Wire size for residential sub panel in garage.

S

steve61

licensed Electrical Contractor
Location
Nebraska
Occupation
Electrician
What column in table 310-16 do I use for a 100 sub panel in a residential garage ? If I use the 60 degree column,the wire size I come up with seems pretty large. Thanks for any input,Steve Wiese. Also,do I need to run a ground wire if I use emt or other metallic raceway ?
 
You won’t be able to use the 125A breaker for a 100A panel.
#2 CU in 60 degree if calculated load is less than 95A. Or 1/0 AL.
EMT can serve as an EGC.
 
You can most likely use the 75C column if you choose wire rated at 75C or higher. If using conduit, choose THHN or XHHW would work. If the run is long, using the sizes in the 60C column helps for voltage drop.
 
Start with the wiring method and then go from there. As mentioned NM or UF are only 60° C wiring methods. With metal raceways you can use 75° C. Copper or aluminum? Also a metal raceway may be able to be used as the EGC depending on the type of raceway.

For 100 amp feeder minimum sizes Table 310.16:

AL @ 60° C = #1/0 {100 amps}
AL @ 75° C = #1 {100 amps}
Cu @ 60° C = #2* {95 amps}
Cu @ 75° C = #3 {100 amps}

*#2 Cu is good for 95 amps next size up rule would allow these condcutors if the calculate load is 95 amps or less.
 
Check the buss on the panel. It may be rated 125 amps. Unless you are using NM cable to feed the panel then you could use the 75C column. De-rating may be necessary but that depends.

Is the garage detached? If not then no main breaker is required
 
Biggest mistake I see around here is contractors using the same wire size given as if it was a dwelling unit with 83% allowance making it undersized. "Give me a 100A wire." 310.12 vs 310.16 (2023 NEC)
 
Fred B said:
Biggest mistake I see around here is contractors using the same wire size given as if it was a dwelling unit with 83% allowance making it undersized. "Give me a 100A wire." 310.12 vs 310.16 (2023 NEC)
Click to expand...

Undersize but compliant since 4/0 = 180 amps --- next size up 200
 
Dennis Alwon said:
Undersize but compliant since 4/0 = 180 amps --- next size up 200
Click to expand...
Correct, with #4/0 you're possibly covered under both the standard 200 amp feeder size and the next size up rule. As Fred alluded to for 100 amp feeders the 83% will bite you. :giggle:
 
steve61 said:
What column in table 310-16 do I use for a 100 sub panel in a residential garage ? If I use the 60 degree column,the wire size I come up with seems pretty large. Thanks for any input,Steve Wiese. Also,do I need to run a ground wire if I use emt or other metallic raceway ?
Click to expand...
U can use #3 copper or #1 Alum, as long as the terminals are rated for 75 degree
 
steve61 said:
What column in table 310-16 do I use for a 100 sub panel in a residential garage ? If I use the 60 degree column,the wire size I come up with seems pretty large. Thanks for any input,Steve Wiese. Also,do I need to run a ground wire if I use emt or other metallic raceway ?
Click to expand...
2023 NEC
310.12(B) (C) Feeders. A feeder ran to another panelboard may be used.
I don't think the 83 % rule would apply to your installation.
Suggestion: Read below.
Do you really need a 100 amp feeder to the garage? What kind of a load do you have ,how big is the garage?
Since this is a romex job, you could use 4 AWG copper and put a 70 amp CB for the feeder.
You will need an EGC sized as per T,250.122, an 8 AWG EGC copper.
That will give you about 17,000 VA load.
So, first determine what you will need for the garage. Use nameplate labels.
Calculate using volt-amperes, Volts X Amperes = VA.
Then use Ohms law and convert to amperes.

Thanks for reading
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TX+MASTER#4544
 
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