Running line from main to subpannel in a garage

[Yuri Sadykov]

I am in preparation to move in new house with two car garage. I am planning to put 100 amp subpanel there. I will hire an electrician to do the job, but curious if the other end of the line is connected somewhere in main panel? Main panel is 200 AMP and if subpanel will consume in 100 amp range, will it drain main panel too much to feed rest of the house at the same time? What is your thinking, experience?

[Matt Krusen]

I did the exact same thing for my shop. 100 amp subpanel in the shop fed from the 200 amp main in the house. As a one man shop, I have experienced no problems at all. Maybe there'd be an issue if I was running my table saw and DC while my fiancé had a roast in the oven and was drying clothes and watching TV? Even then, I don't think there'd be an issue.

[Andy Fox]

curious if the other end of the line is connected somewhere in main panel?

Yes, the electrician will install a double-wide, 100 amp, 240V breaker to feed the subpanel. It will be like what is used for an electric range or dryer.

Main panel is 200 AMP and if subpanel will consume in 100 amp range, will it drain main panel too much to feed rest of the house at the same time?

The subpanel won't consume anything. The tools, DC, and lights will only consume what they normally consume, except that this will be coming through the subpanel first. A 100 amp panel and breakers are used to ensure they don't become the limiting factor, they're common, and to leave plenty of amps and breaker space in the subpanel for upgrades.

(I'm not an electrician. This is just my understanding as a fellow hobbyist woodworker.)

[Von Bickley]

That will be perfect. That is what I would do and I am a retired electrician.....

[Yuri Sadykov]

The subpanel won't consume anything. The tools, DC, and lights will only consume what they normally consume, except that this will be coming through the subpanel first. A 100 amp panel and breakers are used to ensure they don't become the limiting factor, they're common, and to leave plenty of amps and breaker space in the subpanel for upgrades.

I understand that the panel itself does not consume anything, but I am thinking about the situation when I run table saw, dust collector, have lights on and my (future) mini heatpump heating the shop.

[Dan Hintz]

I understand that the panel itself does not consume anything, but I am thinking about the situation when I run table saw, dust collector, have lights on and my (future) mini heatpump heating the shop.

It's no different than if you tried to do all of those things and power them directly off of the main panel. The main panel limits you to 200A, the subpanel limits you to 100A. Even if you were pulling the (subpanel) max of 100A, does it matter if it came directly from the main panel or first through a secondary panel? Think of the subpanel a matter of convenience... you don't have to remove the wires going to a socket upstairs just so you can connect a socket downstairs.

[Chris Padilla]

Add up the total current draw you think you will have at any given time. If you're under 100 A, then you're fine. Odds are excellent you are WAY under 100 A. I doubt you'd go over 50 A with a table saw, dust collector, lights, and a heat pump all working at the same time.

[Steve Meliza]

100A is about 24,000 watts. If you are in the garage using 24kW and someone else is in the house also using 24kW then you might trip the main breaker. In that case tell the people in the house to turn off the 10 microwave ovens and 40 grow lamps till you're done running two 5HP table saws, two 5HP dust collectors, lights, and heat pump simultaneously. I'm joking, but that is pretty much what you could power simultaneously.

I can't fit or afford that level of equipment in my garage so I get along just fine with a 60A sub-panel to power my 1.5-2HP machines even if the air compressor happens to cycle on while I'm making a cut.

[Todd Burch]

As a side story to this, I just had a house rewired - completely. When I was doing the demo, I was BITTEN by something that I suppose was improper wiring. (BITTEN = almost pooped my pants and ruined a $20 pair of wire cutters by melting the cutters cutting 240V hot wires).

The house had a main panel and then a sub panel off of it. I turned off all breakers in the main, including the main. However, I did not check the sub panel. The sub panel was adjacent to the main panel. All the sheet rock was off, so I could see the path of the wires coming into the sub panel and out to the rest of the house.

After "shutting off all electric", as I was cutting the old dryer cord, I got the SHOCK!! I didn't have voltage go through me, but it sure was a wake up call.

What had happened was that even with the house main off, the sub panel was still hot. The sub panel was not on a breaker off the main panel. Scared the heck out of me. Shame on me for not testing the wires with a meter, but who would have thunk? I sure didn't.

[Doug Ladendorf]

You should be fine with that set up. I have a 60Amp sub-panel and my concern would be if I got a 240v electric heater. If that was on while I had the cyclone on, 3-5HP machine on, and the compressor kicks on. That would put me over 60 Amps but even that should be under 100 Amps. Now if you had more than 100 Amps running in the house between heating and appliances you could be closer but it would take an awful lot to get there. Talk with your electrician but it sounds like you will be fine.

[Charles R Johnson]

Todd, You definitely should have been ok but as you learned it's always good to check. Hopefully during your house rewire the sub panel got reworked to be fed from the main panel. There should be one main disconnect for all electrical coming off the meter.

[John Dougherty]

Rewiring a house can be a real puzzle palace experience. My own place was built in 1929 and had the original knob-and-tube insulators and some very scary, crumbling old sheathing on the wiring. We never were quite certain just how the place was really wired and there are still some anomalous hot leads in the old wiring we couldn't track to a known, hot connection. But, those anomalies are still off if the main breaker is thrown. In your case, if the main panel was off, the subpanel should have been off as well. It is after all a "sub"-panel. You should probably look very, very closely at your meter area and look for an additional lead going somewhere you don't expect.

[Ole Anderson]

As others have stated, coming off the main panel shouldn't be a problem as long as it still has the capacity. An alternative is to go back to the meter and run a separate line to the new panel. installing a disconnect. I have done that twice when wiring up a 50 amp spa circuit where the main panel was not easily accessible or was already tapped out. Here you really want an electrician to do it as pulling a meter and working next to live incoming wires isn't for the faint of heart.

[Roy Turbett]

The only thing I'd add is when in doubt, go bigger on the feed wire. The last thing you want to do is run new wire because you went too small the first time around. I ran three strands of #2 copper to my shop and am glad I did. I have a 150 amp main in the house and a 100 amp sub in the shop and have had more than 10 guys working in the shop at one time. We've dimmed the lights in the house a few times but have always had enough juice in the shop. My shop is about 250 feet from the house which is another factor when considering what size wire to buy.

[Larry Whitlow]

I have what I hope is not too dumb of an add-on question. What is the advantage of coming off the main as opposed to coming off an existing sub panel? I'm assuming it would have to do with the size of the circuit (100 amp) feeding the new sub in the garage.