Receptacle Roundup Ic: Co-plugging

Boy, we just can’t get away from 3-prong 120V, can we?  Of course not, it’s the standard consumers’ power interface.  Woe be he who underestimates consumer power.


So if the standard electrical receptacle isn’t doing it for you, how about two of them, or more?  It’s an obvious enough question, and no, neither you nor I were the first to think of that:

1.  Double Your Current

If what you need is just twice the current, that’s easy… kinda.  Plug into two outlets, if your vehicle supports the dual 120V connections.  The uneasiness comes in amperage limitations.  No receptacle supplies limitless current; the circuit breaker will stop you from pulling too much, hopefully before the wires in the wall melt or ignite.

In most residential construction, the NEMA 5-15Rs are ganged into circuits, each with a circuit breaker of 15 amperes; the copper in the walls is 12-14 gage.  Thus, neither individual outlets nor their circuit as a whole can exceed 15 amps, individually or together.  (In practice, even this is for load surges only; sustained current should be downrated 20% from this, or no more than 12 amps.)  Some newer, upscale houses may have some 20A circuits, so two outlets can each draw 10A (…peak, of course; 8A sustained).  A well-constructed duplex fitting might take both 10A loads without overheating, but to be sure I’d split that into two receptacles down the wall from each other.  However, some older homes may only have 12A breakers and wiring.  In all of these cases, it’s still your responsibility to make sure no other loads on the circuit sum up to 15A or 20A or whatever, and trip the breaker.  For this reason, homebuilders give many of the larger loads (A/C, dryers, stoves) outlets on dedicated circuits- one receptacle, no confusion.

Electrically, it’s probably easier to find two different circuits of your house.  If you plug through two completely different circuits, their breakers are less likely to trip on your vehicle.  You might then max out at 2x12A or even 2x16A.  Of course, the outlets are probably farther apart, and need longer cords.  And now you have more outlets to keep from over-drawing, unless you’ve plugged into a dedicated circuit.  My house has good circuits, electrically separate but physically close; don’t know if I’m just lucky that way.

Outside your own home… good luck!  You generally won’t be able to access strange breaker panels, to check their circuit ratings; asking a janitor or manager or resident or such will probably be met with a blank stare.  You might have luck with workplace charging, since you only need to check the rating once, to charge day in and day out.  Commercial circuits are also more likely to be 20 or 30 amps or more.  Still, you’d need to make sure other users don’t plug in anything big while you’re charging, and trip the breaker.  Again, good luck.

Just like NEMA 5-20, what’s physically possible with two 5-15 outlets isn’t confident and quick, unless you have some rights to the circuit.  So… what is “quick”?

2.  Double Your Voltage

In many cases, you also want more voltage than 120V.  Electrons want to flow “downhill,” from high voltage to low.  You want to charge a battery from a source greater than the pack’s specified potential.  Preferably a good amount more- you also need to overcome miscellaneous losses around the system.  All wires have some voltage drop; chargers add switches and other components.  If necessary you can step up your voltage using a properly-designed charger, but why would you?  This adds even more components and losses, and more energy isn’t making it into the battery.

Okay, but why would a battery and other components have high voltages in the first place?  First off, high voltages reduce losses in general.  For the same work done, wires lose less energy to heat when at high voltages than at low.  In motors, high voltage also broadens the powerband.  Broad powerbands reduce or eliminate transmission gearing, and their energy losses.

My Zero motorcycle has eliminated multiple gears at an operating potential of 67V.  This is because motorcycles are light, and didn’t need widely-spaced gear ratios in the first place.  Cars, on the other hand, need low gearing to accelerate at first.  All that steel just won’t get up by itself.  That low gear is then unlikely to work on the highway… unless the internal-combustion engine has a shifter, or the electric motor has very high voltage.  Cars, then, have electric drives of a few hundred volts; 67V or even 120V would be undriveable through one gear ratio.  (And even the 2013 Zeros have risen somewhat from 67V.)  Since automakers would like to eliminate transmissions where possible, cars like to charge at above 120V.

How do we do this?  Two 120V receptacles can be ganged together if you’re lucky.  Households generally receive power from the utility as 240V-nominal alternating current.  At the breaker panel, the house is split into two “buses” by defining ground to be right in the middle of 240.  One bus divides the potential into +120V, relative to the middle.  The other bus is then -120V from ground, or 240V from the first bus.

A thingie can recombine both buses into 240V.  A “quick 240” or “quick 220” looks like a giant Y.  One plug leads to an outlet on the positive bus, the other to the negative bus.  The junction of the Y contains some circuitry to handle the merge.  One thing it can’t handle sometimes is GFI.  Because you are now on multiple circuits of your house, on multiple buses, there’s a possibility of slight mismatches.  The two circuits just can’t be expected to be perfect mirror images of each other.  If the outlets you use have GFI protection, mismatches may be read as short circuiting, and an outlet will trip.  Fortunately, these 5-15 receptacles are cheap.  Once you find two suitable outlets (not in the kitchen or bathroom), you can replace their GFI sockets with plain 5-15s at low cost.

It’s a bit of a kluge lying around the floor, sure.  But now you have a third leg, yielding 240V and substantial current.  Hmmm, what would we plug into at 240V…


11 thoughts on “Receptacle Roundup Ic: Co-plugging

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