Calculating subpanel/feeder size

[BMWpowere36m3]

I'd like to run a subpanel for my garage, entrance hall, mud room and bathroom. In the future I want to run another subpanel off this one for a detached garage/shed.

For calculating load, or to properly size the subpanel and feeder.... I have a few questions regarding NEC methods.

There is the "3VA/sqft" rule (GP lighting and outlets), 1500VA for small appliance/laundry, and name plate rating.

So I figure:

Bathroom ~64 sqft, 192VA

Mudroom 1500VA (laundry branch) and 5000VA (dryer)

Entrance hall ~100 sq, 300VA

Garage... air compressor (biggest single load), T8 lights, lots of 20A outlets for various tools, nothing permanent. Calculate compressor load (nameplate) and use 3VA/sqft for GP outlets/lights or what?

 

[bartstop]

The NEC calculations are for service size. You are talking about a subpanel.

 

[Sponsor]

 

[BMWpowere36m3]

The NEC calculations are for service size. You are talking about a subpanel.

Is there a seperate NEC article for calculating the feeder to a subpanel? I know I can just put in a big subpanel (~100A) and not use its full capacity... but I still need to figure out the load for both the wire size and feeding breaker.

 

[bartstop]

I'm not trying to be an ass but I think you're over analysing this. If you're going to go through all the trouble and expense, why not just install a 100 amp panel? Unless the feeder is going to be really long, you're not going to save much by going with a 60 amp or smaller. I personally would use a Square D homeline and feed it with #4 SER copper and be done. There's nothing wrong with having extra capacity. Who knows you might need to plug in an RV or welder someday.

 

[JWelectric]

, why not just install a 100 amp panel? feed it with #4 SER copper and be done. There's nothing wrong with having extra capacity. Who knows you might need to plug in an RV or welder someday.

If you use #4 copper protect it with an 80 amp breaker as 4 is only good for 85 amps

 

[BMWpowere36m3]

If you use #4 copper protect it with an 80 amp breaker as 4 is only good for 85 amps

Newer NEC has lower ampacities, right? I was told somewhere if the cable is run indoors, then you must size the wire per the 60*C column in Table 310.15(B)(16) (regardless if you use 75 or 90*C cable) like typical NM (14/12-2).

 

[Vegas_sparky]

Newer NEC has lower ampacities, right? I was told somewhere if the cable is run indoors, then you must size the wire per the 60*C column in Table 310.15(B)(16) (regardless if you use 75 or 90*C cable) like typical NM (14/12-2).

What are the ratings at your termination points(lugs/breakers/device)? The point with the lowest rating is what we use to determine which ampacity column is used for a given circuit. 90°C equipment is fairly uncommon.

 

[BMWpowere36m3]

What are the ratings at your termination points(lugs/breakers/device)? The point with the lowest rating is what we use to determine which ampacity column is used for a given circuit. 90°C equipment is fairly uncommon.

I have a GE PowerMark Gold main panel (200A service) and I was planning on using a similar GE panel (either with a main breaker or not). GE lists the THQP/THQL series breakers (used in the PowerMark panels) at 65/75*C… so based on that I could size the feeder cable off the 75*C column, right?

 

[Norcal01]

If you use #4 copper protect it with an 80 amp breaker as 4 is only good for 85 amps

A 90A breaker is fine, as it's allowed to upsize to the next higher ampacity when the rating of the cable does not correspond to a standard rating.