Tesla asks: How big a charging circuit can you put in your house?
In his latest post on the Tesla blog, Martin Eberhard poses the question how big a charging circuit can you install in your house? To help readers determine the answer to this question, Martin sat down with a copy of the National Electrical Code to try and determine what the rules say. If you've ever tried to read a standard like the NEC, you'll quickly find your brain tied in knots trying to figure it out. Fortunately Martin has distilled the most relevant parts out and created a spreadsheet that can be downloaded from the Tesla site. There is no simple answer to the question of how much EV charging capacity your house can support. It depends on a multitude of variables such as what appliances and other loads you use and in what combinations as well as the wiring setup of your house. Martin would like readers to download his spreadsheet, fill it out and post the results in the comments of his post. The results will help Tesla determine what kind of charging rate potential customers can support when designing new products. If you have an older house with corresponding wiring, you may well find that you need to upgrade your home wiring before you get an EV.
[Source: Tesla]
Reader Comments (Page 1 of 1)
Phil L. 10:25PM (9/05/2007)
I planned ahead: During a home renovation project, I added a 240V, 100 amp capacity panel in my garage, currently nearly unused. I had a number of purposes in mind (ability to run an air compressor without lights dimming; a possible future welder), but an EV was also in the back of my mind.
Now if I only had a Tesla-sized budget...
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Jim 2:41AM (9/06/2007)
The answer as to how big a service you can get to your house is not a question for the NEC, but for your pocketbook.
The code places no limit on maximum service size only on minimum.
The local utility and your favorite electrician will supply you with the answer in the form of dollar signs.
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rgseidl 8:30AM (9/06/2007)
At the very least, you should use a dedicated circuit with good wiring. However, at just 110V and 15amps, a regular household circuit can deliver just 1.65kWh per hour. A 13kWh charge-up will then take a full 8 hours, which is fine if you hook up your EV at night. Note that a fraction of the electrical energy will be lost as heat during the charge process.
For greater flexibility, you'll almost certainly want to upgrade the drop into your home. A 240V 100amp drop can deliver 24kWh per hour, so assuming the cable, plug, charger and batteries can all handle that much power, charge time drops to ~40 minutes. Charging efficiency is lower than before.
There is, of course, one major snag with this. Short charge times will encourage you to recharge during the day. This means you'll probably drive more miles rather than plan ahead to minimize them.
Just as bad: raid charging means you'll be taxing the grid during the day, when it may already be near full capacity (especially in the middle of summer). If the EV market takes off, additional investments will be required for both grid and generating capacity - probably in the form of gas turbine gensets that can be fired up in a matter of minutes. That means natural gas and daytime electricity prices will go up for everyone. Capacity utilization at night will be even worse than it already is.
Ergo: EVs should only be charged slowly and only at night. If you really need greater daily range than an affordable EV can provide on a single charge, you may want to shop for an efficient regular HEV or perhaps a PHEV instead. In some cases, a conventional subcompact with a small - perhaps turbocharged - engine will be the best choice, especially if you decide to invest the difference in purchase price into insulation, double glazing and, energy star heating and/or a/c for your home.
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Christine Smith 8:44AM (9/06/2007)
Just a note---they need to add an option that says something like: "cannot change electrical wiring because I live in an apartment".
I think this will be something that EV vehicle companies will have to anticipate/address in some way as they roll out this new technology more widely.
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KC 1:54PM (9/06/2007)
Wow, so in the real world - outside the wet dreams of EV enthusiasts - the infrastructure to support pure EV vehicles is just as non-existent as the infrastructure to support hydrogen fuel cell vehicles.
Plus the fact that battery technology is decades away from supporting all the current abilities of the internal combustion engine (like range).
Looks like we are stuck with ICE for the foreseeable future of our yet to be born children.
Hydrogen is to hard to transport and store, batteries are based on chemical processes and are not able to charge quickly and cannot deliver anywhere near the range of an internal gasoline combustion engine,and matching a diesel is pipe dream. The day we have a 1000+ Km range pure EV is the day world peace will be declared and men will finally achieve equal rights with women (yes, you read that correctly)
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rgseidl 1:56PM (9/06/2007)
@ Christine -
alternative scenario: upmarket apartment complexes will install suitably secured and metered outlets in their car parks and advertise this as a premium feature. As with most new technologies, the early adopters will be in the upper income brackets.
Besides, it's not like you would want to hang an extension cord out of an open window at night in the middle of winter, anyhow.
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rgseidl 2:12PM (9/06/2007)
@ KC -
the comparison with hydogen is very wide of the mark. You absolutely can trickle-charge a BEV overnight off your household wiring if you have a garage. It might be a good idea to have an electrician come around and check the wiring to make sure you don't suffer an electrical fire but that's just common sense. In many cases, the utility drop will be in or near the garage anyhow, so even a brand-new circuit will cost peanuts. The technology and infrastructure for producing and distributing inexpensive electricity to retail customers exists and is currently underutilized at night. The only issue for PHEVs and BEVs is the on-board battery pack.
Hydrogen production via steam reforming of natural gas and/or electrolysis using (nuclear) electricity is anything but cheap, but it is at least feasible at industrial scales. It would be merely be a question of expanding capacity. By contrast, retail distribution and storage of hydrogen - both stationary and on-board - still represent far more serious technical hurdles than anything on the battery front does. Not to mention the still sky-high cost of the automotive fuel cell stacks themselves.
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_Jon 2:57PM (9/06/2007)
In the City of Detroit, most of the houses are over 40 years old and only have 60 amp service. You cannot get a meter for more than 100 amp service from the energy company (DTE) in most areas of the city.
It is likely that, in order to properly charge the car, you will have to install an auxiliary circuit like is needed for Central Air Conditioning. In many cases, the AC service is interrupt-able if the demand goes too high. A similar feature may be required for an auto recharge circuit.
On the other hand, a "refueling station" that provided hook-ups and full recharges in less than an hour could become social-centers. Coffee shops, restaurants, and malls could offer hi-capacity plug in posts (like parking meters) that would attract EV customers.
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Bill 4:24PM (9/07/2007)
Easy.
Time of use meter for overnight charging - rate is a fraction of daytime rate (for commercial HVAC systems that make a huge block of ice overnight, rate differential may be 3 cents/kWh night vs. 11 cents/kWh day)
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