Receptacle Roundup Ib: NEMA 5-20

‘Wait,’ you might be thinking.  ‘Isn’t this the same as the last one?’  Look closely-


…the neutral (left) slots will accept horizontal blades, so this is NEMA 5-20R.  The slot is still T-shaped, so it will still take a regular 5-15 plug; the combo is referred to as 5-20 by default.  Meanwhile, a 5-20 plug (below) won’t go in a 5-15R and fry it.  Unlike the 5-15’s maximum current of 15 amperes, 5-20 will give (surprise!) 20A.  (At least, it should- read below.)  The voltage is still unchanged at 120V nominal; like 5-15, actual volts can be 110-125V while still being in spec.  Basically, 5-20 is an industrial-strength 5-15, and is then found in factories, shops, and some offices.  You probably walk by a lot of ’em and don’t even realize.  This is its greatest advantage and, it turns out, its greatest weakness too.


At 20 amps, one 5-20 would give 2.4 kilowatts (20A x 120V).  But most electrical engineers choose to size their continuous loads to 16A (1.9 kW), to be safe.  Demands may spike, and supplies may droop, so 16A gives a margin of 25%; only brief loads should even approach 20A.  Needless to say, the circuit breaker and the wiring in the wall should be rated to at least 20A, and that’s for a single receptacle.  Duplex outlets or circuits with multiple boxes installed should of course be even higher.

That would still beat a standard 5-15 socket, clearly.  Either 2.4 kW beats 1.8, or with the 25% knockdown factor, 1.9 kW still beats 1.4.  In practice, this shortens overnight charging by a few hours.  This could mean the difference between not completely charging, and easily charging every time.  Or between not completely charging every time, and charging plus firing up the heater or A/C ahead of time for a comfy commute.

Let’s put some hard numbers on that.  Let’s say you plug in before you go to bed at 10, then pull away at seven.  Nine hours of charge at 16 amps gives 17.3 kWh (that’s 120V x 16A x 9); if you’re confident in your electrical system, maxing out at 20A gives 21.6 kWh (120V x 20A x 9).  An electric car may yield 3 to 5 miles per kWh, depending on model, whether you’re a conscientious driver or a leadfoot, etc.  So your sleepy-time at a 5-20 outlet would give you anywhere from 52 to 107 miles of weekday driving- more than enough for the majority of the population, and noticeably more than a 5-15 wall plug.  Of course, this neglects max AC/heat, leadfootedness, safety margin, etc.  But it also neglects evening charging as soon as you get home some days, and any charging during the day (such as within your job’s 8-hour stretch, maybe 9 with lunch).

In some regions, 5-15 charging may be barely acceptable, as I described in the 5-15 Roundup… but the extra boost at 5-20 may keep you within your utility’s time slots for off-peak electric rates.  In this case, you’d set a timer to start the charge at just after midnight or whenever, and then a stop time whenever the utility had set.  All the name-brand electric cars available in North America come with timers (though my Zero motorcycle didn’t); many also have a smartphone app, which also handles their pre-heat and pre-cool times.

All this, and the receptacle’s common and cheap- what’s not to love about NEMA 5-20?  Notice I keep saying “should,” “would,” “may,” “if.”  Just because you see a 5-20 doesn’t mean you’ll actually get those extra amps.  Many architects, builders, and electricians simply go with 5-20 receptacles out of habit; that doesn’t mean the installed circuit breaker and the copper in the wall are actually more capable than on 5-15.  If you then try to draw more than 15 amps, you’ll trip the breaker, and everyone’s pissed.

Unless you can check the circuit breaker (good luck with that), you might as well assume the outlet is effectively 5-15, for safety.  Even after you check, you’d need to make sure no one else plugs into that circuit, if the circuit has more than that one socket.  You certainly won’t be able to look in the walls and check the wire gage in someone else’s building.

And that’s why neither I nor the EV manufacturers have much confidence that NEMA 5-20 charging will catch on.  Its greatest strength- lots and lots of outlets in the wild already- actually turns out to be somewhat useless.  I see one possibility.  Supporters of EV charging might inspect their circuits, mount a nameplate that says “Yes, REALLY 20 amps” or such, and also unhook or blank off any other outlets on the circuit.  Like I said, don’t have much confidence in that.  Take an even closer look at the installation above:


The horizontal slots (the very thing that make this a 5-20) are full of dust.  No one’s plugged in at 20A in years, if ever.  This might as well be a 5-15.

NEMA 5-20 Plus

-Receptacle is backward-compatible with our 5-15 standard

-Already widespread in industry, dirt cheap, and thus might continue spreading

-Can charge a noticeable amount faster than 5-15 (“Level 1”)


-Little confidence in existing receptacles thus means no confidence

-Little plug-side support from EV or accessory manufacturers

-Still just 120 volts- fine for motorcycles & scooters, but skimpy for cars sold today


-No more (or less) safe than 5-15; may or may not need to add GFI and weather enclosure

NEMA 5-20 Ground

Good to install in your own garage (no extra weatherproofing!), though you’d have to wire a new cord with a 5-20P on its end.  Not so good for existing, public charging.  Some potential for new public installs if done right.


8 thoughts on “Receptacle Roundup Ib: NEMA 5-20

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