Last Update: 2021-01-03T23:53:32.513Z

Electric car charging within electrical code and power outlet limits

When are electric car extension cords required? Is it safe to charge an electric car in the rain?

The electrical code isn't an arbitrary set of nanny-state rules. It was developed through decades of experience and application of electrical engineering principles. It's possible that most rules in the electrical code came from a specific electrocution or electric fire.

As we've said elsewhere in this book, we buy electric cars partly to help the environment. That's a great goal. Accidentally creating an electrical fire tends to erase that gain thanks to the negative publicity because oh my gosh an electric car caught on fire. It doesn't matter that gasoline cars catch on fire every day, with 250,000 per year in the US alone, sometimes causing death or serious injury. Unfortunately any electric car fire seems to cause outsized over-hyped news coverage.

Safely charging our electric cars means understanding enough of the electrical code and available power outlets. We don't have to become electricians ourselves. We need to know how to recognize the capability of a given power outlet, how to choose appropriate wiring (including extension cords), and we must understand the words required to talk with electricians.

A brief summary of the Electrical Code

The "80% rule" says that continuous power through a plug/socket has to be 80% of the rated value. Electric car charging fits the definition of a continuous load, and therefore we must constrain ourselves by this rule.

In practice this means we cannot use a NEMA 14-50 outlet for a 50 amp charging station. The 80% rule says to limit it to 40 amps continuous.

Ampacity is the measure of the maximum safe current (amps) we can use in a wire of a given thickness. Generally, the thicker the wire the more current it can carry. Wire thickness is measured in "gauge", and you may have seen this number used. A typical home extension cord is 14 gauge, for example. You'll see in the following chart that's completely inadequate for 120 volt 12 amp electric car charging.

The smaller the AWG (gauge) the thicker the wire. The chart shows that the higher the current the thicker the wire must be.

Voltage is not a factor, only current (amps). A 10 gauge cord can carry only 12 amps at up to 20 feet distance, and it doesn't matter whether that's 120 volts or 240 volts.

Current equates to the number of electrons flowing. If you like analogies, imagine trying to jam too many electrons through a skinny wire, or having too many people walking through a tunnel at a subway station. Just as the subway tunnel gets jammed up during rush hour, a wire with too many electrons gets jammed up, and in both situations problems or slowdowns can occur.

The rating for an ELECTRICAL CIRCUIT must be limited to the rating of the weakest device in the circuit. It doesn't matter if the circuit breaker and wiring is rated for 50 amps, if the power outlet is only rated for 20 amps. Try to run 40 amps continuous through that 20 amp outlet and you'll have a fire.

There are three columns of importance on this chart. The 120 volt line-cord charger usually runs at 12 amps, and the chart shows you must have a 10 gauge extension cord. The 6 kiloWatt charging stations run at 32 amps continuous, and therefore you must have a 6 gauge extension cord.

Common power outlets

Generally speaking single phase AC power is what we'll use to charge electric vehicles. It comes in either 120 volt or 240 volt varieties, either 50 hertz or 60 hertz frequency.

In all cases the power outlet and matching sockets will have either 3 or 4 wires. The 3-wire connectors have two "hot" lines and a ground line, while the 4-wire connectors add a "neutral" wire. There are also ungrounded 2-wire plugs.

In some cases we need to build adapters to use a device having a certain plug with a different power outlet. For example the portable 6 kiloWatt charging stations come with a NEMA 6-50 plug, but we commonly see NEMA 14-50 outlets at RV parks and the like. Both offer 240 volts AC power and are rated at 50 amps peak (40 amps continuous), but the 6-50 has only three wires while the 14-50 has four. An adapter is simply built from a matching plug and socket, and connected together using a cable with thick 6 gauge wiring. Wire the hot line at one end to the matching hot line on the other end, then wire together the ground lines. Do not wire the neutral line if it is present.

Choosing the correct plug

We've shown here the NEMA 14-50, but you should choose the plug which fits the power outlets available to you. There are several considerations to be aware of.

The "80% rule" says that continuous power through a plug/socket has to be 80% of the rated value. The NEMA 14-50 is rated for 50 amps, but the 80% rule says to limit it to 40 amps continuous. That means all the charging stations shown above must be used with at least a 40 amp plug/socket combination, and 40 amp circuit breaker (for a 32 amp charging station).

The 14-50 is a commonly available outlet - for example at most RV parks - making it not only widely available, but powerful enough for the charging stations shown above. But maybe the power outlet you'll be using isn't wired with NEMA 14-50, but some other 40 or 50 amp or more outlet. If so, get a matching plug or else build an adapter between 14-50 and this other plug.

We said this earlier, but don't plug a 32 amp charging station into an outlet rated for 20 amps. A 20 amp outlet can only be used for 16 amps continuous, and the 32 amp EVSE is twice that amount. The circuit breaker should do it's thing and trip before a problem occurs. On the other hand, a charging station with adjustable power level can be used on a lower power outlet.

Suppose you do arrive at a 20 amp power outlet? Most of the charging stations above don't allow you to adjust the power level, and therefore you'd be stuck unable to charge. But a charging station supporting adjustable power is helpful, because you simply turn the knob to match the power capability of the outlet you're using.

Another factor is the charge rate supported by your car. Many cars, like the Chevy Volt or Chevy Spark EV, only support a 3 kiloWatt charging rate. You could buy a 6 kiloWatt charging station, and plug it into a 50 amp outlet, but that doesn't magically make your car charge at a higher rate. Even when using higher power equipment, cars supporting 3 kW charging will use at most 3 kW of power.

United States of America, maybe Canada and Mexico

There are two basic classifications of NEMA device: straight-blade and locking. The straight-blade 5–15 and 5–20 are found nearly everywhere in countries using the NEMA standards, and are intended for supplying lighter-duty, general-purpose electrical devices. Twist-locking types are used for heavy industrial and commercial equipment, where increased protection against accidental disconnection is required. Numbers prefixed by "L" are curved-blade, twist-locking connectors; others are straight blade and non-locking. The metal conductive blades are sometimes informally called "prongs" (as in "3-prong plug").

These two charts show the common single-phase AC power outlets. There are additional plugs for 3-phase AC and for 277 volts, but those are uncommon.

120 volt locking	120 volt, 15A, NEMA L5-15	120 volt, 20A, NEMA L5-20	120 volt, 30A, NEMA L5-30
120 volt flat blade	120 volt, 15A, NEMA 5-15	120 volt, 20A, NEMA 5-20	120 volt, 30A, NEMA 5-30	120 volt, 50A, NEMA 5-50
250 volt locking	250 volt, 15A, NEMA L6-15	250 volt, 20A, NEMA L6-20	250 volt, 30A, NEMA L6-30
250 volt flat blade	250 volt, 15A, NEMA 6-15	250 volt, 20A, NEMA 6-20	250 volt, 30A, NEMA 6-30	250 volt, 50A, NEMA 6-50
250 volt locking 4-wire	250 volt, 20A, 4 wire, NEMA L14-20	250 volt, 30A, 4 wire, NEMA L14-30
250 volt flat blade 4-wire	250 volt, 30A, 4 wire, NEMA 14-30	250 volt, 50A, 4 wire, NEMA 14-50	250 volt, 60A, 4 wire, NEMA 14-60

Great Britain and the former British Empire

Much of the former British Empire adopted the British electrical plugs.

This plug, and the matching socket, handles 240 volts at up to 30 amps. The largest shown here is 15 amps.

These are rated for 240 volts 13 amps. Some variants let you open the plug to rewire it, while other variants have fuses in the plug.

Europe, the Europlug

Common across Europe are plugs with two or three round pins like this:

Adapting a charging station to NEMA 14-50 or other common plug

Many of the charging stations don't come with a plug on the end of the power cord. The manufacturer's manual will tell you the charger must be hard-wired to a junction box, instead of connected via a plug. It's safer to hardwire the thing - a plug you're constantly plugging in and unplugging will eventually go bad, right? And, a plug that's gone bad is a fire waiting to happen.

On the other hand, hard wiring your charging station means you can't carry it along on a trip. Hence, if you want portability and flexibility, add a plug to your charging station, or select one that has one already wired up.

Let's take a look at what's required to attach a NEMA 14-50 to a typical charging station.

This is the standard wiring for a 240 volt device hard-wired to the power system. Two lines are "hot," meaning they carry the AC power, the third wire is ground, and the fourth wire is neutral. Between the two AC wires is 240 volts. Between either of the hot wires and neutral is 120 volts.

For our purposes we're only going to consider the two hot lines and the ground line. That's because the power cord on every charging station uses three wires, two hots and a ground. The standard color coding for the wires is that GREEN is ground, and the two hot lines are RED and BLACK.

This diagram should help clarify the wiring.

Once you get a plug in your hand, it'll be obvious how to match the three wires with the pins on the plug. All you do is to carefully strip the insulation from those wires, then attach the wires to the matching connections on the plug. For most plug designs you loosen a screw, stick the wire into a hole, then tighten the screw.

Adapters

Unfortunately there are a plethora of power outlets for 240 volts, in the U.S. and perhaps elsewhere. It means that while all single phase AC power outlets have the same kind of power, if the plug on your power cable doesn't match the power socket you're out of luck. Unless you build an adapter.

That's what we see in the picture above. The Golden Gate Electric Vehicle Association (GGEVA) meets at Lucious Garage in San Francisco. The fine management of Lucious Garage goes beyond the call of duty to support the meetings. One example is that during meetings we're allowed to plug in to charge. Each of the car lifts have a NEMA L6-20 outlet, supporting 240 volts and rated for 20 amps. As we saw above, that means a maximum 16 amp charge rate.

The charging station is an OpenEVSE which has been set for 16 amps. It supports a maximum 40 amp charge rate, using components rated for 50 amps, and has a NEMA 14-50 outlet. Obviously an adapter is required. That's the grey cord between the outlet and the NEMA 14-50. The grey cord came from a "range cord", from which the plug was removed. The three strands of the cord were separated and the ends stripped. The stripped ends were inserted into holes in the NEMA 14-50 socket, and the NEMA L6-20 plug. Each of those includes a strain-relief system holding the cord ends in place.

Fortunately all single phase AC power outlets use three wires (sometimes four) whose purpose is the same: Two wires are "hot" (carry the AC voltage), one wire connects to ground, and the optional fourth wire is neutral. As we said above, for our purpose we'll only focus on the two hot lines and the ground line, and will ignore the neutral line.

Generally speaking, building any adapter follows these steps:

Get a short length of 3-wire cable of the appropriate gauge, the thicker the better
Strip the ends so you have bare wire - for some adapters you'll only strip one end
Open up and study the plugs for each end of the adapter, making sure you understand where the two hots and ground are
Insert the stripped ends into the plugs, tightening down screws, making sure the connectors on each end match up correctly
Tighten down strain relief, and otherwise reassemble the plugs at each end

That's what one end of a fairly typical adapter might look like.

And this is a couple completed adapters. At one end is a NEMA 14-50 enclosure - because the charger system in my car uses NEMA 14-50 plugs. The adapters shown here are for the L6-30 (240 volt 30 amp) and 6-20 plugs (240 volt 20 amp).

NEMA 6-50 to NEMA 14-50 adapter for common charging stations

Many of the charging stations shown above have a NEMA 6-50 plug on the end of the power cord. That plug supports 240 volts at up to 50 amps (40 amps continuous), which is great except for one thing. The typical outlet at an RV park is a NEMA 14-50, necessitating an adapter. Which gives us a chance to go through a practical demonstration of building an adapter cable.

The 14-50 connector uses four wires, while the 6-50 connector uses three. In wiring the adapter we'll simply skip wiring the neutral line of the 14-50 to anything.

This is the wiring connections to make - ground-ground, hot1-hot1, hot2-hot2. In fact, these are the wiring connections for any plug/socket adapter.

At the end of the charging station power cord is a 6-50 plug, meaning one end of the adapter must have a 6-50 socket. At the RV park is a 14-50 socket, meaning the other end of the adapter must have a 14-50 plug.

In the shopping guide below are two choices with a 14-50 plug already attached to a length of heavy-duty power cord. This makes the adapter trivial to build, because you only need to procure a 6-50 socket and wire it to that cord. Which, you'll find below.

You'll also find a couple pre-built adapter cords with a 6-50 socket and 14-50 plug all wired up and ready to go.

About the Author(s)

David Herron : David Herron is a writer and software engineer focusing on the wise use of technology. He is especially interested in clean energy technologies like solar power, wind power, and electric cars. David worked for nearly 30 years in Silicon Valley on software ranging from electronic mail systems, to video streaming, to the Java programming language, and has published several books on Node.js programming and electric vehicles.

Electric car charging within electrical code and power outlet limits

When are electric car extension cords required? Is it safe to charge an electric car in the rain?

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