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Question DetailsAsked on 3/4/2018

Running a line underground for a 240 volt outlet 200 feet.

I need to run a line underground 200 feet from the breaker box to install a 240 volt 6-20 outlet to charge my Chevy Volt. I only need 16 amps. What kind of wire do I need?

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1 Answer


I assume that 16A is the maximum charging amperage seen by the system, not the battery draw (charger demand will be more than battery draw because of losses and amperage limiting control power in the charger) - which means you need a 20A rated circuit minimum and maybe more.

Wire and breaker size depends on the peak and maximum sustained loads, whether the wire is copper or aluminum, whether open air overhead or, in your case, if in conduit and conduit size or if direct burial, and what type wire and insulation it has - that latter factor alone can make for a 25% or more difference in rated capacity. For instance some heavily protected direct burial cables can have a significantly different rated amperage than say a THWN in a buried conduit. BTW - for that distance run, I would personally be looking at burying a suitable size PVC conduit so the wire can be pulled out in the future if needed - also likely to be quite a bit longer life and a lot cheaper than direct burial rated cable. And of course make sure burial depth meets code requirements - probably minimum 24" cover required - 18" allowed in certain circumstances for conduit.

Looks like, assuming 20A maximum draw and 16A maximum sustained draw, though Chevy Volt website warns some non-GM chargers may pull more than that early in the charge cycle, and especially much more - one reference said up to 30A - if the car is remote started while plugged in, looks like you are going to need an absolute minimum #8 if copper or #6 if aluminum wire for a 200' run - at a bare minimum, and that would likely be only for an overhead run.

However, that does not provide significant overload capacity, does NOT provide capacity for remote starting, assumes car is at normal 60 degree design temp (amperage will possibly be higher if real cold depending on whether charger is "built" for a fixed amperage or actively compensates for variable temperature effects, like in unheated garage), nor does it account for any required direct burial or conduit-run derating, and assumes normal 3% energy loss in the wire run (which does not include any losses in the charger or connections nor in the lines to the breaker box from meter or in the breaker box itself - alone commonly 1% range) is acceptable. With an electric car charger you might well prefer to have well below 3% line losses by going with a larger wire size to minimize the power cost you will be throwing away as wire heating during charging.

Also - for long-term sustained loads like this, while not rigidly required by code (as far as I know) there is a guideline to not exceed 70% of rated breaker/wire rating rather than 80% - which also reduces the change of accidental breaker trips. So that would make your circuit design amperage at least 23A rather than 20A.

Also, for sustained load applications like this, there is another guideline - that the wiring should be rated for at least what the breaker is (not be lower). This is sort of backwards from normal design criteria, where the breaker is usually rated at (or closest higher available size) the wiring capacity. Making sure the wiring is rated to handle everything the breaker can take prevents wiring overheating in the event of a circuit overload (not an uncommon thing with car chargers like if charger or battery shorts out). So - in this case the breaker might be 25A (though not all companies make 25A breakers for all boxes) or 30A maybe if overloads and derating is taken into account or 25A is not an available breaker size for your panel. Perhaps even higher if car starting is provided for, though that would be (presumably) a short-term load so not subject to the 70% rating guideline.

But by good practice the wiring and circuit components should be rated for what the breaker capacity is so if the breaker is overloaded to capacity the wiring should be able to safely handle it OK till the breaker trips - especially since this is likely to be predominately a nighttime load, while you are asleep. Also, remember, breaker actually trips at something higher than the rated amperage - typically 15-25% or so for long-term sustainted load, as high as several times rated load for a short-term load.

Might also be good time to think about putting in an overload/power loss alarm on the circuit ? - and fire alarms over the car / charger repeating to the house ?. And if car is 200' from house, you might want motion-sensor activated lights to cover that interval, automatic lights in the garage - and don't forget garage door opener if not already wired for those. For possibly 25A or 30A circuit design capacity, that might well (depending on sustained load derating and conduit/burial derating and line loss provisions and such) put you into more of a #4 range wire.

So - I am guessing after considering all those, you will actually need wires one or two sizes larger than the normal "rated capacity" of 16A load - like maybe into #4 wire size range orpossibly (especially if aluminum) maybe even #2 - which may also mandate a junction box at the charger and electrical panel because those are larger wire sizes than normal breaker and connection lugs can handle (those are transmission, not distribution wire sizes).

Oh - one more complication - I am 99% certain that with the charger being in a different building not to mention 200 feet away, that the feed to the garage has to have a breaker in the garage too - either running a large enough wire from the existing panel or main breaker box as suitable and allowed to power the car and any existing panel in the garage, or a separate breaker in the garage. If the charger has a built-in breaker, this might or might not be required by local code or NEC, but a good ldea anyway for safety purposes in case the cr or charger starts overheating/battery frying (so I would not put it right by the charger).

Since you asked this question obviously you are not highly electrical handy, so you need an electrician to do this work - who can also size the wiring correctly, as well as providing for any heat or ancillary lighting or security system (since 200' from house) or such.

Answered 2 years ago by LCD

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