Workshop Subpanel Amps

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I am in the process of installing a subpanel to consolidate the wiring in my workshop. The panel is rated for 150amps, but I purchased a 125 amp to install in the main panel. I additionally have a 100amp breaker as well. The largest bound copper cable I can get without a special order from the local electrical supply house is 2/3 which will handle up to 95amps. I would prefer not to wait for the special order if I don't have to. 50amps is the next size down at the local Home Depot, ( I need to see what the electric supply house has, I assume I can get a 70, 80, or 90)
My questions are:
1. I am thinking that I really don't need 100amps, so I wanted to query others to see what they have installed. (I am in the process of totally up the equipment and amps that I would expect would be needed at a given time to make sure I have all that I need, but that is not complete) I do have a fair amount of machinery and can see 2 - 3 users larger pieces of equipment plus auxillary air compressor, dust collection, air filtration, general power consumption that will add up. In my mind, it's not that much more expensive to add the extra amps at this point and would rather overplan than be under powered at a later date which would be much more expensive to upgade, if I needed it.
2. I can purchase Aluminum to handle the 125amp's but I have heard bad things about the aluminum connection corrosion. I know I can put the special grease on the connections to prevent the corrosion. Any thoughts or comments?
David
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DL wrote:

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Unless the insulation is rated at only 60 deg C, a 2/3 cable ought to handle a lot more than 95 amps. Where are you getting that figure from?
You can do 100A in a 4/3 cable if the conductor temperature is rated 85 deg C or higher....

I would think so. I'm surprised you didn't find a 60 at HD.

You certainly have the right idea about the cost of building bigger than you need vs. the cost of upgrading later. But I think you're going bigger by a larger margin than necessary. 60A at 240V will provide more power than you're likely to need in a home shop.

Under _no_ circumstances would I ever install an aluminum feed to a subpanel in my home, or advise anyone else to. It's just too risky, and for what benefit? Sure, you might save a few bucks, but it's a whole lot less safe than copper unless it's installed _exactly_ right. Trouble is, it's a whole lot more difficult to install correctly than copper is. It's not just a matter of adding anti-oxidant compound to the connections. Bolt lugs must be tightened with a torque wrench, both to make sure that they are tight enough, and to make sure that they are not _too_ tight, because aluminum will cold-flow away from an overtorqued connection, loosening it, and causing it to arc. All equipment used must be rated for use with aluminum. And so on.
Don't do it. It isn't worth it.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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I'm betting his whole house runs on an aluminum drop to his main panel. I can't remember the last copper service I saw. With a big feed like that, no sustained heavy loads, and no connections buried in the walls, I wouldn't worry. But for a short run, copper might feel better, and be easy to handle. Wilson
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wrote:

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Note that if your copper cable will *not* handle it, neither will an aluminum cable. If using aluminum [which I do *not* recommend], you need substantially larger conductors to have the same ampacity as copper.
To carry 100A in a 3-conductor cable, you need at minimum: 4ga copper or 2ga aluminum, conductor temp 85 deg C or higher 3ga copper or 1ga aluminum, conductor temp 75 deg C or higher 1ga copper or 2/0 aluminum, conductor temp 60 deg C or higher [National Electrical Code, Table B-310-3]
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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Yes, of course the whole house runs on an aluminum drop.
The difference is that the service entrance almost certainly was installed by a professional who has the skills and knowledge to install it correctly - skills and knowledge not possessed by the average homeowner.
_When_properly_installed_ aluminum wiring is as safe as copper. The trouble is that aluminum is much more difficult to install properly than copper is. It's just plain foolish for Joe Homeowner to take chances with it.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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While I do generally agree with what you've said about the advantages of copper over aluminum in this whole thread Doug, I do disagree with what you say here. There's just too many home across America that had their service entrance installed by the home owner with absolutely no problems and no reason to believe there will be any probles, for your assertion above to be correct. It really does not take a professional to install a service with aluminum wire. It really does not take any special skills or sage wisdom. It's just a connection. The average homeowner is very capable of posessing these skills and knowledge. Not to say that all are of course, but that'd be another thread. Sometimes we can overly mystify this stuff.
--

-Mike-
snipped-for-privacy@alltel.net
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I'm sorry, but that's not true, not completely, anyway. No, it doesn't require a professional to install aluminum wire [and I did not say that it did], but it *does* require special knowledge, if not special skills, and tools that are not in the average homeowner's toolbox.
To install aluminum safely, connections must be coated with anti-oxidant compound. This is not necessary with copper. And not everyone knows about the difference.
Lugs holding aluminum wire must be tightened to a specific torque. Too loose, and the wire arcs, or comes out - but too tight, and it cold-flows away, and arcs. Again, not everyone knows this. And the average homeowner does not have a torque wrench in his toolbox.

No, it's *not* "just a connection". It's a connection that needs to be made with substantially more care than connections involving copper wire. And not everybody knows this.

I never said that the average homeowner was not *capable* of possessing the requisite skills and knowledge. I do claim, though, that the average homeowner possesses neither those skills, nor a torque wrench.

I'm not trying to make mysteries out of anything. Just pointing out that installing aluminum wire correctly is not as simple as installing copper wire correctly - and since *any* incorrectly installed wiring presents potentially lethal hazards, Joe Homeowner is better off using copper simply because it's harder to screw up.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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wrote:

I agree with Mike that it is not a overly complex connection to install as long as someone has the proper knowledge, tools, and respect for the potential dangers of not doing it correctly. I also agree with Doug that the are inherent issues with the Al connection that don't arise with the copper installation. When I added an addition on our old house, I needed to redo a significant amount of the wiring the previous owner did when he added a small addition on the house because it was installed wrong.
Also, the fact that I would need to go to #00 Al to get the same load capacity, it as the #2 copper, going with the cooper will just be easier to handle when I am routing the cable from the main to the subpanel.
It's great to know that I am not an average "Joe" homeowner. Not only do I have a torque wrench and know how to use it, I have three of varing sizes :D
David

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wrote:

The sales rep at the electrical supply stated the 95 and when I did research on the internet for loads on copper wire, it said the same, even the board at HD said the same as well. I didn't look in the temperature rating though. I plan to call the county to see what they will approve. Simplest would be that the 2/3 would be accepted for the 100amp breaker that I currently have so I can make the connection and move on with the electrical work.

I will check with the electrical supply to see what the temp rating is on the 4/3 and 2/3 they stock.

I expected to see the 60 and 70 as I have in the past. It could be possible that they are starting to phase out the GE panels for the CH ones so the selection of breakers is smaller.

Given that I have already have the 100 and 125amp breakers for the main panel which I would prefer to use, the cost difference in the wire is minimal given that we are talking < 50' distance. If I am correct, just like the main panel at 200amps, if you only put one 20amp breaker in, there won't be a problem having too many amps being fed into the subpanel, the equipment will draw the necessary power it needs from the individual breakers in the subpanel.
This does go to the point of equipment overload. If I have a piece of equipment that draws 25amps and have a circuit consisting of a 50amp circuit breaker, 6/3 wire from the breaker to the outlet, and a 50 amp outlet, I should still be fine, isn't the logic the same? or do I need to worry about overloading the equipment. It has always been my understanding that equipment / motors draw the power they need from a circuit. The breaker is designed to kick off when too much current is required thus causing a situation where the wire would heat up and become unsafe. (goes to your temp rating statement above)
Also, the rotary phase converter I have requires 70-80amp breaker IIRC (I have to get with the manufacturer to confirm), though I am not exactly sure why a 7.5hp 3 phase motor requires that much. I would figure that while it doesn't draw those amps all the time, it is needed at some point, possibly startup.

What I expected as a response, just wanted to confirm. The extra money of the cooper wire is not the issue here. I am trying to get things done and didn't want to have to wait for a special order if I didn't need to.
Thanks for the advise,
David

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Well, it's not just the cost. Heavy cables are a PITA to work with. They don't bend easily.
One thing I meant to mention in my first response to you, that I think I omitted: make sure that the cable you use to feed the subpanel has a grounding conductor. You need 2/3 WG (With Ground), not just 2/3. In the subpanel, the equipment grounding bus bar and the neutral bus bar must *not* be connected.

Yes, that's correct.

Your understanding is correct: the equipment draws the power it needs. The purpose of the circuit breaker is to protect the wiring and the receptacle, *not* what's plugged into the receptacle.

I think you have that right, too.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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wrote:

The cost difference isn't that much so I would rather go with the heavier wire. As for the bends, I am lucky that the routing of the wire has just a few bends in it that can be done at a gentle curve. Once I find out wire temp rating and what Amps the county will approve on rated 2/3, I plan to go with the 2/3 and buy the the highest amp breaker if the 100 or 125 are too much. Hopefully they will approve the at least the 100amp. This will cover me for anything I have.
Thanks for the help,
David

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You're welcome. You did catch the part about needing three conductors PLUS ground (below), right?

-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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wrote:

Yes I did, it's 2/3 with ground, and I do understand that the neutral and ground bars are not to be connected in the subpanel as the are in the main panel.
Thanks again,
David

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DL wrote:

on 3 #4's and a #6. I could have used less but I had the breaker. If it's a one man shop then anything over I think would be overkill. All I would say is stay away from aluminum, even if it's just for a feed.
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DL wrote:

For a 120/240 VAC service, a 2P-60A main kit installed in a 125 MLO, 12/24 panel will handle everything a one man shop can throw at it.
The largest motor you can run on single phase is about 5HP, cap start, cap run, normally found on an air compressor.
A 2P-40A branch c'bkr will handle the compressor.
Everything else incuding a table saw and a welder will be smaller loads.
HTH
Lew
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Lew,
Thanks for the input.
David
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Lugs on panels and breakers are either rated copper only, or copper and aluminum. If you use aluminum wire make sure the lugs on the panel or breaker specifically are rated for aluminum.
It'll never hurt to oversize the wire. Larger sizes will lower the voltage drop over distance.
If you are at all unsure, hire an electrician, or get advice from one. I would not count on advice from your electrical supplier, any HD employee, or even reccomendations for well meaning and mostly intellegent members of this group.
my $0.02.
Alan
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"DL" wrote in message

Take Lew's advice to heart and follow it ... I wired my shop according to his suggestions a few years back and have not regretted it. Best advice I ever got in exchange for a recipe, IIRC. ;)
--
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Last update: 4/14/05
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