SER vs NM-B cable for 90 amp sub-panel

[TheKnout]

I am converting my garage to a small apartment for my brother to live in. Based on load calculations the max draw is 72-80 amps, so I am going to install a 90 amp subpanel to service the garage apartment

I believe I need a 4-4-4-6 SER cable to run from my main panel (125 amps) to the garage sub-panel, but my friend (he is a low voltage installer) says a 4-4-4-6 NM-B cable would be just fine and is offering to get me said cable (he owes me a favor).

If I am correct due to ampacity the NM-B cable will not be enough to support my sub-panel due to the 60-degree derating and not 75-degree that you would get with an SER cable. Can anyone confirm this for me?

 

[Bert_]

I run aluminum SER. #2 instead of #4 copper but it's 1/3 the price.

 

[Terry D]

You are correct, you have to use the 60 degree column for Nm-b. So 70 amps for #4

Defiantly use a SER cable. #2 Aluminum would be cheaper, but #4 copper would work also

 

[wyliesdiesels]

Is this garage attached or detached?

If detached, SER and NM-b are not permitted underground. NM-b is also not permitted outdoors

 

[TheKnout]

The garage is attached. The cable would go through the crawl space under the house to the garage then through a stud cavity into the garage attic across part of the garage and drop down in another stud cavity to the sub-panel.

 

[AntonLargiader]

2-2-2-4 Aluminum SER is a pretty standard way to get 90A to a subpanel. It's right at 1" diameter, for planning.

 

[MerlinsBeard]

I am converting my garage to a small apartment for my brother to live in. Based on load calculations the max draw is 72-80 amps, so I am going to install a 90 amp subpanel to service the garage apartment

I believe I need a 4-4-4-6 SER cable to run from my main panel (125 amps) to the garage sub-panel, but my friend (he is a low voltage installer) says a 4-4-4-6 NM-B cable would be just fine and is offering to get me said cable (he owes me a favor).

If I am correct due to ampacity the NM-B cable will not be enough to support my sub-panel due to the 60-degree derating and not 75-degree that you would get with an SER cable. Can anyone confirm this for me?

Check out 310.16 for the wiring ampacity, and 110.14(C)(1)(b) for when you can use the 75 degree terminal ratings. Note that 310.16 is based on the conductor insulation in isolation, but the terminal ratings depend on the type of end points.