commercial service wire size

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keith gigabyte

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New Kensington, Pa
I have a commercial building I am installing a 150 amp three phase service in. Its 220 no neutral. Loads are (2) 50 amp air conditioners. No additional load expected. Can I run 2/0 xxhw aluminum terminated to main breaker rated at 150 amps. 2/0 is good for 135 amps and next trade size breaker is 150. The calculated load is well below 135 amps of wire rating. or do I need 3/0?
 
keith gigabyte said:
I have a commercial building I am installing a 150 amp three phase service in. Its 220 no neutral. Loads are (2) 50 amp air conditioners. No additional load expected. Can I run 2/0 xxhw aluminum terminated to main breaker rated at 150 amps. 2/0 is good for 135 amps and next trade size breaker is 150. The calculated load is well below 135 amps of wire rating. or do I need 3/0?
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May want to also check and make sure you don't have high leg coming in.

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got the service inspection for this job. The neutral does not need to be in panel. Ground does. not sure why you would suggest a non center tap delta would need a neutral.
Inspector said 2/0 is perfect but cited main breaker as a switch and not an over current protection device. He did mention I realy didnt need a main because it would be under the 6 or less disconnect rule and could get away with a main lug panel. I still think the 150 amp main is ok because it is not protecting the service conductors from a short between power company and lugs on breaker. It only protects if buss fails or someone adds too much amperage on the branch circuits. either way of thinking..the 2/0 is good.
 
keith gigabyte said:
got the service inspection for this job. The neutral does not need to be in panel. Ground does. not sure why you would suggest a non center tap delta would need a neutral.
Inspector said 2/0 is perfect but cited main breaker as a switch and not an over current protection device. He did mention I realy didnt need a main because it would be under the 6 or less disconnect rule and could get away with a main lug panel. I still think the 150 amp main is ok because it is not protecting the service conductors from a short between power company and lugs on breaker. It only protects if buss fails or someone adds too much amperage on the branch circuits. either way of thinking..the 2/0 is good.
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The "grounded" service conductor must be brought to service disconnecting means if the system is grounded, 3 phase 3 wire can be corner grounded. If it is an ungrounded system you must have ground fault detection/indication equipment.

Is your main a switch or a circuit breaker? Like you said you may not need a single main because of the six disconnect rule, but a switch does not provide any overcurrent protection, a circuit breaker does. The service main overcurrent device does not protect service conductors from short circuits or ground faults ahead of the device, it does protect them from overload conditions though.

And back to one of your main questions - you are correct that 2/0 (aluminum) ampacity is 135 and next standard size higher can be used which is 150 as long as the connected load doesn't exceed the conductor ampacity of 135. If using copper conductor you could use 1 AWG which is good for 130 amps and still use the next size up 150 amp overcurrent device so long as your load is not more than 130 amps.
 
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keith gigabyte said:
not sure why you would suggest a non center tap delta would need a neutral.
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confused by your statement.....:? Is there a GROUNDED conductor from the service or not...? Center Tap delta or corner grounded delta? If so...250.24(C) requires you to bring it to the service disconnect. Inspectors are wrong everyday.......
 
Canton said:
Inspectors are wrong everyday.......
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some of us, more than once :D
keith gigabyte said:
. It only protects if buss fails or someone adds too much amperage on the branch circuits. either way of thinking..the 2/0 is good.
Click to expand...
I find a bit of humor here considering you are using 135 amp conductors. If you feel the 150 amp breaker is necessary to prevent future overloads, then a 150 amp conductor would be more appropriate :D
 
Canton said:
confused by your statement.....:? Is there a GROUNDED conductor from the service or not...? Center Tap delta or corner grounded delta? If so...250.24(C) requires you to bring it to the service disconnect. Inspectors are wrong everyday.......
Click to expand...

If it is corner grounded he very likely has the grounded conductor there, but it still needs to be bonded to the service enclosure as well.
It can be switched, simultaneously with the other two phases, but can not have fuses in in that conductor.
 
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