Dedicated Power Service for a small fleet of electric car chargers

  • Thu 27 June 2019
  • misc

I've previously written about choosing an electric car charger (EVSE) and thinking about it in terms of how long someone stays at a location and how much of a charge they can expect to get during their visit.

A friend is the proprietor of a retreat center and has owned a Leaf in the past (currently owns a plug-in hybrid) and was eyeing my Bolt a couple of months ago as we discussed electric car charging infrastructure for the retreat center.

One of the difficulties once you start talking about having more than one charger opportunistically installed is that it's really easy to run out of electrical service in the building where you're putting them if it's not a super big industrial place. One way to handle this is to make the amount of power they can deliver smaller (say, a 15 amp 120v receptacle instead of a 40 amp 240v circuit), but that's got a downside to it in that it takes the amount of power one can onboard way way down, to something like 5 miles per hour of charging from 25 miles per hour. Great for an event that lasts a week worth of camping, a lot less great for a weekend afternoon activity wherein a visitor might want to pick up a decent number of miles in a relatively short number of hours.

In the interest of maximum flexibility, my recommendation to him is that he pick up a dedicated 200 amp service from Delmarva Power (the local power company) and hang his car chargers off of that. That'll end up costing $10 a month or something like that as the minimum charge to have an electric meter in the system but how many hookups can that support and what should he install?

The first question is of course going to be "how many amps can we pull from that" and the obvious answer is "200" but that is not correct. As anyone who has felt the wires going to a toaster or waffle iron when company's over can tell you, running heavy loads over a wire will cause the wires to get warm. No magic here, even fat wires have resistance and voltage drop to them; it's Ohm's Law at work and the electrical losses manifest themselves as heat.

Cracking open the National Electrical Code, we discover that for "continuous loads" (defined as more than 3 hours), there must be a 20% derating of ampacity (that's a term of art that means amp capacity) for conductors, buses, OCPDs (over current protective devices - circuit breakers and fuses to you and me), etc. It's quoted in a bunch of places; Articles 210.19, 215.2, and 690.8 are a few of them. This is quoted variously as "the 80% rule" or "the 125% rule" (these are reciprocals of each other so it is a glass half-full/half-empty thing).

So, that 200 amp service is only good for 160 amps of actual load. Note that you don't have to derate twice - you can fit five 40a circuit breakers, each with a 32a expected load, not just four. 5 x 40 = 200; the 80% rule carries all the way through and you don't have to duplicate your work.

I know what you're thinking though... The circuit breakers in the big electrical panel in your house add up to a whole lot more than 200 amps once you factor in the heat pump, the range, the hot tub, all that stuff... so can't we squeeze in a sixth or seventh plug? After all, the vast majority of time either not all the chargers are plugged in or there's at least one car sitting there that's done charging and the only load that's on it is the green "ready" light...

Not so fast. You have to size for maximum likely simultaneous load. The term of art for this is "demand factor". You get away with a lower demand factor for, say, residential lighting, because people turn off the lights when they leave the room. Likewise, you're probably not running the heat full tilt, the hot tub full tilt, and the oven, simultaneously, for hours on end.

Not so for charging a car! The demand factor for electric car chargers is 100%, and it's not difficult to understand why if one takes a moment to consider several people showing up at the start of an event and plugging in their cars all at once.

Now that we've determined that one can get five 240v 40a circuits off a 200a service, what kind of charging equipment should he install?

Going back to the early days of modern electric cars, campgrounds were the only place where one could reliably find a place to plug in while on a road trip, so when Tesla started selling electric cars en masse they threw in an adaptor for their Mobile Connector (EVSE) that would plug into a NEMA 14-50 receptacle since that's the campground standard.

That choice has made the 14-50 the de-facto standard for electric car charging receptacles despite the fact that the neutral serves no useful purpose here - a 6-50 (which is the welding shop standard, but doesn't have a neutral pin) would have served as well. My mother-in-law had a house built last year and an electric car charging receptacle was a checkbox option and was delivered as a 14-50.

Fast forward to April 2019 and it looks as if Tesla is discontinuing shipping the adaptor with their cars since it's used less frequently now that public chargers are more plentiful. But they're just $35 on the Tesla Store so maybe keeping a spare on hand would be a neighborly thing to do.

My recommendation to him is that he install NEMA 14-50 receptacles and call it a day, for a couple of reasons. First, they are cheap and deliver a usable solution for a minimum amount of cost. An outdoor-rated receptacle in a box is $35 from Lowes. Second, they can be easily swapped out for a hardwired charger whether it be a J1772 style or an HWPC from Tesla (I hear Tesla is subsidizing their destination chargers, though being listed on the destination charging map could be a problem for a private retreat center). Signage indicating that the receptacles are breakered at 40 amps and rated for 32 amps constant ought to provide sufficient notice for someone with an older or oddball charger that they should manually throttle back.