Nec 2020 310.12 subpanel feeder size question

[spk6543]

I have a 100 amp subpanel I have installed for an addition. I have installed a 90' run of 4-3 nm-b in the attic to feed the subpanel. it is protected by a 100amp breaker in the main. My inspector has told me the Max breaker I can use to protect the 4-3 nm-b is 70 amps. Am I incorrect to use 100amp breaker with 4-3nm-b feeder looking at table 310.12 in the 2020 NEC?

 

[wwhitney]

What size is the residential service?

Cheers, Wayne

 

[spk6543]

Hi Wayne I have a 200 amp main service. After searching the forums it appears 310.12 only applies to feeding the entire load of the dwelling. But now I am trying to figure out if I can at least go to to 90 amps with a 4-3nm. I have a 125 amp main lug Siemens panel for the sub and the 100 amp Siemens breaker I installed in the main is rated for 75degrees.

the subpanel is feeding:
1x washer
1x electric dryer
1x 25 amp minisplit system
2x 1500 watt 240volt heaters
2x in-tile heating Maybe 6 amps total
1x bathroom
2x bedrooms
1x recroom
and a garage.

I think 70 amps may be pushing it a little bit

 

[wwhitney]

I have a 200 amp main service. . . . But now I am trying to figure out if I can at least go to to 90 amps with a 4-3nm.

No, you are limited to 70A as the inspector indicated.

Cheers, Wayne

 

[electrofelon]

Next time use SER cable. Much cheaper and better ampacity.