Need 240V 20AMPs in detached garage for 16 AMP Level 2 EV Charger (Chevy Spark) at my MIL's home.

1200 sqft House, has a 100 AMP Panel. Furnace, Dryer, and Stove are Gas. All circuits are 20AMPs, and currently unlabeled. Only 240v appliance is 20AMP for central AC.

Garage is currently fed by 20AMP circuit from the main panel via Underground conduit run (probably about 25-30feet). Feeding a couple lights, outlets and 2 car garage door opener. No heavy equipment in garage

Here's what I'm thinking:

  • On Main Panel: Upgrade 20AMP single to 30AMP double
  • Pull out existing Romex run, upgrade to 10/3 to feed garage Sub Panel
  • Garage Sub Panel: 1 20AMP double(Dedicated for EV), and 1 15AMP single(outlets, lights, etc)

Updated Information

  • The Main Panel has 12, 20 Amp Circuits @ 120(Including the garage I want to switch to a subpanel 30 @ 240). And 1, 20Amp @ 240 for Central AC.

  • Pipe feeding the garage is 1 inch.

  • If it can support running a 50AMP subpanel for futureproofing Im all for it, but if its a difference of upgrading the main panel or not, I'll take the less intrusive option.

  • I used the calculator located here Electrical load Calculator and it output Total VA Load = 12600 Total Amp Load = 53.

  • 2
    A minor point, but I'd recommend installing breakers and wiring to support at least 50 Amp @ 240VAC. Your planned EV charger is at the very low end of Level 2 and someone may wish to upgrade cars in the future. (By comparison, Tesla wall chargers use the equivalent of NEMA14-50) . Commented Aug 16, 2019 at 17:17
  • Is the total output capacity of your 100 Amp main panel , considering all existing breakers&house circuits, such that going to 30 Amp-240 will not exceed design? Commented Aug 16, 2019 at 17:19
  • What is the size of the conduit running to the garage?
    – prl
    Commented Aug 16, 2019 at 18:22
  • What size is the conduit? Also, think about running THHN or THHW.
    – JACK
    Commented Aug 16, 2019 at 18:26
  • 1
    Its a vastly easier pull than cable, especially if you use stranded wire. Commented Aug 16, 2019 at 23:50

1 Answer 1


1/2 inch conduit would accept 4 #10 thhn /thwn conductors for your original 30 Amp design.

3/4 inch would accept 4# 6 thhn / thwn for your 60 amp design, but not certain of your local code, where in some places, it will require 1".

What size panel is your MAIN panel ? If it's 100 amps, I believe you are beyond the design limits. If your MAIN panel is 200 amps, you'll be fine with a 100 Amp Sub panel, as long as your MAIN it isn't loaded up with other stuff ...

The OTHER thing you're going to have to do when you wire that new sub panel...

REMOVE the tie ( jumper ) between the ground and neutral in the sub, if it has one.. ...Unlike your MAIN has got, any sub panel must have that ground-to-neutral jumper removed.

This is for when you hook back up your existing 115v. circuits, if there is ever any faults to ground, they do not travel up the neutral that is in your sub panel. All ground faults will travel through the grounding conductor in your sub to your main.

  • It's 1in. Conduit feeding the garage. Main Panel is 100 Amps. It has 12, 20amp Breakers @ 120 + 1 20 Amp breaker @ 240 for AC. All Gas appliances, no dishwasher, disposal, or basement. 3 small bedrooms 1 bath. Commented Aug 17, 2019 at 0:25
  • Well you can get a Maximum of 4ea #4's thhn /thhn in a 1 inch PVC schedule 40..Which will put the circuit ampacity at 85 amps ACCORDING TO THE SIZE of the #4... Commented Aug 17, 2019 at 4:28
  • 1
    This forum's comment allowance design will presently NOT let me take over 5 minutes to do so,..Strange. Here's what you should do... Your 1 inch conduit will accept 3ea #4 thhn/ thwn ( black, white, red ) AND 1ea #6 thhn/ then ( green) Install those 4 conductors in your 1 inch conduit. Install a 60 amp breaker in your main panel to feed your new sub panel. Get a 60 amp sub panel with a minimum of 4 spaces. In your sub panel, get a 30-40 amp ( 30 amp = #10 / 40 amp = #8 )breaker to accept your minimum EV 240 volt charging design. The remaining spaces in your panel can accept the rest 120v. Commented Aug 17, 2019 at 14:13
  • Don't forget to remove any connections ( jumpers) that exist between the ground and the neutral busses in your new sub panel if any exist. The reason for this is that any possible ground faults in your sub panel's branch circuits will traverse through the grounding conductor and not neutral, (which is called the "grounded conductor") Commented Aug 17, 2019 at 14:18

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