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Wiring Setup for Attached Garage

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Forum topic by Oosik posted 02-10-2015 02:28 PM 1118 views 0 times favorited 28 replies Add to Favorites Watch
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Oosik

126 posts in 1142 days


02-10-2015 02:28 PM

Been inactive for a while due to a new child but I’m finally going make the plunge and pick up a cabinet saw at the end of the month but that requires some 220 outlets!

Apologies if my understanding of electrical wiring is off.

My house currently has an exterior Main Panel that I believe is 225 amps with 180 amps worth of breakers on it.

The sub Panel in the garage for the house is 125 amps but connected to a 70 amp breaker on the Main.

I like the idea of a new sub panel for the table saw and future larger tools (Jointer, Dust Collector, Band Saw, Compressor, etc).

So my question is, what is the best way to size a new sub panel and will my main panel need to be upgraded to handle the new sub panel?

Any other recommendations to find the most efficient and future proof wiring for my 3 car tandem garage shop would be great.


28 replies so far

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soob

223 posts in 668 days


#1 posted 02-10-2015 02:41 PM

It’s not the panels but the wires that are going to be the limiting factor. Find out what the gauge of the wire that feeds that panel in the garage is. It may be fed by 70 amps because the wire limits it to that size—that would be the max for.. #4 copper I think? The feed breaker is the one that matters not the panel’s main breaker. That’s basically just a disconnect.

Load calculations are complicated, too complicated for me. But as a practical matter, figure out what you expect to be doing all at once and make sure it won’t exceed the capacity. Realistically you’re just going to be running one power tool, the dust collector, and maybe a compressor all at the same time. Remember that 110v counts for just half of its rated amps.

70 amps is not a lot for the house if the major appliances are fed from that panel. If not (and I’m guessing not) you can probably just feed from that one. If it doesn’t have the spare capacity you might need to run a new wire to the garage for another subpanel—assuming you have free space at the main panel (if not you might need to add TWO subpanels). If you can spare, say 35 amps from that garage subpanel, you should be fine for just about anything unless you want to start welding. 20-25 might even be enough depending on the size of the compressor and dust collector you have in mind.

Sorry if this is rambling. I’m not an electrician but I’ve done my fair share of wiring. It’s not hard once you have the basics down.

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agallant

530 posts in 2346 days


#2 posted 02-10-2015 02:57 PM

It depends on how crazy you are going to get. My shop has two feeds in to it that are 30 amps each. Considering it powers just the shop its enough for my lights to be on, dust collector and 240V table saw. After all how many tools am I going to run at once. In my new house that we will be moving in to shortly I have

1X 20 amp breaker feeding wall outlets
1X 15 amp breakier for lights and extension rheel on cealing
1X 20 amp breaker dedicated for compresson (overkill)
4X 15A breakers dedicated to 2X 220V outlets

Tools I have are:
3HP Delta Unisaw
Harbor Freight special Dust Collector
Shop Vac
Router Table
Jet 16-32 drum sander
10” sliding CMS
Bench belt sander
Drill Press

It really comes down to how many of these things I am running at once. With the exception of the lights, what ever battery charger is plugged in and the compressor kicking on once in a blue moon on its own its never more than two additional things. The Dust collector and table saw or drum sander.

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TheFridge

5764 posts in 945 days


#3 posted 02-10-2015 02:58 PM

I wouldn’t upgrade. None of your equip will be running at the same time.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

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soob

223 posts in 668 days


#4 posted 02-10-2015 03:09 PM

I should add that when I said to figure out what “you’re” going to be doing, I meant everyone in the house. If he has a wife ironing and six daughters blow drying their hair at the same time off that 70-amp panel he might be in trouble. Think about things like refrigerators, space heaters, microwaves etc. Realistically though most people don’t approach the capacity of their panels through outlets and lights. It’s appliances that get you, and most of them are probably fed separately (from his description).

Also, for panel capacity you don’t add up the breakers, just look at how much you expect to be actually using at the same time.

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Oosik

126 posts in 1142 days


#5 posted 02-10-2015 03:46 PM

This is the garage panel. I’ll have to look at the major appliances and their max load to come up with a rough calculation. Problem I’m concerned with is I was told it is preferred to keep panels at 80% of the max load and if there is room to expand.

Whats the easy way to figure out the wiring to the interior sub panel?

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soob

223 posts in 668 days


#6 posted 02-10-2015 04:01 PM

I can’t read the labels on the picture so it’s hard to guess what the draw is going to be. My feeling is that you’ve got plenty of spare capacity for your garage shop tools even if it’s just a 70-amp wire.

There’s lots of physical room in that panel to add circuits.

To determine the feed size, take the cover off and look at the big wires that are feeding the panel. It should either be printed on them, or, you might have to deduce it from the diameter.

Do you have easy access to that panel from above? I.e., is there an attic over your garage? Or do you have a crawlspace under it? If so it will be very easy to add circuits.

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Oosik

126 posts in 1142 days


#7 posted 02-10-2015 04:18 PM

Major appliances with dedicated circuits:

Freezer, Washing Machine, Furnace, Dishwasher, Refrigerator, and Microwave.

The only other major power drawing items would be a desktop computer that pulls 900w max, and 2 plasma TVs that I can lookup the model number later and of course the wife’s hairdryer.

I read that some people use the sqft of a room to help calculate a theoretical draw?

My needs are pretty standard I’d think, need the dust collector or shop vac running with one tool at a time and whenever the compressor kicks on when I upgrade to a larger unit.

Yes, It’s easy to get to the panel from the attic. Luckily it is in the middle of the house near the garage door to the house, so no fighting with the pitch of the roof.

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soob

223 posts in 668 days


#8 posted 02-10-2015 04:50 PM

Sounds like you’re fine.

Adding circuits is very easy DIY. I am not going to describe it in detail since there are lots of resources for it, but don’t be afraid to do it yourself. Basically all you need is a fiberglass rod, a drill bit, and a saw for cutting holes for outlets in the drywall. Poke the rod through, tape the wire to it, and pull it back up. Very easy. 12/2 wire for 20-amp circuits, 10/2 for 30-amp.

If you have any specific questions post them here or PM me and I’ll do my best to help out.

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Sparks8286

72 posts in 948 days


#9 posted 02-10-2015 04:55 PM

You can’t add up the amperage on the breakers in the panel to figure out how much electrical ‘space’ you have left. You have to figure out what’s called demand factor which is part of the load calculations. A 70A subpanel for your shop isn’t bad and should be plenty provided you don’t have a lot of commercial equipment running. Most of the time you’re only using 1 tool at a time with maybe 1 or 2 others running in the background. If you’re the kind of guy that prefers to have the planer and drum sander going at the same time and really ripping through some work while your son is doing something over on the table saw, then you’ll likely want to consider upgrading some wiring.

Turn off the 70A breaker in the main panel that feeds the subpanel. Then go open up the subpanel in the garage and see what size wire it is. If it’s copper it’s probably #6 AWG. If it’s aluminum (which I bet it is) It’s either going to be #4, #3 or #2 AWG depending on what type of insulation it has. If it’s one of those then you’re fine for a 70A subpanel. If you don’t have a clamp on ammeter, see about borrowing one or picking up an inexpensive one from Sears. With the 70A breaker on for the garage panel, turn on some stuff; compressor, sanders, washer and dryer (if they’re fed from the subpanel) and take a reading on the feed wires for the subpanel just to see where you stand now. Try to find a combination of things that are fed from that panel that are likely to be running at the same time. Remember your 70A panel can’t be loaded more than 56A (70×80% = 56). That’s going to be your best method of finding out how much ‘space’ you have in your panel.

Remember when you’re talking about 220 outlets that the 220 refers to the voltage, not the amperage. All homes are wired for 120/240V AC (alternating current) and current code requires at least a 200A electrical service for residences. You can have a panel rated for more than that of course, but you can’t have less unless the house was built before code updates. Voltage only refers to the insulation on the wire. Most of what you’ll see is either going to be THHN, THWN or UF inside a house. Read the nameplate on your saw to find out the amperage you need for that particular 220V circuit. One thing to remember, a circuit can’t be loaded more than 80%. For example, if you have a 20A circuit then you can’t put a load greater than 16A on it (20×80% = 16A). So if your saws nameplate says 17A then your overcurrent protection (circuit breaker) will have to be rated at 30A. Then you would also have to change the wire size you’re running. #14 is use for 15A circuits, #12 is used for 20A circuits, #10 for 30A circuits and so on. If you want to add additional circuits in your shop, don’t waste your time with anything less than a 20A circuit. We’re always moving things around in the shop and plugging things in at different places. Some circuits rated at 15A and others rated at 20A will only increase the chance of tripping a breaker sooner or later and that’s just annoying.

I hope this helps (at least a little) rather than confuses you.

-- If you can't fix it with a hammer, you have an electrical problem.

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soob

223 posts in 668 days


#10 posted 02-10-2015 05:10 PM

The 80% rule is for continuous loads. None of your shop tools are continuous loads and 99% of your household stuff is not either. So I don’t think that’s a real consideration.

Also I don’t think you’re going to see THHN, THWN or UF inside a house. It’ll be romex (NM) or service entrance (SE).

Edit: let me clarify. My understanding of the 80% rule is that you need to reserve 20% (i.e., multiply by 125%) the amps for continuous loads when doing calculations. So if you have a refrigerator that’s 15 amps you need to count it as ~19. Same with lights, freezers, anything that’s going to be on for more than 3 hours at a time. But that’s not most of your stuff.

Also you might see THHN-type wiring (i.e., separate conductors) in a conduit but for in wall wiring it has to be jacketed, NM or SE. UF you should never see inside a house unless the wire has other ratings which I don’t think it usually does.

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Sparks8286

72 posts in 948 days


#11 posted 02-10-2015 07:22 PM

Soob, just want to clarify a few things.

I am references the 2008 National Electrical Code because that’s the one I happen to have in front of me right now.

The 80% rule is not for continuous loads. The 80% rule is for any circuit. [Table 210.21(B)(2)]. I’ve referenced the code section related specifically to receptacles and cord-and-plug related loads because that’s what were talking about in this thread.

A load is considered continuous when it runs for 3 or more consecutive hours. (Article 100 I) It’s not likely that any tools in a home shop would be considered continuous duty, but I don’t know everything he has and how much he likes to use it. Now that he knows what continuous duty is he can make that distinction for himself.

Your edit about continuous loads is correct. Oddly enough the 80% circuit load maximum and the 125% continuous duty calculation requirement are the same mathematically. For some reason, NEC requirements still separate them.

Romex is a brand name. Code refers to it as non-metallic sheathed cable or simply NM as you stated. If you look at the individual insulated wires inside that cable you will see stamped on the side of each insulated conductor THHN. THWN is virtually the same as THHN, but with minor differences that really don’t mean much of anything to anyone other than the people that make it. It’s still electrical insulation that’s used for 90 degree C ambient air temperature applications. Whether it’s already inside a PVC jacket like Romex or pulled in a conduit as individual wires is irrelevant. It’s still the same insulation.

SE cable is very similar to Romex or NM cable. The individual wires use an XHHW-2 or THHN/THWN-2 insulation and the cable has a PVC jacket for protection. SER cable is the same way. It just has more conductors.

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet273

UF (underground feeder) cable is a direct-burial cable that does not need to be put inside a conduit in order to be buried. It can also be used above ground because it is sunlight resistant also. It can be used in wet or corrosive environments. Many people use it for lamp posts they want to install along their driveway or maybe to power a small pump in a fish pond. It can also be used to provide power to a subpanel in a detached garage or work shop.

http://www.paigeelectric.com/specs/P7295D.pdf

Almost every house in America has THHN/THWN insulated conductors and many also have Type UF cable as well.

-- If you can't fix it with a hammer, you have an electrical problem.

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WhyMe

608 posts in 1020 days


#12 posted 02-10-2015 08:37 PM


Soob, just want to clarify a few things.

I am references the 2008 National Electrical Code because that s the one I happen to have in front of me right now.

The 80% rule is not for continuous loads. The 80% rule is for any circuit. [Table 210.21(B)(2)]. I ve referenced the code section related specifically to receptacles and cord-and-plug related loads because that s what were talking about in this thread.

- Sparks8286

You are mixing apples and oranges. 210.21 is for outlet devices, not the circuit. Example, if you have a 15A duplex outlet you are not to plug in more than 12A of load on that one outlet device. Circuit sizing and amp capacity is based on 100% of noncontinuous and 125% of continuous loads (NEC 210.19).

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Sparks8286

72 posts in 948 days


#13 posted 02-10-2015 08:54 PM

I stand corrected about referencing the wrong code section. I was thinking about receptacles. However, I did a little more research. Take a look at 384.16©. 100% is allowable provided the circuit breaker is rated for 100% load rather than the standard 80% load capacity.

So the way I understand this now, 100% is allowable provided it’s not a continuous load and the circuit breaker is rated for 100% capacity.

-- If you can't fix it with a hammer, you have an electrical problem.

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soob

223 posts in 668 days


#14 posted 02-10-2015 08:56 PM

Edit: I am too slow. Serves me right for commenting at work. Here is my response below, though:

210.21(B)(2) is by its own terms applicable only to plugs and cords. You said he’s limited to 56 amps on that 70-amp-fed panel, which I believe is erroneous. It’s not an outlet so 210.21(B)(2) does not apply. 56 would only be the limit if the load was continuous which it isn’t. Also, 210.21(B)(2) applies only where there’s more than one outlet on the circuit. So if you just have one 220v plug on the circuit (which is often the case) you can still use the full rating.

Fair enough about the THHN/etc. thing, but the way you said it was confusing, I thought. I wanted to make sure he knows he can’t use separate THHN-type wires in his walls. That has to be in conduit.

Honestly, I am not so sure now about the UF. My understanding was that you can’t use it indoors, that it has to be terminated outside, but as I said in the other post, I’m not so sure. So I will defer to you on that, but I still would not recommend UF for this guy’s application.

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WhyMe

608 posts in 1020 days


#15 posted 02-10-2015 09:09 PM

Honestly, I am not so sure now about the UF. My understanding was that you can t use it indoors, that it has to be terminated outside, but as I said in the other post, I m not so sure. So I will defer to you on that, but I still would not recommend UF for this guy s application.

- soob

UF can be used indoors and is installed just like NM. USE is the one that is to be used outside the structure.

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