SEC for 300 ATS > 225 Load Center

C

COcoppersmith

Member
Location
Colorado
Occupation
Electrician
Having some grey area with my inspector in CO on sizing service entrance conductors from a 320A continuous / 400A max meter socket > 300A Service Entrance Rated ATS > 225A load center. Question is sizing based on 300A ATS or 225A load center. Because I'm only feeding a 225 amp main breaker, that should be what determines my wire size, not the maximum rating my equipment, correct? The code says in [230.42] "Service Entrance Conductors shall have an ampacity of not less than the maximum load to be served." That should be the 225A load center breaker and not that it passes through a 300A ATS, right? Section [310.12] also goes into services for homes and gives a table that says that you use 3/0 copper for a 225 amp service. Simply put, thanks for confirming size of service entrance conductors from meter socket to the 300A ATS, and then service entrance conductors from the 300A ATS to the 225A load center. THX.
 
COcoppersmith said:
Having some grey area with my inspector in CO on sizing service entrance conductors from a 320A continuous / 400A max meter socket > 300A Service Entrance Rated ATS > 225A load center. Question is sizing based on 300A ATS or 225A load center. Because I'm only feeding a 225 amp main breaker, that should be what determines my wire size, not the maximum rating my equipment, correct? The code says in [230.42] "Service Entrance Conductors shall have an ampacity of not less than the maximum load to be served." That should be the 225A load center breaker and not that it passes through a 300A ATS, right? Section [310.12] also goes into services for homes and gives a table that says that you use 3/0 copper for a 225 amp service. Simply put, thanks for confirming size of service entrance conductors from meter socket to the 300A ATS, and then service entrance conductors from the 300A ATS to the 225A load center. THX.
Click to expand...
Meter to SE ATS, 300A.
Conductors leaving ATS need to be rated 300A, unless you use a 240.21 tap rule.
 
COcoppersmith said:
300A Service Entrance Rated ATS
Click to expand...
If that means the ATS has a 300A OCPD in it, and that is your only service disconnect (the downstream 225A panel is not a service disconnect), then your service entrance conductors must be sized to that 300A OCPD, see NEC 230.90(A). As you are supplying a 225A rated panel, that panel would need a 225A rated OCPD in it (or between it and the ATS).

If you can change the ATS to have a 225A OCPD in it, your service entrance conductors would only need to be rated for 225A, and your downstream panel would not require additional OCPD.

Cheers, Wayne
 
wwhitney said:
If that means the ATS has a 300A OCPD in it, and that is your only service disconnect (the downstream 225A panel is not a service disconnect), then your service entrance conductors must be sized to that 300A OCPD, see NEC 230.90(A). As you are supplying a 225A rated panel, that panel would need a 225A rated OCPD in it (or between it and the ATS).

If you can change the ATS to have a 225A OCPD in it, your service entrance conductors would only need to be rated for 225A, and your downstream panel would not require additional OCPD.

Cheers, Wayne
Click to expand...
Thanks Wayne,

The 225A Leviton Load Center does have an OCPD in it. Because it does, does that allow ATS>Load Center w/ OCPD in it to be sized for 225A? Also in the manual for the Generac RTSW300A3 (300A ATS) it states "conductor sizes must be adequate to handle the maximum current to which they will be subjected, based on the 75C column of tables, charts, etc. used to size conductors." I assumed that would be 225A as the load center is 225A. I assumed using 4/0 Cu would suffice as at 75C it's rated to 230A. If I'm wrong, that means I size to 350 MCM Cu to meet the 310A based on 75C. Just seems so large for the 225A load center w/ OCPD in it. Thx for confirming @wwhitney
 
COcoppersmith said:
Thanks Wayne,

The 225A Leviton Load Center does have an OCPD in it. Because it does, does that allow ATS>Load Center w/ OCPD in it to be sized for 225A? Also in the manual for the Generac RTSW300A3 (300A ATS) it states "conductor sizes must be adequate to handle the maximum current to which they will be subjected, based on the 75C column of tables, charts, etc. used to size conductors." I assumed that would be 225A as the load center is 225A. I assumed using 4/0 Cu would suffice as at 75C it's rated to 230A. If I'm wrong, that means I size to 350 MCM Cu to meet the 310A based on 75C. Just seems so large for the 225A load center w/ OCPD in it. Thx for confirming @wwhitney
Click to expand...

Remember that the general rule is conductors must be protected at the beginning of the circuit. An ocpd downstream from the start of the conductors will protect the conductors from an overload, but will not protect the connectors (upstream) from a ground fault or short circuit. Now there are some exceptions to this, like service conductors where all you can do is protect them from overload. Another exception is tap conductors where you can have the ocpd downstream from the tap. In both of those cases, the ocpd only protects the conductors from overload. There is risk of a short sort of ground fault upstream from the ocpd but you have more stringent installation rules, particularly short length, to minimize that possibility.

Are you familiar with the tap rules? Look over them and see if the conductors between the ATS and your panel will apply.
 
I agree that common sense would indicate that a 225A breaker at the tail end of it all should suffice that the wire could be rated for 83% of 225 amps, but that is just too many bridges to cross.

How many feet are we talking about here?

See if you can find a 225 breaker for the ATS, or one that could be adjusted down to 225 amps and locked/covered per the NEC.
 
You must log in or register to reply here.
Top