How to Power the Plug

By Bill Moore

Posted: 03 Nov 2009

This month's ePoll question (check the home page, right-hand column, about half way down) asks whether or not readers have access to 240 volt electric power to more quickly charge a future battery or plug-in hybrid car. Not being an electrician, I incorrectly phrased the question by asking if you had access to 3-phase, 220 V electric power. Turns out, there is no such animal. 3-phase is strictly an industrial rating to run big motors, welders and such and it comes in flavors of 240 or 480 volts. Typical residential wiring is single-phase 240 and 120V, as Mike Harrigan notes in an email he sent me this afternoon. I found it so helpful that I decided to reproduce it here, along with the link to Martin Eberhard's blog and spreadsheet.

So, with my apologies for the confusion, here is Mike's explanation of what you need to be looking for when figuring out how you'll someday recharge that Volt, Leaf or Tesla Model S.

I just read your latest EV World update and have a comment regarding 230v power. You don't need 3-phase power to charge a car battery. Your article implies that 230v and 3-phase are synonymous, which they are not.

Few, if any, homes would have access to 3-phase power. The power available in residential homes is typically 120v and 240v, both single-phase. Most homes have either 100 amp (older homes), 200 amp (newer, normal size homes), or 400 amp (newer, larger homes) 240v single phase service. The transformer that feeds your house has three wires, let's call them A, B, and N. The A & B wires are fed from the two ends of the transformer winding and provide 240v service. All of your high power appliances such as stoves, ovens, clothes dryers, pool pumps, air-conditioning compressors, etc. use the 240v service. The rest of your house electricity is 120v service that is provided by either an A-N or B-N connection. The N (neutral) wire comes from the middle of the transformer winding, therefore 120v (half of 240) is provided across the A-N and B-N connections. Often the A, B, N connections are confused with 3-phase power, but it is actually completely different. The transformer feeding your home is using only 1 of the 3 phases that are generated by electric generation plants. Commercial establishments that use high power often have real 3-phase power and it certainly is possible to develop a car charging device that uses 3-phase power, especially if we are going to have high-power chargers that provide 100+ kW of charging power.

OK, so that's probably more than you wanted to know about power. However, here is something you will find useful and you might want to share with your readers. Martin Eberhard (Tesla founder and former CEO ) developed a spreadsheet that allows you to do a survey of your home electrical system to determine how much spare capacity you have for charging your EV. Since I was the VP of Service and Customer Support for Tesla at that time, we distributed the spreadsheet to our customers in preparation for the installation of their EVSE (charger system). Please note that it's not as simple as adding up all your electric loads. The NEC (National Electric Code) includes duty cycle data that takes into account the fact that your electrical appliances aren't all on at the same time.

OK, spreadsheet is attached. But for complete information you should steer your readers to the blog that was created by Martin which also has a link to download the spreadsheet. The blog explains in more detail much of what I've already said in this email plus has instructions on how to use the spreadsheet.

Mike is the VP of Sales & Marketing for Atieva Inc., but formerly worked for Coulomb Technologies and prior to that Tesla Motors.

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