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Old 03-10-2013, 12:48 PM   #1
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EV charging loads and time switches - rating question


I have gotten questions about controlling EV charging loads using time switches to take advantage of cheaper time-of-day rates.

One particular model of EV presents a 40A continuous load on a 50A circuit via 14-50R. Intermatic T100 series documentation lists the load rating at 40A:

http://www.intermatic.com/~/media/In...ries_2pgs.ashx

My assumption would be that given continuous load rules, the switch's rating wouldn't be sufficient (and would need to be 50A). Correct? Or does the listed "load rating" consider continuous loads?

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Old 03-10-2013, 06:21 PM   #2
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EV charging loads and time switches - rating question


Use a contactor controlled by a timer with a manual switch override for the other times.

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Old 03-10-2013, 06:42 PM   #3
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EV charging loads and time switches - rating question


Quote:
One particular model of EV presents a 40A continuous load
Why the vague description ? A make and model generally helps.

But I would guess a Leviton/Toyota. You might ask them about timers and if they have any objection to their regular use. I know they will cause a cold load start which could delay your starting time up to 15 minutes.

I would also be careful about time of use rates. Time of use rates vary by utility, so location is important.
But you need to look at all uses by time and calculate the net change in billing. Low EV charging rates can be lost thru higher rates during peak periods. (eg high peak period air conditioning rates)
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Old 03-10-2013, 06:50 PM   #4
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EV charging loads and time switches - rating question


Why? Because it's immaterial.

Tesla Model S. Its UMC plugs into a 14-50R and draws 40A continuous on a 50A circuit.

I can build a contactor driven by a timer, but that's not what I'm after. Those less inclined to build such a device are looking for more turnkey.

I'm not analyzing the various rates - I have a single flat rate which is just fine. The others are left to consider their options.

I'm just looking for an answer to my question, which is whether the Intermatic documentation should be construed to support 40A intermittent only, or if it can support 40A continuous loads as well?

Last edited by flasherz; 03-10-2013 at 06:55 PM.
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Old 03-11-2013, 01:35 AM   #5
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EV charging loads and time switches - rating question


The 125% factor for continuous loads applies to conductor ampacities and overcurrent protection device ratings. The NEC seems to apply the factor specifically in these situations, and I don't think the derating would be applied to device ratings unless there is some other article that does so. So the breaker and wire must be rated for 50A, but the timer and any other devices in the circuit could be 40A rated.
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