100 Amp adequate wire sizing

[americanthunder]

Did some searches on here and I'm still scratching my head I'm currently building a new shop that will have a 100A Feed to a subpanel (a 40ft run) in the new garage. Everything I have read points to need #1 Copper or 1/0 Alum from what I can tell. I was looking at the wire left for me (doing a self install) and its 2-2-2-4 which appears to be this stuff: http://www.wireandcabletogo.com/2-2-2-4-****-Underground-Secondary-Distribution-Cable.html

I asked the inspector about it and his response was "thats whats on the approved plans".

In any case I know he'll approve it but is it really enough? I don't want to rock the boat with the inspector but I want it done right the first time. Just doesn't feel right telling an inspector you want to do more than they ask!

Thanks in advance for your opinions!

 

[Norcal]

#2 aluminum for a shop is 90A max, it's only allowed for 100A when it supplies the entire load of a dwelling, no subpanels, no feeders to outbuildings qualify for that exception.

 

[americanthunder]

#2 aluminum for a shop is 90A max, it's only allowed for 100A when it supplies the entire load of a dwelling, no subpanels, no feeders to outbuildings qualify for that exception.

Thats what I thought after everything I've read on here and using any wire sizing calculator. So some 1/0-1/0-1/0-2 should do the trick or get a 90A breaker for the main panel.

Thanks for the response!

 

[CNGsaves]

+1 to leave the 100A breaker in the subpanel (garage), and use 90A breaker in the main panel that actually feeds your run. Doubt that you'll ever need the 90A in one-man garage and the MHF 2-2-2-4 Al wire is most affordable solution. No need to get copper.

If garage is detached, you'll need 2 ground rods or have ufer in the slab.

Good luck and post up pics of the shop build. Heck, start a thread in the Gallery section and take a whole slew of PICs. Everyone on GJ just loves new builds !!

 

[pattenp]

For 100A it's #3 copper and #1 aluminum. You can use the 75 deg C column to determine conductor size for the type of wire you're using. The sizes you listed are 60 deg C. Also the wire you linked to is URD is is not approved to be used inside the structure it has to be terminated outside. Make sure the cable you use is rated for inside use if going inside the structure to the panels. Mobile Home Feeder (MHF) is also rated for inside along with being a direct bury cable, but needs to be in conduit above ground and when installed inside.

 

[americanthunder]

+1 to leave the 100A breaker in the subpanel (garage), and use 90A breaker in the main panel that actually feeds your run. Doubt that you'll ever need the 90A in one-man garage and the MHF 2-2-2-4 Al wire is most affordable solution. No need to get copper.

If garage is detached, you'll need 2 ground rods or have ufer in the slab.

Good luck and post up pics of the shop build. Heck, start a thread in the Gallery section and take a whole slew of PICs. Everyone on GJ just loves new builds !!

I agree however after a few calls to local electric houses, home depot, and lowes no one has 2-2-2-4 MHF only the what I've already got which has the URD designation as pattenp pointed out cannot be used indoors.

Got the ufer installed and ready for the pour. Thanks for your input!

For 100A it's #3 copper and #1 aluminum. You can use the 75 deg C column to determine conductor size for the type of wire you're using. The sizes you listed are 60 deg C. Also the wire you linked to is URD is is not approved to be used inside the structure it has to be terminated outside. Make sure the cable you use is rated for inside use if going inside the structure to the panels. Mobile Home Feeder (MHF) is also rated for inside along with being a direct bury cable, but needs to be in conduit above ground and when installed inside.

Noob question. What is the designation for inside use? Every place that has 2-2-2-4 has the URD designation. Maybe I should just use #3 THHN copper in the 2" PVC already installed?? If that is the case what size does the ground need to be? Sorry this wiring thing has me baffled but I willing to learn! Thanks

 

[pattenp]

Most URD cable has only a USE-2 rating which is not a fire resistant insulation. Cable such as MHF looks a lot like URD but also carries a RHH/RHW-2 along with the USE-2 rating which allows it to be installed inside. THHN/THWN-2 is allowed inside because it has a fire resistant insulation, so does NM-b (Romex). Look at the jacket/covering of the wire, if it's only labeled as USE/USE-2 then it's to be used only outside.

If using copper the equipment ground for 100A can be as small as #6.

I think Cu THHN/THWN-2 is your best choice.

 

[bmxdad]

I just used #2 copper THHN in conduit and a 100amp breaker. Your only 45 feet? ... so cost shouldn't be that big. I've got several 30 and 40 amp circuits that could be pulling a load at the same time, so I'm pretty comfortable on what I can handle.

 

[wyliesdiesels]

Heres the MHF home depot carries:

http://m.homedepot.com/p/Southwire-2-2-4-6-Aluminum-MHF-Wire-By-the-Foot-30163099/205001794/

Maybe they can order it for u...

 

[theoldwizard1]

I agree however after a few calls to local electric houses, home depot, and lowes no one has 2-2-2-4 MHF only what I've already got which has the URD designation as pattenp pointed out cannot be used indoors.

Slow your roll !

Uncoil some of the wire, at least 10', and carefully inspect it for all markings. f you don't understand them, post a picture here !

Your other alternative is a weatherproof splice box (12x12x4 ?) on the outside of each building to a short length of wire to the load center in that building that is rated for indoor usage. Make sure to use splice that are rated for aluminum wire.

 

[jgorm]

I just went through this with my 100a panel. It ended up being 90a, because my main panel said 90a max for the breakers. Might wanna check that. I called 5 electric supply houses looking for 2-2-2-4 Cu SER, and nobody had it so I figured I was doing something wrong. I ended up going with 2-3 romex under the house, converting to 2g THHN/THWN to go underground in conduit. I would much rather have some extra capacity / lower temp ratings, than use 2224 MHF aluminum wire that is right on the limit. 90C is damn hot!

 

[pattenp]

I just went through this with my 100a panel. It ended up being 90a, because my main panel said 90a max for the breakers. Might wanna check that. I called 5 electric supply houses looking for 2-2-2-4 Cu SER, and nobody had it so I figured I was doing something wrong. I ended up going with 2-3 romex under the house, converting to 2g THHN/THWN to go underground in conduit. I would much rather have some extra capacity / lower temp ratings, than use 2224 MHF aluminum wire that is right on the limit. 90C is damn hot!

What are you talking about? 90C? 90C is used only when derating. MHF is normally sized at 75C. Romex has to be sized at 60C.

 

[wyliesdiesels]

Awww the age old confusion on temperature ratings...

 

[ishiboo]

#2 aluminum for a shop is 90A max, it's only allowed for 100A when it supplies the entire load of a dwelling, no subpanels, no feeders to outbuildings qualify for that exception.

Why can't it feed a sub panel? I have a 100A panel outside under my meter, and a panel inside... I ran 2-2-2-4 SER. It still meets the entire dwelling rule!

 

[bmxdad]

I'm a nube at this, so I go by the lowest amp rating ... can't loose that way

Actually, this helps explain it for me. Forgot where I got it, but saved in my notes ...

NEC Table 310.16 defines the current-carrying capacities (sometimes called ampacity) of different gauge wires, in aluminum and copper, for wire temperature ratings of 60°, 75°, and 90° C. The higher the temperature rating, the greater the ampacity for a given AWG size (gauge) of conductor. This table also lists the wire types for each temperature rating (THHN, THHW, etc). It covers conditions where up to three current-carrying conductors are installed in a pipe or wireway. NEC table 310.15(B) (2) (a) gives the derating factors for ampacity when more than three current-carrying conductors are installed in a pipe or conduit. This is the table of "normal" derating factors.

 

[wyliesdiesels]

Why can't it feed a sub panel? I have a 100A panel outside under my meter, and a panel inside... I ran 2-2-2-4 SER. It still meets the entire dwelling rule!

Not quite sure what youre asking here but #2 can definitely be used to feed a subpanel. It just needs to be protected by 90a OCPD. Are u saying u used #2 AL SER to feed your shop protected by a 100a breaker? That would be incorrect as a shop IS NOT considered a dwelling by code. The reason for the 100a allowable ampacity for dwellings is because dwellings usually have limited loads in most circumstances!

Ive got #2 AL SER feeding my condo and the most it sees is about 50a and thats with the AC, dryer and electric range on...And the range and dryer go on and off intermittenly during their cycling.

I'm a nube at this, so I go by the lowest amp rating ... can't loose that way

Actually, this helps explain it for me. Forgot where I got it, but saved in my notes ...

That has to do with # of conductors in pipe. Different subject...

 

[americanthunder]

Thanks for everyone's suggestions. I went ahead and just purchased the copper. Now if I can just get the darn building built

 

[Norcal]

Why can't it feed a sub panel? I have a 100A panel outside under my meter, and a panel inside... I ran 2-2-2-4 SER. It still meets the entire dwelling rule!

It really does not because of the panel outdoors, but your situation is a spot where there is nothing gained by increasing the wire size, but still wrong.

 

[ishiboo]

[/B]

It really does not because of the panel outdoors, but your situation is a spot where there is nothing gained by increasing the wire size, but still wrong.

Where is it made illegal?

 

[Norcal]

Where is it made illegal?

The outdoor panel.

 

[wyliesdiesels]

The outdoor panel.

I thought SER can be run outside in damp locations when not subject to damage just not in underground conduit which is a wet location...

 

[alfredeneuman]

There's no definition of "damp location" in the NEC.

Only wet and dry locations. Any damp location is deemed to be wet.

(Light fixtures are often labeled "suitable for damp locations", as opposed to weatherproof but that's a different thing entirely.)