Wire size for sub-panel

[Aaron Neve]

I have 200 amp service in my basement. I want to put a 100 amp subpanel in my garage. It's about 60 to 75 feet run for the wire in between. I'm going to run it in conduit, what size copper wire should I be using? And I need two 50 amp breakers in my main panel correct? Also I should have 2 hots, a neutral, and a ground?

[Dick Thomas]

Aaron
I assume you will want 220V available in the garage. Usually the rating given for a panel/subpanel is it's max 220V input. A given panel can be used for a lower capacity, usually isn't, unless reserving space for future upgrades. If you want 100A of 220V capacity in your garage subpanel, then you need a 100A 220V breaker in the main panel, which will have a 100A breaker on each hot leg, which are mechanically tied together into a single unit. An electrician would probably run a 1/0 (One Ought) aluminum for 100A service.

If you want 100A of 110V service, then you could use a 50A 220V breaker in the main, with possibly 8ga, but more likely 6ga leads.

Strongly recommend you have a licensed electrician do this work, if only for the fact that 1/0 wiring is a bear to work with without the proper tools!!!

Good luck.

[Don Buck]

I second the previous comment about getting a licensed electrican to run the primary feed to the sub panel. This past summer I needed to get 100 amps to my detached shop about 25' from the house. I rented a ditch witch and had a 24" deep trench ready for the electrician. He came out prior and made measurements and brought out the materials (I believe 3/0 aluminum in conduit for the feed) and he installed a 100 amp breaker on my main breaker and energized the sub panel. Although not required, I had a main breaker installed on the sub panel. I agreed to be on site to help pull the 3/0 wire as it was very stubbon. Not including the rental for the ditch witch, I spend about $800 for parts (nearly 100' of 3/0 to go breaker to sub) and labor to get the sub box hot. I have been wiring the shop myself and have just one line to complete before I get it inspected. I never wired before but got a couple of good electrical wiring books and talked with several "experts" to gain knowledge and confidence.

I probably saved $800 running the circuits myself and by taking my time I have it layed out just as I want including outlets for a future air cleaner, 5 x 220v circuits for current and future equipment. I saved some additional money by getting a great deal on a used 125 amp breaker panel at the local Habitat for Humanity store for $10 and it was filled with breakers! My box has 20 slots and I've just about filled it out so a larger box would be highly recommended.

Good luck.

Don

[Brent Ring]

I ran 6-0 for mine. 100 AMP capable panel - 50 AMP main breaker. Not going to ever need more in this current shop

[George Bregar]

I have 200 amp service in my basement. I want to put a 100 amp subpanel in my garage. It's about 60 to 75 feet run for the wire in between. I'm going to run it in conduit, what size copper wire should I be using? And I need two 50 amp breakers in my main panel correct? Also I should have 2 hots, a neutral, and a ground? 4-4-4-6 copper and a single 100A breaker in the main. 2 hots, neutral, and ground is correct. Make sure that you separate the neutral and ground bus in the sub-panel. PS: Not an electrician, there are several members who are, so maybe thye can chime in. But I think this is accurate to wire size.

[David Castor]

A 100 A sub-panel requires a two-pole 100 A breaker - if you want 100 A. You can use #4 AWG 75 deg C or 90 Deg C copper wire in conduit for a 100 A breaker provided the breaker is labeled for use with 75 deg C wire. A #8 copper ground is adequate for a 100 A feeder If running NM cable or SE cable, you would need #2 copper, I believe.

The sub-panel does not require a main breaker at the sub-panel, since there will be a breaker protecting it and the feeder in the main panel. You can put a main breaker in if you want to.

As mentioned already, you can buy a 100 A sub-panel and feed it with a smaller 50 A or 60 A circuit as long as you use the proper size breaker in the main panel.

[Aaron Neve]

Ok thanks for the info guys!!! A couple more questions. should I be using stranded thhn cable? Home depot dosent carry the aluminum cable. Some one told me (who has electrical knowledge) that i dont need a ground from the main panel to the sub panel if i use conduit, because the conduit bonds it. Is this true? He said I only need 2 hots and 1 Neutral, He also told me to use #2 guage thhn cable, (Is that overkill?)The home depot guy said (which I take with a grain of salt) That I need to put a grounding rod outside and tie to that, I have not heard that before. He also said I could get away with #4 Thhn cable. Hiring a electrition is out of the question ($), I have no problem adding new circuts and outlets and switches. But I just want to make sure I go big enough so to speak with the sub panel. I have read alot of information on installing a sub-panel, But it seems everyone has a different opinion. I plan on having 5 220 outlets and a couple 110 outlets and some shop lights. It seems to me that I need a 100 amp breaker in the main panel, 2 hot cables a neutral cable and a ground going to the sub-panel? Or is the ground optional with conduit? Once again thanks for your time and your knowledge!!!

[Bob Deroeck]

Hi Aaron,

I ran a 100 amp service out to my detached garage/workshop last summer, a total distance of 170' from my service panel. I recommend you discuss this with your electrical inspector. I found my inspector a bit more conservative than the NEC requires, but on the plus side, he gave me lots of useful advice that made the job easier. Plus, since he had essentially blessed the design (with his added on conservative elements) this avoided the risk that he would not signoff on the final installation.

He required a 4 wire conductor from house panel to workshop subpanel (2 hots, a neutral and a ground). He also required a separate ground at the garage/workshop. Everything is in 2 inch PVC conduit. NEC required only 1 or 1.25 inch PVC (I forgot which), but he suggested I use 2" since the cost of PVC is cheap and it made pulling the 2-2-2-4 AWG copper much easier. He also recommended installing a second PVC conduit for future cable installation since the trench was already open. I put in a 1 inch spare conduit for very little money. You will probably need to use expansion fittings on the PVC where it comes out of the ground to counter frost heaves, since you're in Illinois.

I also suggest you ask around for where the contractors buy their large cable. A friend of mine directed me to a place 50 miles away where the price was half of that at Home Depot.

Good luck.

Bob DeRoeck

[Aaron Neve]

Thanks Bob... I think I will see what the inspector gadget says. When you are refering to 2-2-2-4 are you implying #2 guage for the 2 hots and the nuetral and #4 guage for the ground?

[Barry Lyndon]

Aaron, I don't know if you have more than one Home Depot within reasonable distance to you but if you do, you might call another one and see if they have 2-2-2-4 aluminum cable. I have 4 home depots that I frequent around me and 2 of them had the cable and 2 of them didn't. I just had a 100 amp sub panel put in the shop by an electrician and then I wired in a bunch of new circuits. The 2 electricians I had helping me both suggested using 2-2-2-4 aluminum since it was much cheaper than copper and would do the job no problem (I bought all the material and they installed). I didn't use conduit (mine was all inside so it wasn't needed) so I can't comment on not needing the ground (though it doesn't sound correct). They also put a 100 amp breaker in the main.

[Joe Angrisani]

Aaron.... Any particular reason for conduit? Can you run and secure romex, or is everything going to be surface-mounted, hence the conduit? I just did the almost the same distance/amperage as you are looking at, and I'd strongly suggest 2-2-2-4 aluminum romex cable if you can. No reason for copper unless you're pulling and want (slightly) easier pulls and smaller conduit.

[Barry Lyndon]

Thanks Bob... I think I will see what the inspector gadget says. When you are refering to 2-2-2-4 are you implying #2 guage for the 2 hots and the nuetral and #4 guage for the ground?

Yes, 2-2-2-4 means 2 gauge hots and neutral and 4 gauge ground.

[Joe Angrisani]

and.... 2-2-2-4 refers to romex cable, not individual wires. A great big version of the 14/2 or 12/2 or 12/3 stuff you run to your lights and outlets.

[Aaron Neve]

In the garage everything will be surface mounted... But my basement is unfinished, and I guess I just thought romex was a no-no. Everything they ran when they built the house was in conduit in the basement. I.e. Basement lights and any runs to the kitchen or other rooms. If i could do romex that would obviously make it alot easier. Is it fine to just attach it to the underside of the joist?

[Joe Angrisani]

You can't use romex in an exposed position on the surface. It has to be in between or through the joists or walls, and secured every four feet in most jurisdictions. Do you see "regular" romex runs when you look up in the basement?

Potentially, you could run the 4-4-4-2 aluminum through the joists and in the garage wall to get into the back of a surface-mounted subpanel, then run your surface conduits out of the subpanel as needed. Just FYI: You can't run romex in conduit, even briefly to enter the subpanel.

When you figure out how it can be done, I'd suggest going to the Building Dept that covers you and run your idea by them and clarify any areas that you have questions about. The inspectors here were VERY helpful in guiding me in my project.

[Bob Deroeck]

Hi Aaron,

The 2-2-2-4 cable I was referring to was 4 individual THHN cables, two hots, a neutral and a ground with the ground the 4 AWG cable. The reason I went with 2 AWG copper for the 100 amp subpanel service was for voltage drop considerations for the larger machine motors over the long 170' run to the workshop. With this cable I should be able to run 8-10 HP worth of motors simultaneously. That may sound like a lot but it's only a 5 HP dust collector plus a 3-5 HP table saw. I could have tried to squeeze the amps through 2 AWG aluminum conductors or 4 AWG copper, but I just know that if I did, I'd regret it later when some beautiful old 5 HP machine was being sold on Craigslist for small money, but I couldn't tolerate the amps.

Bob

[Dennis McCullen]

Getting all the correct jargon down..... wires are individual, some stranded and some solid. "Cables" are things, such as Romex where 3 or 4 "wires" are encased in a common sheath. Refer to cables as 2-2-2-4 and refer to "wires" and three #2s plus a #4 ground. Trying to learn all this before I start wiring to new shop.

[Tom M King]

Ok thanks for the info guys!!! A couple more questions. should I be using stranded thhn cable? Home depot dosent carry the aluminum cable. Some one told me (who has electrical knowledge) that i dont need a ground from the main panel to the sub panel if i use conduit, because the conduit bonds it. Is this true? He said I only need 2 hots and 1 Neutral, He also told me to use #2 guage thhn cable, (Is that overkill?)The home depot guy said (which I take with a grain of salt) That I need to put a grounding rod outside and tie to that, I have not heard that before. He also said I could get away with #4 Thhn cable. Hiring a electrition is out of the question ($), I have no problem adding new circuts and outlets and switches. But I just want to make sure I go big enough so to speak with the sub panel. I have read alot of information on installing a sub-panel, But it seems everyone has a different opinion. I plan on having 5 220 outlets and a couple 110 outlets and some shop lights. It seems to me that I need a 100 amp breaker in the main panel, 2 hot cables a neutral cable and a ground going to the sub-panel? Or is the ground optional with conduit? Once again thanks for your time and your knowledge!!!

I'd run the ground wire too, rather than rely on the conduit. You can run into all sorts of weird trouble if for some reason you end up with separate grounds at the main and sub panel. No problem with a ground rod at both places (some inspectors even require it), but the whole ground system needs to have every part of it bonded together.

Wire at dedicated electrical supply stores costs about half what it does at the big box stores. Around here, the cheapest wire you would get for this run is commonly called "mobile home service entry wire"- comes in a few different gauges, and has four separate conductors including one with a yellow stripe and one with a green stripe (or maybe a white instead of one of those depending on wire manufacturer), and is okay for direct burial, so you really only need conduit where it is exposed above ground. You can buy it by the foot in electrical supply stores too.

Dig any trench first, and THEN measure for the wire. Get enough to make sure you have a few feet to throw away, rather than running short.

A box end wrench over the wire can help for tight bends, but sometimes you still end up having to hit it with a hammer.

Make sure to get plastic bushings for anywhere the wire enters or exits a metal opening. Use some De-Ox coating on any aluminum wire before inserting it into the connector.

2-2-2-4 would be the size, and the name also lists the sizes of the conductors. It looks like this: http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet15

All connections damtite. I use a 1/4" drive breaker bar with a short extension, and an Allen socket.

Here's a pdf for the latest Electrical Code: https://archive.org/details/nfpa.nec.2014

[James Tibbetts]

I'm in the same boat to run a sub panel. I found this site for wire: http://www.paigewire.com/products.aspx
Looked like good prices compared to HD and Lowe's.

[Rollie Meyers]

To any posters saying that 4 AWG is OK for 100A this is in error, the ONLY way this is allowed is the service entrance conductors that carry the entire load of a residence, a subfeed does not meet the criteria allowing undersizing the conductors to comply w/ code & use 4 AWG a 90A breaker is the maximum allowed, for 100A 3 AWG is minimum, this assuming copper conductors.