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Old 01-07-2011, 07:20 PM   #1
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New guy needs to rewire his garage!


Hi. I'm new. Well, to this site and to home wiring (anything more complicated than rebuilding a lamp or replacing a power cord). I have done some household wiring projects before without issues so I have some idea of what's what. Having an automotive background helps as well, as I can basically rewire anything in, on, or around a car (and will be doing so shortly to mine).

Here's my problem. Basically, my home is a couple centuries old (no, not kidding), and the wiring was half-baked in the retrofit. I need to run about a 30-35A power source to a sub panel in my garage (detached), as I just bought a welder that was supposed to be a 15A draw that turns out to be a 24A draw, which will overtax the circuit (current 15A air compressor occasionally blows the breaker as it is). The underground cable is shot... There is a switch in the laundry room that controls the outlet and light in the garage that we have to keep turned off when the ground is wet, for about a week after the weather clears or the ground freezes, or the breaker blows.

I mentioned that our house wiring is half-baked... One light blows and every light in the house goes out (as one example). The garage is in a circuit with the TV, cable box, and VCR in the living room as well as the refrigerator.

This is what I'm up against. Between the half-baked wiring and the bad cable, I'd like to get the garage on an independent circuit. I CANNOT go any more than 40A (using approximately 260A on a 300A load center as it is). I'm planning a 4-breaker box tied to the main panel by an 8-gauge conductor (yes, I've done some research), using at least a 25A breaker for the welder and the compressor (one at a time), another breaker for another outlet setup for a drill or saw, and another for a set of (at least, maybe 4) 3, 4' fluorescent lights (only have 1 60W bulb in there right now and it gets dark pretty easy). The garage is basically big enough to squeeze in 2 cars (no trucks, not enough height) and that's pretty much it (and you would have to take just about everything out of the garage to get both in there), for an idea of the square footage. I'm going to get some plastic conduit for the buried cable (unless someone could tell me a way to run an overhead), but I'm not sure as to whether I should use flexible conduit or rigid. Also, the easiest way to run the conduit/cable is about 3 times the shortest distance to the garage from the basement due to the driveway, sidewalks, patio, and the outdoor basement stairs (which is surrounded by concrete blocks), which I think is about 80 feet. Yes, I am aware I will need to bury the cable a minimum of 18" and probably deeper.

Any help is appreciated. In the mean time, I'll be doing more research...

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Old 01-07-2011, 07:26 PM   #2
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Its better to run a 60a sub at a min
How did you come up with using 260a on a 300a load center
What did you use to do a load calc ?

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Old 01-07-2011, 07:51 PM   #3
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Simple adding up all of the current breakers... It's at least 220 and there seem to be 2-3 unlabeled breakers in the box. I suppose I could put my 400A clamp meter around the service cable and measure, but not everything is turned on at the moment.

Forgot to mention we can't afford a contractor either, and I NEED to use the welder as it's the smallest I could find and I need to replace a quarter panel on the car.
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Old 01-07-2011, 07:58 PM   #4
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IMO there is no way your home is loaded to 260 amps. I'd be surpised it's even loaded to 150 amps.
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Old 01-07-2011, 08:03 PM   #5
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IMO there is no way your home is loaded to 260 amps. I'd be surpised it's even loaded to 150 amps.
Between the 2 30A and 15A breakers, that's over 100 right there, then there's a couple 20s and a few more than that. I just checked about 20 minutes ago to make sure I had a good count and also the load of the Main breaker, and I got about 210-220 without the unlabeled ones.

:edit: Just counted again just to make sure... 2 30A.. 60A. 3 20A.. 60A. 8 15A... 120A. Total of 240A, not counting the unlabeled ones. Not sure if all breakers are used to capacity, 2 of them seem to blow the most, and I think they're a 15 and a 20. 3 holes unused in the panel, 2 on the right and one on the left. Now I'm not entirely sure about the capacity as it may already be full and I might be SOL on this...

Last edited by martianent; 01-07-2011 at 08:24 PM.
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Old 01-07-2011, 08:15 PM   #6
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New guy needs to rewire his garage!


You do not add the value of the breakers ti figure load.
You may well have over 300-400 amps if you add all the breakers.
How big is the main breaker in the panel?
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Old 01-07-2011, 08:17 PM   #7
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Originally Posted by martianent View Post
between the 2 30a and 15a breakers, that's over 100 right there, then there's a couple 20s and a few more than that. I just checked about 20 minutes ago to make sure i had a good count and also the load of the main breaker, and i got about 210-220 without the unlabeled ones.
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Old 01-07-2011, 11:42 PM   #8
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Quote:
Originally Posted by martianent View Post
Simple adding up all of the current breakers... It's at least 220 and there seem to be 2-3 unlabeled breakers in the box. I suppose I could put my 400A clamp meter around the service cable and measure, but not everything is turned on at the moment.

Forgot to mention we can't afford a contractor either, and I NEED to use the welder as it's the smallest I could find and I need to replace a quarter panel on the car.

as others have said, you don't add the breakers. An easy way to think of it, is that your range is probably on a 50 amp breaker. On bake, it probably draws about 10amps and it probably wouldn't draw 50 if you had all 4 burners on, plus both elements. In my house, i ran a dedicated 20 amp circuit for each bedroom. All that's plugged in is a cell phone charger in 1 of the bedrooms. That is certainly not an 80 amp load.

in other words, you can put in as big of a circuit as you want. I would recommend no smaller than 60amps, as would most other people. We just need to know how you plan to get from the garage to the main panel. Pictures would be the most helpful, take some pictures and we can give advice
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Old 01-08-2011, 12:12 AM   #9
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New guy needs to rewire his garage!


Speaking from similar experience, I'm going to make some guesses about the equipment you're talking about for your garage. If you say the welder and compresser were expected to be 15 amp equipment, I'll make the further guess that these are 120 volt pieces of equipment.. I hope the welder was Miller or Lincoln, I have a very nice Miller I'm happy with for now. These should be fine on a 20 amp circuit.

If you've got a more serious compressor or welder that takes 240 volt, I'm guessing you wouldn't be able to run either in your garage. That's where I'm planning to get to eventually myself.

Your problem isn't that you need more than a 15 amp or 20 amp circuit, it's the underground feeder cable for the garage is probably underrated. My detatched garage is bad news. I don't know what I have underground, but I know inside the garage there is a receptacle serving the door openner that is fed by lamp cord, and there are additional outlets in the workshop part of it which operate off a swich that is fed by lamp cord. This lamp cord is probably 16 or 18 gage, which is undersized for 15 amp - and futher, it's multistrand cable which probably at its age has broken strands and as such isn't even equivalent to the 16 or 18 gage it started out at.

I'm sure you're planning to do this on permit, if you were undecided or weren't - I strongly recommend it. You'll probably find that your garage outlets will be required to be GFCI by current code, you'll want an underground rated cable to go from your main panel to your garage subpanel, I think you're supposed to put in a ground rod for the garage sub and I wouldn't be suprised if the current wiring doesn't have one. You'll need to dig a trench for this, your sequence of events will be that you get the permit before you start any of this, you'll dig the trench and your permit will require that the trench be inspected before cable is buried as part of rough inspection, so be prepared also to keep anyone from falling in. You'll get rough inspection after you run your wires and junction boxes but before you cover these or install outlets. Then you finish everything and get final inspection.

Let's talk about gas piping. If you're considering at any future point adding heat, it may be convenient to get the gas piping put in the trench while it's open, if not then keep carefully measured diagrams so you can keep from hitting your wires when you dig another trench for pipes (by you, this may mean the contractor you hire to do the job)

Also, if there's already a heater installed, even if it's a fire hazard like I have, you'll want to know where the gas pipe is run so you don't burst it while digging. Your utility may be able to help you with this.

The cable... Look for UF cable, you can bury it directly without a need to put it in conduit - just use rigid conduit where it goes up. And when you get a permit ask the inspector anywhere you're unsure about stuff like that.

First time I did an underground cable run, I was wiring an outlet for a pool pump. I dug a trench, ran UF cable in PVC conduit and called for the rough inspection. Inspector okay'd it, but said I didn't need the conduit with UF. Wish I'd known, it's no easy task pulling 100' of cable through conduit even just doing 10' at a time.
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Old 01-08-2011, 12:19 AM   #10
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New guy needs to rewire his garage!


Quote:
Originally Posted by jbfan View Post
You do not add the value of the breakers to figure load.
You may well have over 300-400 amps if you add all the breakers.
How big is the main breaker in the panel?
This!
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Old 01-08-2011, 12:29 AM   #11
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Quote:
Originally Posted by jbfan View Post
You do not add the value of the breakers ti figure load.
You may well have over 300-400 amps if you add all the breakers.
How big is the main breaker in the panel?
I'll add my 2 cents on this too.. My current project, I'm adding a subpanel which I'll feed off the main panel with a 60 amp breaker. The main panel has about 20 breakers most of which are 15 amp. These add up to over 300 amps. It's a 100 amp panel.

What I'm a little concerned by is you seem to be saying you have a 300 amp panel, yet your whole house has issues if one bulb burns out???

My house was bulit in 1917, so it's younger than yours. It seems clear to me it was originally wired with knob and tube and it was probably all on one circuit originally.

Extrapolating my experience to yours, you probably have most of your house on one circuit that is the old knob and tube wiring. And I'd guess the garage might be fed off this too, and maybe certain other parts of the house were modernized with the upgraded panel. If this is the case, you may need to modernize the rest of the house even more than the garage.
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Old 01-08-2011, 02:43 PM   #12
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Well, if I don't add the breakers, how do I figure out what the load really is? The Main is 100A (supposedly). We have gas-fired appliances... ALL of them... Furnace, stove, water heater (2 when we get a new second one), except the electric dryer and also the central air. And, if the load is less than 100A, why is there 240+ amps worth of breakers in the box?

As I said before, when the garage goes out, the refrigerator and the TV in the living room also go out, nothing else, no lights, no loss of the washer or dryer or furnace or anything, just those.

The welder is a Chicago Electric that I only plan on either using or having long enough to fix my quarter panel and floorboards in the car as well as fixing a couple of broken lawnmower housings. The manual says 20A dedicated circuit MINIMUM. My Craftsman compressor draws 15A at start (3HP peak, according to the nameplate). There is one light and one outlet in the garage, both running off of both the garage power switch and the switch in the laundry room (basically a 3-way circuit which is apparently on a separate circuit from the rest of the laundry room because the porch light, laundry light, and exterior porch light (when working) will work when the garage doesn't).

I have a new idea. There is a dedicated outlet to the sump pump right next to the main panel. It's currently fed by a 15A breaker (and is listed as such on the outlet box cover. If I were to put that on it's own circuit (say 5A or whatever the sump requires as I haven't looked), rewire the garage to have a 20A circuit, and then put in a dedicated outdoor box for about 20-25A, would that work? I don't necessarily need to run the welder in the garage (car shell is in the driveway due to lack of space), but I do need to at least replace the underground cable to the garage. And the extra outlet outside would be good to have for outdoor power tools (weed eater, etc).

Yes, our house is a big mess, in more ways than one, and there really isn't much to do about it without tearing it down and starting over again. The main panel was replaced from a standard fuse panel in 1990 when the bathroom was renovated and the furnace and A/C were put in. We don't even have standard 6" thick interior walls on anything that was there when the house was built (seems that the dining room, front porch, back porch, and a couple other things were added on later)... The 2x4 studs are flat (wallboard attached on the wide side of the studs).

So, I guess another question would be as to why there seems to be so much wasted space in the main box when it seems that most everything is tied together? And is there a way to fix this? Or should I just give up and sell the welder to buy a gas setup?
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Old 01-08-2011, 02:50 PM   #13
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There are load calcs to use on the internet
I have 2 electric stoves = 2 complete kitchen setup
Electric dryer, water heater, pool, plus more
I have a 200a service & my load calc comes out around 140a
I have over 1000a of breakers between the main panel & 3 sub panels
There are min amount of circuits for different areas that must be run

Here's one load calc as an example
http://www.electricalknowledge.com/SFDLoadCalc.asp

Another:
http://www.zenfixit.com/load_calculations.shtml

Last edited by Scuba_Dave; 01-08-2011 at 03:30 PM.
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Old 01-08-2011, 03:17 PM   #14
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Probably a newb question, but how do I figure the "small appliance" circuits for the load rating calc? By room?

Also, the welder and compressor are both 115VAC fed. I'd love to have a 230V version of both, but that's $$$. And I think that the garage circuit is currently on a 20A breaker, but I'll have to confirm that when it pops again.
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Old 01-08-2011, 03:19 PM   #15
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(2) Small appliance circuits are required for the kitchen
I have 3 for the main kitchen, 2nd kitchen will also have 3

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