Service Size

[Designer69]

I have come up with 345A of demand load for a new 120/240 1-ph service.

Can I get by with a 400A service or do I need 500A? IE- if the main breaker is supposed to be loaded to 80% then 320A would be the max. my demand amps should come to.

Thanks

 

[Dennis Alwon]

Is this a residence. Then it is not an issue. If the loads are continuous at 345 then you would use an 80% calculation and there would be a problem.

Is the demand load the calculated load or the real time measurement?

 

[Designer69]

thank you. It is a new apartment building type R-2 occupancy.

the demand load is the calculated load

 

[infinity]

The calculated load is used to determine the service size. If it's 400 amps you need a minimum 400 amp service.

 

[Designer69]

The calculated load is used to determine the service size. If it's 400 amps you need a minimum 400 amp service.

thank you. My calculated load came in at exactly 345 amps.

does this mean I don't need to apply any other multiplication like 1.25 to the 345A or make sure the calculated load doesn't come in at a max 320A for a 400A service (400 x 80% = 320)?

 

[infinity]

thank you. My calculated load came in at exactly 345 amps.

does this mean I don't need to apply any other multiplication like 1.25 to the 345A or make sure the calculated load doesn't come in at a max 320A for a 400A service (400 x 80% = 320)?

You already did the calc, 345 amps is the minimum service size.

 

[kwired]

If you have a single 400 amp overcurrent device for service disconnect - you must supply it with 400 amp conductor.

If you had two to six service disconnecting means grouped in one location - say they were all 100 amp, and a load calculation of 345 - your common service conductor only needs to have an ampacity of 345.

 

[packersparky]

If you have a single 400 amp overcurrent device for service disconnect - you must supply it with 400 amp conductor.

If you had two to six service disconnecting means grouped in one location - say they were all 100 amp, and a load calculation of 345 - your common service conductor only needs to have an ampacity of 345.

I think he could use 500 kcmil copper (380 amps) and use a 400 ampere overcurrent device per 240.4(B).

 

[kwired]

I think he could use 500 kcmil copper (380 amps) and use a 400 ampere overcurrent device per 240.4(B).

To a single OCPD yes. What I was getting at is for multiple service disconnecting means, a common supply conductor only needs to be sized to calculated load. He could have six 100 amp breakers for service disconnecting means - but only required to have 345 amps of service conductor. If he hits a splice box or gutter he needs 100 amp conductor to each individual overcurrent device though.