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Old 06-18-2012, 07:09 PM   #1
mjbdesigns
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Default 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

Sorry guys I could not find a clear answer on this one through search feature.

I am in process of powering up the 30 x 40 shop and will be running 2" Gray Electrical PVC 20 inches underground (18" code here) roughly 70ft distance from 200amp house panel to 100amp Shop sub-panel. I was going to order the wire for this tonite and wasn't certain on guage needed. I beleive #4 awg (stranded copper)will pass for this but #3 awg (stranded copper) sure seemed closer to the amp rating? A local small town electrician whom I do not know from Adam told me that #4 awg is all that code required? Would this be correct? and/or would running #3 be beneficial in some way if I never plan on upgrading service anyway? I sure appreciate any help from qualified folks here....

My load will be as follows:
Miller 252 MIG Welder (220 volt)
Quincy 60 Gallon 5hp Compressor (220 volt)
Plasmacam CNC Plasma Table (110 volt)
Hypertherm Powermax 60 Plasmacutter (220 volt)
Standard typical lighting and 110 outlets with common hand tools etc.
Other than compressor and plasma cutter nothing else will run at same time
(One man shop)

Last edited by mjbdesigns; 06-18-2012 at 07:13 PM.
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Old 06-18-2012, 07:40 PM   #2
Norcal
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

If you use a 90A breaker, 4 AWG is fine but if you want 100A then 3 AWG..
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Old 06-18-2012, 07:52 PM   #3
mjbdesigns
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

I suppose if I opt for the #3 I will be covered either way, I was trying to maintain my budget but don't think wire size is the place to try and save pennies. The difference between the 2 sizes in $$ is about $65 total. Thanks for the quick input....
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Old 06-18-2012, 08:06 PM   #4
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

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Originally Posted by mjbdesigns View Post
I suppose if I opt for the #3 I will be covered either way, I was trying to maintain my budget but don't think wire size is the place to try and save pennies. The difference between the 2 sizes in $$ is about $65 total. Thanks for the quick input....
Use 1/0 aluminum if you want to save money. Your pipe is plenty big enough.
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Old 06-18-2012, 08:40 PM   #5
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

Definatly compare the cost between AL and CU. Price of CU has been on the rise again.
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Old 06-18-2012, 08:57 PM   #6
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

Aluminum will definitely save you some $$. If you want to be good for the full 100A with no "if's and's or but's", use the 1/0 AL as suggested by Aceman.
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Old 06-19-2012, 02:37 AM   #7
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

I thought #2 would be required if using the lower termination rating.
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Old 06-20-2012, 11:08 AM   #8
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

Here's what I would do:

I would go with a 100A breaker and size the wire for that. If you sized your wire for 90A you wouldn't find a 90A breaker at the big box stores. You can find a 100A breaker at the big box stores. My experience is that the big home improvement stores have 20-40% lower prices on breakers and load centers than the electrical distributors. The only problem is that these stores only carry the common breaker sizes.

OK, so now with a 100A breaker I would go with aluminum and save a bunch of money. Use that money to buy more tools. For a 100A breaker and ASSUMING your breakers at both ends have terminations rated for 75C, I would use 1AWG Aluminum THWN. IF you have an older panel or one only rated to 60C then you'd have to use 1/O aluminum.

I would use individual conductors of 1awg, 1awg, 1awg, 4awg. The 4awg being the ground. That should easily fit inside your existing 2" conduit. You can't buy the individual aluminum conductors at your home improvement store but most electrical distributors carry it. In Ohio I've found that most suppliers in my area carry the Southwire SIMPull THHN (dual rated THWN-2) aluminum wire. Call around and compare the price to copper and you'll be surprised at the cost savings. It doesn't pull any harder than copper and actually bends easier. Today's Aluminum wire is much better than that 30+ years ago and a completely different alloy. The NEC would allow you to size your ground smaller but then you'd have to buy green wire because 4awg is the smallest you're allowed to relabel the wire as green with electrical tape. The aluminum wire doesn't come in green, Thus I recommended the 4awg ground. You could also reduce the size of the neutral if properly calculated, but some savvy inspectors might question your calculation (for good reason), and if done improperly could cause a fire hazard so I recommended a full sized neutral.

here's a link to the Southwire site for reference:
http://www.southwire.com/ProductCata...rodcatsheet327

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Old 06-20-2012, 03:08 PM   #9
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

Don't all the breakers readily available have multiple temp ratings, i.e, 60-90? If so why does that beaker not default to a 60 degree termination rating when put in a panel with other breakers?

Why don't we have to use the 60 degree column here for breakers sized 100 amps and under?
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Old 06-20-2012, 03:13 PM   #10
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

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Originally Posted by bjcouche View Post
Here's what I would do:

I would go with a 100A breaker and size the wire for that. If you sized your wire for 90A you wouldn't find a 90A breaker at the big box stores. You can find a 100A breaker at the big box stores. My experience is that the big home improvement stores have 20-40% lower prices on breakers and load centers than the electrical distributors. The only problem is that these stores only carry the common breaker sizes.
I found the same thing out when I did my garage. Menards had a 100a breaker in stock dirt cheap, but no 90a breaker. Also, Menards didn't stock #3 wire and getting it anywhere else would have cost more than buying #2 at Menards, so I ran #2 copper and used their cheap 100a breaker.
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Old 06-20-2012, 03:37 PM   #11
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

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I found the same thing out when I did my garage. Menards had a 100a breaker in stock dirt cheap, but no 90a breaker. Also, Menards didn't stock #3 wire and getting it anywhere else would have cost more than buying #2 at Menards, so I ran #2 copper and used their cheap 100a breaker.
Menards will special order the 90a breaker for $25 more than the 100A. I ran into this when I ran my sub panel with 2-2-2-4 SER. It's just so damn cheap/convenient to wire subs with #2 aluminum as its so cheap/readily available.
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Old 06-20-2012, 04:28 PM   #12
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

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Menards will special order the 90a breaker for $25 more than the 100A. I ran into this when I ran my sub panel with 2-2-2-4 SER. It's just so damn cheap/convenient to wire subs with #2 aluminum as its so cheap/readily available.
But, why pay an additional $25 for a breaker and settle for less amperage when I can buy the 100amp breaker and get more for less?

And, I'm one of those old diehards who has wasted time fixing problems with aluminum wire and don't want to spend anymore of my life doing it again. I've never had problems with copper, so I use only copper.

In the end, the cost was about the same to use #2 copper and a 100amp breaker than if I had "settled" for 90amp service and aluminum.

By the way, my 100 amp breaker was under $9 on sale.
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Old 06-20-2012, 04:40 PM   #13
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

I pulled 2/2/2/4 and fused it at 75. Not counting a breaker, for your run the price would have been $102.9 for the wire. I'd bet you spent over 3 times that for CU. I left HD with 20 slot box w/100A main and 6 breakers, 75A breaker for feed panel and 130' of wire and didn't spend $200. Just for comparison. Mine is 960 sq/ft and I figure that if I kick a 75A breaker, I'm gettin' it on big time. Air, machines, etc - should be no problem.

And don't say I should have shopped Menards - the closest store is 1000 miles
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Old 06-20-2012, 05:00 PM   #14
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

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But, why pay an additional $25 for a breaker and settle for less amperage when I can buy the 100amp breaker and get more for less?

And, I'm one of those old diehards who has wasted time fixing problems with aluminum wire and don't want to spend anymore of my life doing it again. I've never had problems with copper, so I use only copper.

In the end, the cost was about the same to use #2 copper and a 100amp breaker than if I had "settled" for 90amp service and aluminum.

By the way, my 100 amp breaker was under $9 on sale.
I bought the 100 amp breaker and ran it with the 2-2-2-4 :P If it ever has to be inspected, I'll have to switch to the 90a.

I have no worries using aluminum and never have, done correctly (simply proper torque and some no-ox) the services and large current supplies are pretty much bulletproof, and almost every service around here is done with aluminum. To do copper, I would have had to get the conduit shoved through TGIs with 3 feet open on one end, down through a 12" thick oak beam, and over to the panel, which would have required four 90s and overall be a PITA. The SER was much easier to run, cheaper and has plenty of current for what I need even at 90A.
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Old 06-20-2012, 07:19 PM   #15
mjbdesigns
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

Thanks for all the input, just took awhile to read through all the various opinions. This is what I ended up with and hope no qualified electrician will find isssue with what I have done or am in the process of doing.

Running 55ft. from House 200 amp main panel using #3awg copper THHN (One Red, One White One Black and One Green) 4 wires total running through 2" gray electrical conduit with minimum depth to top of pipe at 18" below grade. Both ends of PVC will have an Expansion sleave installed above grade as per Michigan state electrical inspectors requirement. From there into PVC LB junction boxes and into house basement and shop wall respectively. The expansion sleave seems a bit overkill but maybe it will save my siding thru the winter/summer cycles? I chose copper over aluminum because it seemed the only reason to go other way was $$ cost and I got what I feel was a great deal on the #3 copper wire. Oh, I will put a 100 amp breaker in the 200amp house panel to feed the 100 amp shop panel and the next question will be wether to use the boding screw or not in the sub-panel? and should I run a seperate grounding rod for the shop?

Appreciate the attention to my questions and the time taken to teach an old dog new tricks....
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Old 06-20-2012, 07:21 PM   #16
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

A GM Electrician buddy suggested running the 4 different colors for no real reason other than he said "Can keep them all seperated and easier identified" If that makes sense? But I figured it couldn't hurt and cost no more money...
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Old 06-20-2012, 07:55 PM   #17
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

Don't connect the bonding screw. Throw it in the trash. Add a grounding bar to the load center for your grounds. Most load centers don't come with a separate ground bar, you have to buy one extra. You have to keep the neutral and ground separated. You'll also have to drive 2 quantity two, ground rods and terminate it to the ground bar. Unless that is you have a UFER ground but that's another topic and I think you'd have asked about it if you had one.

I did the math on copper vs aluminum before reading your last post.

1-1-1-4 aluminum $131.25
3-3-3-6 copper $280.50

Either one would work just as well. I like your choice to use the 4 individual colors. That makes a neater installation and I do the same whenever I can.

Brian Couchene
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Old 06-20-2012, 10:04 PM   #18
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

Gotta jump in to ask whats UFER?
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Old 06-20-2012, 11:39 PM   #19
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

For UFER ground, try wikipedia or this link:

http://www.psihq.com/iread/ufergrnd.htm

Codes vary from location to location on how the foundation rebar is tied together and stubbed up through the foundation leaving an exposed place to connect the electrical ground. In my location just a piece of 20' rebar tied to the cross rebar sections along one footer then bent up to exit through the concrete floor was sufficient.

Brian
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Old 06-21-2012, 07:43 PM   #20
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Default Re: 100 amp Sub Panel # 3awg THHN or #4awg THHN that is the question

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Originally Posted by bjcouche View Post

1-1-1-4 aluminum $131.25
3-3-3-6 copper $280.50

Either one would work just as well. I like your choice to use the 4 individual colors. That makes a neater installation and I do the same whenever I can.

Brian Couchene
Curious where you were quoted those prices? I did make the mistake of getting #3 for the ground as well when I find out now I could have used #6 which would have been a slight savings.
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