100/125 Amp Sub Panel Questions

[MN4x4]

I know this gets asked from time to time, but these circumstances might be different so here goes. I'm only looking for answers from licensed electricians, as I want the info to be ABSOLUTELY accurate!

House is in northern Minnesota. Conventional 2x6 wall construction. Both panels will be under the same roof, i.e. interior wiring,

Main Panel is Square D QO 200 Amp 40 space on exterior wall (but mounted inside) serving house.

Sub Panel will be Square D QO 100 or 125 Amp (spaces unknown for now, but probably 12-20 depending on panel choice) on interior wall serving garage. Slab is already in place.

Distance from Main to Sub panel ~ 50 feet. Wiring will be above ceiling along and across trusses. Ceiling will be insulated (hey, it's Minnesnowta!) but not finished at this time, but may be later on.

Questions (in no particular order, but numbered for reference):

1. Does wiring need to be in conduit? If so, I beleive 2" schedule 40 is adequate?

2. Does the sub-panel require a main breaker? There will likely be 12 or more circuits.

3. Assuming a 100 Amp panel, what wire size is needed? I'm pretty sure I can use 3x 1 awg THHN copper with a #6 ground? If I use AL, what size would it need to be?

3a - Assuming a 125 Amp panel, what wire size is needed? I *think* I can use 3x 1/0 THHN (under the round-up rule) with a #6 ground? If I use AL, what size would it need to be?

4. Sub-panel will be mounted in an interior wall on a concrete slab on grade. No access to soil unless I drill a hole through the concrete. Is a ground rod required?

5. Largest single load will probably be 50 amps for a welder. Other 240 volt loads might be Air Compressor, table saw, etc. The remaining circuits will be 120 volt receptacles and lighting.

Anything else I should have asked but didn't?

 

[Mustang51js]

I can answer a few, no need for pipe as long as the wires aren't where they can be damaged. Your going to have trouble finding a 125 amp breaker to fit in your panel,i would get a 100 amp main breaker sub panel and that takes care of two things. You don't need a ground rod. I would say your ok with wire size but don't have my book to double check. I would go with aluminum cable though since it will be a lot cheaper than copper.

 

[bill in in]

You don't need a ground rod but will need to supply a ground conductor back to your main panel. You don't need conduit, but it might be advantageous on a run of that nature. I won't sleep in a place with aluminum anyplace but as a service entrance termination, is it worth it for 50'? If you buy a panel with a main installed, there isn't an issue- but be aware that you probably don't have 100 amps available at your main anyway- and most branch circuits feeds are rated at 100 Amps anyway....

 

[sberry]

I am not licensed, cant answer.

 

[mr overdunne]

I am not licensed, cant answer.

Lmao!

 

[pattenp]

I've got some news for you. I've seen bad information given by so called licensed electricians on this forum. If you want only information that you think is correct then run your questions by your local electrical inspector, not in some forum on the internet.

I know this gets asked from time to time, but these circumstances might be different so here goes. I'm only looking for answers from licensed electricians, as I want the info to be ABSOLUTELY accurate! .......

 

[Charles (in GA)]

You don't say what kind of wire you will be using for the subfeed, so its impossible to say whether it needs to be in conduit or not.

Assuming it is an attached garage, which it sounds like, no, you do not need a ground rod, nor a main breaker in the subpanel. However, having a main breaker on a distant subpanel is a good thing and I would want one on mine.

Ground rod is only needed if the garage were detached. You do, as already noted, need to run four wires, two hots, a neutral, and a ground to the sub.

No, I'm not an electrician. If you go to an electrical forum and read, you will discover that they argue more than virtually any type of forum on the net. License means you passed a test.

Charles

 

[MN4x4]

You don't say what kind of wire you will be using for the subfeed...

You do, as already noted, need to run four wires, two hots, a neutral, and a ground to the sub.Charles

Actually I answered both in the original post. I said "I'm pretty sure I use 3x 1 awg THHN copper with a #6 ground". Doesn't that answer both type (THHN) and the four wire question (3 X 1 awg hots & neutral plus 6 awg ground) ?

 

[MN4x4]

I've got some news for you. I've seen bad information given by so called licensed electricians on this forum. If you want only information that you think is correct then run your questions by your local electrical inspector, not in some forum on the internet.

Good point, and I intend to VERIFY with a local electrical inspector before buying anything, but figured I could start here.

 

[MN4x4]

I am not licensed, cant answer.

And with 9,000+ posts you obviously don't know anything anyway, right?

 

[Charles (in GA)]

Actually I answered both in the original post. I said "I'm pretty sure I use 3x 1 awg THHN copper with a #6 ground". Doesn't that answer both type (THHN) and the four wire question (3 X 1 awg hots & neutral plus 6 awg ground) ?

THHN always has to be in conduit, box, etc.

Charles

 

[sands35]

Actually I answered both in the original post. I said "I'm pretty sure I use 3x 1 awg THHN copper with a #6 ground". Doesn't that answer both type (THHN) and the four wire question (3 X 1 awg hots & neutral plus 6 awg ground) ?

You can only bond Neutral and Ground at one point. Therefore, you need to run 4 wires. 2 Mains (typically red and black), 1 Neutral (white) and 1 Ground (green) to the sub panel and maintain separation of Ground and Neutral at the sub-panel. (as you have stated)

Conduit size - there are lookup tables available on line. You will need to know how many curves you will have and plan out the pulls. Access needs to be maintained to the pulls (i.e., not buried behind drywall).

http://www.jhlarson.com/resources/electrical/sch40-sch80-pvc.html

You could use 1", but that is a PITA. I'd use 1.5" PVC to ease the pull and the cost difference is trivial.

You can use whatever sub panel you want (size it for how many circuits you plan), you just have to have one breaker that protects the wire with a the proper amperage limit sized appropriately for the wire on at least 1 end. (#4 copper THHN with a 90 amp main breaker with a 200 amp subpanel breaker is OK - volt drop length issue ignored - no need to change the sub panel breaker to a 90 as well.)

http://www.cerrowire.com/ampacity-charts

Unless you are planning on doing a lot of big amp welding or have some big machine tools, 90 amps is more than enough. Lots of folks say that 60 is more than enough.

Just a quick check of online prices at a big box.
#6 is $0.89 / ft = 75 amps
#4 is $1.19 / ft = 95 amps
#3 is $1.39 / ft = 110 amps

As you can see, prices will add quickly for head room you may not need.

So the 1st question is "what are you going to do"?

 

[pattenp]

Those amperage's you listed are @ 90C and are only used for derating purposes.

Depending on the wire type the amperage's are as follows @ 60C and 75C.

#6 55A/65A
#4 70A/85A
#3 85A/100A

...........

Just a quick check of online prices at a big box.
#6 is $0.89 / ft = 75 amps
#4 is $1.19 / ft = 95 amps
#3 is $1.39 / ft = 110 amps
...........

 

[theoldwizard1]

I've got some news for you. I've seen bad information given by so called licensed electricians on this forum. If you want only information that you think is correct then run your questions by your local electrical inspector, not in some forum on the internet.

Yep ! He is the one you have to make happy !

 

[C96]

I want the info to be ABSOLUTELY accurate!

 

[laser3kw]

run your questions by your local electrical inspector,

I'm only looking for answers from licensed electricians, as I want the info to be ABSOLUTELY accurate!

says it all.
what is "ABSOLUTELY accurate" and allowed in Pigpoke NC may be laughed at in Upscale MN.

 

[sands35]

Those amperage's you listed are @ 90C and are only used for derating purposes.

Depending on the wire type the amperage's are as follows @ 60C and 75C.

#6 55A/65A
#4 70A/85A
#3 85A/100A

As I read 310.16, it depends on where you are running the wires.

Attic? Sure, de-rate them.
Basement, conditioned space, interior walls, no, don't bother. (all in conduit for THHN)

(Unless I'm interpreting the tables wrong?)

 

[sberry]

This forum has good advice, its often confusing but gets sorted out in the end.

 

[pattenp]

The tables do have a column that list amp capacities @ 90C, but there's more to it than just what the table list. When sizing wire for circuits the 90C column is only used when calculating a derated amp capacity. There are provisions in the NEC that set the max amps allowed for different types of wiring. As an example NM (Romex) is to be sized based on 60C degrees (334.80). Also device temp ratings such as breakers are limited to 60/75C degrees. So for normal runs of wiring where derating is not a concern the 60C and 75C columns are to be used to determine the amp capacity.

As I read 310.16, it depends on where you are running the wires.

Attic? Sure, de-rate them.
Basement, conditioned space, interior walls, no, don't bother. (all in conduit for THHN)

(Unless I'm interpreting the tables wrong?)

 

[sands35]

Ah - what always confused me is that THHN is listed under the 90*C column.

 

[pattenp]

Yeah.. the THHN is rated as 90C in the table but the NEC provisions don't allow for it to be installed straight out as such. The table is not the end-all of what's allowed. Unfortunately you need to plow through all the "where to's and shall not's" in the code.

Ah - what always confused me is that THHN is listed under the 90*C column.