Wiring Info From Inspector...

[65Stang]

So, I decided to contact the local inspection office to get my wiring figured out.

Here's the scenario: Going about 100' to my new garage from existing house panel. Majority of the run will be in the house crawl space, about 6' under new sidewalk (2" PVC already buried). 100 amps.

Per the inspector, I need to run separate cables due to the run under the sidewalk being considered a wet location, although in PVC. Does this sound correct?

Second question I posed was regarding wire size. Inspector stated that I could use: two 4 gauge for two hots, one 6 gauge for neutral, and one 8 gauge for the ground. Does this sound correct? I seem to keep hearing about how the first three should be the same and for 100 amps, should be #3 or better.

 

[rickairmedic]

Stang I hate to tell you this but he is the electrical inspector for a reason and regardless he will be the one sighning off ( or not sighning off ) on your electrical. He actually is right on all accounts anyways but just in case he wasnt without having a copy of the NEC in front of you and showing him he is stupid I wouldnt do it .

Rick

 

[65Stang]

I just wanted to be sure that what he is saying actually makes sense since there are individuals here who have "been there." My only concern was that what he stated seems to be undersized compared to what I have heard... I would prefer to use the gauges that he mentioned, it would make things a lot easier!

 

[rickairmedic]

Stang 4/6/8 is right for a 100 amp service . I am not a master electrician but I have put in my fair share of panels and thats the right combination for 100 amps.

Rick

 

[65Stang]

Stang 4/6/8 is right for a 100 amp service . I am not a master electrician but I have put in my fair share of panels and thats the right combination for 100 amps.

Rick

Shweet, thanks, just looking for a gut-check. Now to find somewhere that sells W wire (semi-wet locations). No NM here.

 

[Aceman]

Stang 4/6/8 is right for a 100 amp service . I am not a master electrician but I have put in my fair share of panels and thats the right combination for 100 amps.

That would be true if this was a 100 amp dwelling service panel. But since it is a garage, the wire sizes are based off a different chart, making #4 one size too small. #3 Cu is required. For those with access to codebooks, dwelling service wire sizes are pulled from Table 310.15(B)(6) while everything else including garages are pulled from Table 310.16.

Like Rick said though, if he's the one inspecting, it doesn't matter. #4 is normally good for 85 amps but he may have a local amendment that allows it.

 

[sberry]

Toss all that and use a number 2 aluminum quad from the box store. So much cheaper. Your inspector is giving you a minimum of what he will accept, doesnt mean you cant use better.

 

[65Stang]

Toss all that and use a number 2 aluminum quad from the box store. So much cheaper. Your inspector is giving you a minimum of what he will accept, doesnt mean you cant use better.

Had thought about it and the inspector did give me the acceptable gauges to use in aluminum, but it won't fit through my conduit that's under the sidewalk. And the savings wasn't significant enough to be worthwhile, but I appreciate the suggestion!! Just wish I had oversized the underslab conduit when I had the chance.

 

[Skyline]

Had thought about it and the inspector did give me the acceptable gauges to use in aluminum, but it won't fit through my conduit that's under the sidewalk. And the savings wasn't significant enough to be worthwhile, but I appreciate the suggestion!! Just wish I had oversized the underslab conduit when I had the chance.

Why not do a junction box just before you go outside, and switch from the quad #2 to individual wires for the underground run? I don't know what size conduit you have, but if it's at least 1-1/2" you should be fine.

Hopefully one of the electricians here will chime in, but my understanding is if you use individual wires through the crawl space it will have to be in conduit. Whereas if you use Romex type wire, it can be left exposed, just stapled to the joists. Seems to me that's the only way to go. Whether you use copper or aluminum wire, it really does not make much difference (except in terms of cost), and assuming you compensate for the somewhat lower load capacity of the aluminum.

 

[65Stang]

Toss all that and use a number 2 aluminum quad from the box store. So much cheaper. Your inspector is giving you a minimum of what he will accept, doesnt mean you cant use better.

Why not do a junction box just before you go outside, and switch from the quad #2 to individual wires for the underground run? I don't know what size conduit you have, but if it's at least 1-1/2" you should be fine.

Hopefully one of the electricians here will chime in, but my understanding is if you use individual wires through the crawl space it will have to be in conduit. Whereas if you use Romex type wire, it can be left exposed, just stapled to the joists. Seems to me that's the only way to go. Whether you use copper or aluminum wire, it really does not make much difference (except in terms of cost), and assuming you compensate for the somewhat lower load capacity of the aluminum.

In talking with the inspector this morning, if I use THHN type cable, it will need to be in a conduit under the crawl space or buried. If I go with UF cable, I can secure it under the house (to the beams or similar), but do not need to run it in conduit until under the sidewalk.

 

[mrb]

THHN is wire, not a cable type.

I would not derate the neutral.

Use #4 copper THHN in conduit and feed it with a 90amp breaker.

Last time I bought #4 was a couple months ago, I paid $193 for 500ft roll of #4 THHN -so it would probably be cheapest to buy a roll of #4, use your 300+ft of it and sell the rest, and get a cut piece of #8 for your ground.

 

[Stuart in MN]

As usual, Aceman is correct about the sizing. #3 copper would be correct per NEC, but the local inspector is the guy who puts his signature on the final inspection.

 

[mrb]

As usual, Aceman is correct about the sizing. #3 copper would be correct per NEC, but the local inspector is the guy who puts his signature on the final inspection.

but if you can live with 90 amps, you can use #4 THHN (forgetting about increasing size for voltage drop for the moment) and be code compliant.

Someone else will have to confirm, but I remember something about only being allowed to use the 60deg column now for some cables. So your #3 could turn into #2.

 

[stevenson911]

I just did a 90 amp service, 1 1/2" conduit, 70 ft run, 4-4-6-8 THHN. This was from my home main panel, to my new pole building (20' x 48', 12' walls), some under the eave of the house and the final leg underground into the wall of the new building sub-panel. They also required 2- 8 ft ground rounds and #6 solid copper wire to the subpanel. This was inspected in Washington State (confirmed by the inspector before done) so it is legit.

 

[65Stang]

I just did a 90 amp service, 1 1/2" conduit, 70 ft run, 4-4-6-8 THHN. This was from my home main panel, to my new pole building (20' x 48', 12' walls), some under the eave of the house and the final leg underground into the wall of the new building sub-panel. They also required 2- 8 ft ground rounds and #6 solid copper wire to the subpanel. This was inspected in Washington State (confirmed by the inspector before done) so it is legit.

Oh... thanks, what a relief... been waiting for this confirmation for sooo long...

Holy Thread Resurrection Batman!!

 

[StumpjumperDave]

Just dont make the Inspector mad....

 

[Alchymist]

The posts above recommending upsizing to #3 wire are correct. There are some provisions for #4, it was mostly for where service entrance panels were #4; but voltage drop over a long run can be a concern. Another factor that comes into play is the size of the main panel. I won't go into it here but here's a link to wire sizes:
http://www.cerrowire.com/default.aspx?id=46

I personally wouldn't put #4 on a 100 amp breaker.

 

[Falcon67]

I just did a 90 amp service, 1 1/2" conduit, 70 ft run, 4-4-6-8 THHN. This was from my home main panel, to my new pole building (20' x 48', 12' walls), some under the eave of the house and the final leg underground into the wall of the new building sub-panel. They also required 2- 8 ft ground rounds and #6 solid copper wire to the subpanel. This was inspected in Washington State (confirmed by the inspector before done) so it is legit.

Good info - this is almost verbatim what I'm about to do for the new shop. Verbatim for the run and everything LOL. Go figure. I had the utility put a 100A capable box on the house service for the shop, but 90A ought to be more than plenty. Previous testing with my shop equipment showed that I have to run a bunch of stuff that one guy would not be running at once to get anywhere close to 50A draw.

 

[lametec]

Make sure the THHN wire you guys run underground are THHN/THWN rated. Pure THHN can't be run underground since it's not wet location rated. With that said, most THHN is also rated THWN.

 

[gatchel]

Toss all that and use a number 2 aluminum quad from the box store. So much cheaper. Your inspector is giving you a minimum of what he will accept, doesnt mean you cant use better.

Anyone want to take this one?

 

[AussieDan]

The maximum rating for #2 Al quad (typically 2/2/2/4 USE2) is 90A per 310.16 75*C column, you can certainly use it if you use a 90A breaker in the main panel to feed it, and the inspector approves.

It will need to be in conduit within the crawlspace, and the inspector may want to see it dual-rated USE/RHW to allow it to enter the structure.

 

[big.jim]

they cant knock you for oversizing cable if its not too expensive always go a size up

 

[big.jim]

ps as usual BIGGER IS BETTER

 

[Norcal]

they cant knock you for oversizing cable if its not too expensive always go a size up

ps as usual BIGGER IS BETTER

No good deed goes unpunished: NEC art 250.122(B) causes a issue....

Copy & paste from the 2008 NEC.

(B) Increased in Size. Where ungrounded conductors are
increased in size, equipment grounding conductors, where
installed, shall be increased in size proportionately according
to the circular mil area of the ungrounded conductors.

 

[egdede]

In case someone puts in a bigger breaker later.