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I want to add electric heat to my 750 square foot garage by way of an overhead 7.5 or 10 KW heater. Adding a sub panel would allow me other options for power for the following:

  • Heater 220V at either 7.5KW 30Amps or 10KW at 41 Amps
  • Table saw 110V 15 Amps
  • Small Freezer 110V 5 Amps
  • Central Vac 110V 13 Amps
  • Perhaps 8 110V 20 Amp Receptacles I have a 200 Amp Service Entrance Panel (Siemens Model #G4040B1200) Service Entrance panel with 3 spaces left and the proposed garage sub-panel would require 65' of wire to connect to the main.

What breaker do I need in the main? What specific wires to go the 65' to the sub panel? Easy to put these in plastic conduit, so is that okay to do? What else am I missing here?

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  • What make and model is your service entrance panel? Also, how many square feet is the garage? – ThreePhaseEel Dec 13 '19 at 0:08
  • I have a Siemens Model #G4040B1200 Service Entrance panel and the garage is 750 square feet, 8250 Cubic Feet. – Dave Dec 13 '19 at 0:33
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You have to provision 50A for the 10kw heater. We can play some provisioning games on the saw, dust collector, freezer, lights and receps, and allocate them, say, 25-30A. Which puts us at 75-80A.

You could get away with #4Cu / #2Al wire. However, there's very little cost difference just go go all the way to 100A, and that's commonly done.

So, provision 1 AWG aluminum THWN-2 wires with a ground wire of 8 AWG copper bare or 6 AWG Al insulated. There's no reason for a wire size bump at 80'.

This setup would also have the reserve to stick a Tesla charger in there, though Tesla+heater+shop tools simultaneously would be a no-go. Presumably you don't want sawdust on your car :)

1-1/4" conduit should get it done.

Our standard advice is to use a much larger subpanel than you'd ever imagine needing. That's because "buying a subpanel with more spaces" will set you back a couple of Frappucinos, but "having to change your subpanel because you thought too small" is a solid day of work. If the subpanel is attached, even with a breezeway, you don't need a "main breaker" in the subpanel. If detached, you need some kind of shutoff switch, and using a main-breaker panel will only add $20-30 to the cost of the panel. There's no need for the subpanel size to match the feed breaker (100A) size, so feel free to get a 150A-bussed panel with a 125A main breaker if it's on sale :) Also consider "value pack" panels that come with some breakers.

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  • BTW: for panel to panel work, you can use the 75°C column in the ampacity tables, because distribution lugs, as a rule, are slash rated 60°C/75°C, which causes 110.14(D) point 3 to come into play, thus overriding the default rule in 110.14(D) point 1 – ThreePhaseEel Dec 13 '19 at 0:08
  • @ThreePhaseEel Ah you're right, I'm too often assuming NM cable... yes, conduit also allows 75C... – Harper - Reinstate Monica Dec 13 '19 at 15:27
  • This is helpful information. Thank you. Next question. Aluminum vs Copper: I want to make it simpler without having to make the special provision for aluminum, and I feel maybe safer with copper. Your suggestion is to use #1 AL, but I think I'd like to run copper and am willing to accept that added cost. What would 3 wires of #3 Copper and a smaller copper ground give me as far as capacity in the sub-panel? One other question. Tell me about large amp breakers that are AFCI. I've had issues with 15 to 20 Amp AFCI ones. – Dave Dec 13 '19 at 18:18
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    Problems with AL branch circuit wire was with an alloy that was not suited for contact with steel. Alloys now used in feeder conductor perform excellently, and are actually easier to work with. You may need increase conduit one size. Off the top of my head I would probably go with 90A protection, a derated neutral for your 240v loads, 2/2/6/6 in 1" or 1.25" conduit. I would need to verify details. – NoSparksPlease Dec 13 '19 at 22:19
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    @Dave, that's a sad old wive's tale about aluminum. In the 1970s, copper-lug receptacles and switches were pushed to market that weren't adequately tested for aluminum wire, which caused problems. For large feeder like this, aluminum has always been the right way to go. Not least, the lugs are aluminum (because aluminum lugs work fine on Al and Cu both). By insisting on using copper wire, you're creating the very dissimilar metal problem you seek to avoid! Not that it's a problem. Feeder was meant to be aluminum. – Harper - Reinstate Monica Dec 14 '19 at 1:57
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If the overhead heater is meant to warm up working people (and not meant to dry car paint or help to grow plants), an alternate option is the use of normal column-like floor based quartz heaters, combined with normal blowing heaters where needed.

Advantages:

  1. Less power consumation, but higher comfort. 3 quartz units can be placed around the worker thus sending infrared from 3 sides. A unit typically has 3 levels at 400W each, summing up to max 3600W. There is no way to feel cold in the center of these directed rays from 3 sides, since the surrounding air is also warmed up immediately resulting in an invisible warm shelter.

  2. More flexibility. The 1200W heaters can be placed anywhere with direct line of sight to the worker. Additional blowing heaters can be placed underneath and inside cars depending on the work to do.

  3. Less costs. These heaters are produced in very high numbers (price ca. 15-30 Euro), in contrast to overhead heaters. If 1 unit fails, there is plenty redundancy. Lower power saves operational costs.

Disadvantages:

  1. Safety concern. Fluids like gasoline must not come into contact with the heating elements. Fire extinguishers may improve safety.
  2. Additional loose power cables on the floor are an obstacle in working places.
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