Dedicated Circuit vs Oversized Circuit
September 22, 2017 8:52 PM   Subscribe

I just got a plug-in hybrid, but don't have any outdoor outlets at the house. The driveway runs adjacent to the side of the house that already has 240V service for a window A/C on a dedicated circuit. The charger is also supposed to be on a dedicated circuit. I'm thinking of pulling 8ga through that conduit and installing a 40A breaker (Car:16A, A/C:8A). Does having dedicated circuit matter as much if you comfortably oversize the circuit capacity?

Yes, I know I should get an electrician. But if this is doable, it's literally pulling wire through a conduit that ONLY has this one circuit. Thanks, super-restrictive Chicago electrical code
posted by hwyengr to Home & Garden (6 answers total) 1 user marked this as a favorite
Best answer: Is the conduit large enough to pull that size of wire through? It's rare for a raceway to be so over sized.

Putting a 15 or 20A receptacle (for the A/C) on a 40A breaker is generally illegal for good reasons.

The correct way of doing this would be to use your large feeder to service a small breaker panel (Say the SqD QO6L100RB which is outdoor rated) and then drop the two circuits off of appropriately sized breakers in that sub.

Or you can use a cheaper indoor panel if you have access on the inside.
posted by Mitheral at 9:20 PM on September 22, 2017 [3 favorites]

Yes, having a dedicated circuit for that air conditioner matters more than comfortably oversizing the circuit.

Consider your whole-house service limit, voltage drop at maximum load, conduit fill limit,
Article 430 in the NEC for motors for continuous duty application, and locked rotor current for electric motors.

Get an electrician, said a stranger on the internet.
posted by the Real Dan at 9:21 PM on September 22, 2017 [1 favorite]

This ends up violating the UL ratings of the EVSE no matter how you slice it, so it's a bad idea.
posted by doomsey at 9:22 PM on September 22, 2017 [2 favorites]

Best answer: What I'd do is pull the heavy wire through the conduit (8ga copper should be good to 50A), with a big 50A breaker feeding it in the main panelbox, and then put a small subpanel box with properly rated breakers in it at the end of the heavy-gauge cable. From there you can connect to your appliances using whatever gauge (and breakers) they specify.

This is what I have at my house for running outdoor appliances: there's a heavy-gauge feeder running out to a subpanel box on the outside of the house, and it has a couple of breakers in it sized for the actual appliances/circuits outside. If I need more power outside, I have access to the full 50A in theory, but the properly rated breakers ensure I don't have to worry about shorting across a little 15A extension cord out there and running 50A through it, which would be... fire-y.
posted by Kadin2048 at 9:28 PM on September 22, 2017

This is an issue of building codes, not electrical or mechanical possibility. Even if it was perfectly safe, you probably don't want to do something that would, say, let your home insurance get out of paying for a house fire.
posted by ryanrs at 2:20 AM on September 23, 2017 [4 favorites]

I just had an electrician put in a 240V 50A circuit out to the garage for a car charger, with a 6-50 receptacle (this is the kind used for electric stoves, and as an outlet designed for 50A applications it makes sense for a 50A feed and is no suprise to future owners like a lower rated receptacle on a 50A circuit wuld be) even though the charger I bought from will only use about 30A -- but it comes with the 6-50 plug. This is oversized but I wanted future flexibility for the garage, i.e. add a subpanel in the future, plug in a welder instead of the charger, etc. (Clippercreek also sells a dual charger for two cars rated at 40A. It will divide the power between two cars plugged in, or send all power to one car if only one is plugged in, which is pretty cool, but a bit pricey of course.)

So to me having it be both dedicated and oversized is just futureproofing.
posted by thefool at 7:06 AM on September 23, 2017

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