your load is still under ampacity of 4/0. Just something to consider.

Also if this is limited load installation and there is little or no neutral load- the grounded service conductor can be as small as the minimum required grounding electrode conductor.

Thank you very much.