New Service Help Request

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mkgrady

Senior Member
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Massachusetts
I need to run an UG service to upgrade an existing that is currently OH. The service will be 400 amp 120/208 three phase. I plan to install a 400 amp (320 continuos) meter socket on the side of the building and feed it underground. The meter will nipple into a 3R 400 amp MCB right next to it. From there it will go back underground a few feet into an adjacent electric shed where the existing service lives. Here i will install a 400 amp MLO panel to feed the four existing circuits that enter the building which is a restaurant.

I have gone over the job with the inspector and he is good with all of the above but I'm not sure of the right choices for conduits, wire, MCB or meter so please check my thinking.

One 3 inch PVC conduit sched 40 from the trench to the meter with an expansion coupling under the meter. 3 inch PVC nipple to the 3R MCB. One 3 inch PVC 40 back underground from the MCB to the MLO panel with an expansion coupling under the MCB enclosure.

The wire size to the meter and on to the MLO panel inside will be 400MCM CU with XHHW insulation for amperage of 380 amps at 90 degree. The connected load is around 200 amps and I'm thinking a 320 continuous is the right size for now and a little expansion later.

The available fault current from POCO on the new UG will be 15,000 amps. I will look for a MCB that exceeds 15KAIC. The rest of the breakers I assume can be 10KAIC?

I will extend the existing GEC with a compression connector and run it to the MCB.

Any suggestions are welcome. Thanks
 
You need to size your wire using the 75C column not 90C. You need at least 500 CU for the 400 amp breaker.

The GEC gets sized to the service conductors. Since you are upgrading the service I doubt the existing GEC is large enough.

No, you can't automatically use a lower rated breaker. You need to calculate the available fault current on the system at each point. You might be able to use series rated breakers for the new equipment you install but probably not for the existing equipment. Depending on the size and the length of the feeders the available current may be less that 10K. If you are not sure how to calculate and design this system you should really get an engineer involved. I'm surprised the AHJ is not requiring calculations.
 
curt swartz said:
You need to size your wire using the 75C column not 90C. You need at least 500 CU for the 400 amp breaker.

The GEC gets sized to the service conductors. Since you are upgrading the service I doubt the existing GEC is large enough.

No, you can't automatically use a lower rated breaker. You need to calculate the available fault current on the system at each point. You might be able to use series rated breakers for the new equipment you install but probably not for the existing equipment. Depending on the size and the length of the feeders the available current may be less that 10K. If you are not sure how to calculate and design this system you should really get an engineer involved. I'm surprised the AHJ is not requiring calculations.
Click to expand...

The existing gec is 3/0 cu. The service size is being reduced.

I thought if the panel breakers were incapable of interrupting a fault the mcb would operate if it could handle the available current.

How do series breakers function?
 
curt swartz said:
You need to size your wire using the 75C column not 90C. You need at least 500 CU for the 400 amp breaker.

The GEC gets sized to the service conductors. Since you are upgrading the service I doubt the existing GEC is large enough.

No, you can't automatically use a lower rated breaker. You need to calculate the available fault current on the system at each point. You might be able to use series rated breakers for the new equipment you install but probably not for the existing equipment. Depending on the size and the length of the feeders the available current may be less that 10K. If you are not sure how to calculate and design this system you should really get an engineer involved. I'm surprised the AHJ is not requiring calculations.
Click to expand...

Want to try that again, 500 kcmil is rated 380 per the 75? column, the correct size for commercial is 600 kcmil for a 400 amp service.
 
hurk27 said:
Want to try that again, 500 kcmil is rated 380 per the 75? column, the correct size for commercial is 600 kcmil for a 400 amp service.
Click to expand...

Wayne, as long as the connected load is 380 amps or less 500 can be used and protected at 400 amps. This is a very common installation. The OP stated the load was 200 amps.
 
Am I missing something? If the GEC only goes to the grounding electrode, it only has to be #6. 250.66A All other bonding must be sized to 250.66. Right?
 
meternerd said:
Am I missing something? If the GEC only goes to the grounding electrode, it only has to be #6. 250.66A All other bonding must be sized to 250.66. Right?
Click to expand...

You must be thinking of a wire to a ground rod. My reference to extending the GEC was from the new service disconnect to the water main in the building. It happens to be 3/0 CU so I am going to re-use it.
 
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