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Thank you in advance for your advice!

My project is to install a 220v-240v NEMA 14-50 outlet at my house’ s attached garage, which is nearly 65 ft/one way, from the house’ s main electrical panel / newer 4 wires 200amp installation, Grounding is done via 2 ground rods+to the copper main water line. The panel’ s current highest load is a 2-pole 40amp / HVAC, the house is under 1900 sq. ft.

I had access to left-over wires; THWN-2 THHN AWG #8 & AWG #6, so From the main electrical panel thru the attic, I utilized the left over AWG #8 (2hot+1Neutral) and AWG #6 (Ground), via EMT & flex conduit secured to the rafters / 4 ft intervals.

However, rather than sticking to the original plan and finishing with a NEMA 14-50 outlet at the attached garage to conclude the project by installing a 2-pole 40amp circuit breaker at the main electrical panel, I am considering to utilize the pulled wires along with a freebie panel, Cutler Hammer BR816L125RP rated at 125amp, and turn it into a sub panel at the attached garage’s area, and then install a NEMA 14-50 outlet / fed with a 2-pole 40amp circuit breaker at the sub panel. Given that the freebie panel is equipped with only one Bar, I will isolate Neutral from Ground by installing a Ground Bar and connecting ground wire to it / AWG #6.

The reason for the subpanel installation is to maximize the use of the AWG #6 & #8 wires that I have greatly appreciated pulling thru the flex conduit and account for possible future modification, as well as to install couple service outlets in the garage area near the planned subpanel’ s area without having to pull wires thru the finished garage walls from other service outlets.

Given the above information, including the AWG #6 & #8 wires’ length; What is the maximum breaker size that I can utilize at the main electrical panel to feed the garage’ s subpanel, i.e can I install a 50amp or 60amp 2-pole , or even higher rated circuit breaker? While still installing NEMA 14-50 outlet / fed with a 2-pole 40amp circuit breaker at the sub panel

Appreciate your expertise and honest opinion!

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  • Is your new panel a CH or a BR, and how many spaces does it have?
    – ThreePhaseEel
    Commented Jul 14, 2019 at 23:15
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    Are you really using 8AWG for hots and neutral and larger 6AWG for ground? Generally, the ground wire would be same size or slightly smaller since it should only be carrying current during a fault. A real sparky can tell you how much smaller it can go.
    – DoxyLover
    Commented Jul 15, 2019 at 0:19
  • @ThreePhaseEel Is that a degree-C issue?
    – Harper - Reinstate Monica
    Commented Jul 15, 2019 at 1:11
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    Thank you for the feed back, Proposed panel is Cutler Hammer BR816L125RP , Type 3r, rated at 125amp. If wiring is acceptable, I can purchase a new sub-panel, or simply stick to the original plan and only install NEMA 14-50, however, if use of #6 is a real safety and performance issue, then do you recommend that I purchase #8/Ground and pull it thru to replace #6/Ground ?
    – H1991
    Commented Jul 15, 2019 at 19:09
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    @H1991, smaller wire gauge means bigger wire. It's the opposite of what you might expect. So, your #6 ground is bigger than your #8 hot conductors, which is fine, if a bit wasteful of material.
    – Nate S.
    Commented Jul 15, 2019 at 20:42

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Breakers protect WIRE and #8 conductors allow you to use a 40A breaker in the Main panel to feed the sub-panel. That's your only limiting factor here. It makes no difference what the PANEL is rated for so long as it is more than adequate for a 40A circuit, which is is. So get off of that side track.

In the sub panel, you can put whatever else you want in there that is 40A or less. But if your NEMA 14-50 outlet has something plugged into it that is drawing 40A, and you use something else from that sub-panel that pulls any more, the breaker in the Main panel will trip to protect those conductors, so be aware of what you use in that garage.

The tricky part is that a 14-50 outlet is good for 50A, not 40 A, so if you needed that receptacle (i.e. a Tesla charger), ostensibly it means it likely could draw more than 40A which would be a problem for your circuit. If you really needed a 50A circuit, you needed #6 conductors all around.

Edit: Sorry, I was thinking building wire like NM, which is used from the 60C column, you said you have THHN in conduit, so it is good for 50A, not 40A. So you can use a 50A breaker in the Main panel.

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