Using a Contactor to cut off one appliance when another is activated

I've got a situation where a 240V Contactor may be my solution, but I'm out of practice designing stuff like this so I'd like some more experienced eyes on this. Used to be my dad, who was a career electrical engineer, but he's passed and so here I am. In fact, his birthday would have been yesterday.

I just bought an EV that came with a Level-1 / Level-2 charger. It has two dongles, the L1 can plug into a normal 120V wall outlet and the L2 can plug into a 240V NEMA 14-50 outlet. (Level-1 charging on the 120V line gets either 4 or 8 miles of driving range per hour on the charger, depending on amperage settings. Level-2 / 240V charging is more like 25 miles of range per hour charging.) I rigged up a spare dryer outlet on a whip of 14-3, and I can charge on it with no difficulty. HOWEVER, it's on my actual dryer circuit, and of course my 40A breaker can't handle running my dryer and charging my EV at the same time.

I am scheduled to upgrade my breaker panel (all circuits are FULL) and run a fresh line for a permanent EV charger to be installed, but for now I'm using the plug-in EV charger on the extra branch I built off the dryer line. My cunning plan to avoid overloading the circuit is to use a 240V HVAC contactor with a 100V relay to cut off the EV charger outlet when the dryer is activated.

What I'm trying to determine is what is the simplest way to rig it so that the contactor is normally pulled in, but activating the dryer cuts off the contactor. I haven't buzzed out the coil resistance of my contactor or looked up how many milliamps I need to close the relay, I'm thinking about this at lunch at work and all my toys are at home. I'm thinking that adding a small dongle that my dryer plugs into its outlet through is the answer, and that I want a small amount of current coming from that connection to power the contactor coil (siphoning a small connection in parallel to the dryer, going from one leg to the common) - such that when the dryer is activated, the large current it draws causes a voltage drop across the line that feeds the contactor coil, thus releasing the relay and disconnecting the EV charging port. When the dryer finishes its cycle the current will return to milliamps, the voltage will rise to 110V, and the EV outlet will be automatically reconnected.

Will the resistance of the coil be enough? Do I need to put a resistor in line with it to make sure the voltage drops way below the triggering/sustaining threshold of the relay, and so that when the dryer is off the relay isn't basically a short and melts itself? Am I missing something and this is a terrible idea?

 

First, a dryer circuit for a normal dryer is 30 amps and should not be on a 40 amp breaker.
They make devices to do this. e.g. https://www.amazon.com/Splitvolt-Cer.../dp/B0B5FB89LG and https://getneocharge.com/products/ne...smart-splitter and https://www.bsaelectronics.com/

 

14/3 cable is WAAAAAY too small for a 30 amp or 40 amp circuit. You need a minimum of 10/3 for a 30 amp and 8/3 for a 40 amp circuit.

A 14-50 outlet is for a 50 amp circuit. The 30 amp dryer circuit may not be enough to feed the EV at full charge rate.

 

I don't see an automated 240v selector/contactor in your future.
A mechanical transfer switch could be used while you are waiting to run a new 50A circuit.

 

You can get away with one 50 amp circuit (although higher amperage would be better). Run it to a subpanel where a 30 amp circuit branches off to the dryer and a 50 amp circuit branches off to the car charger.

Separate topic. You are trying to sense the power draw of the dryer to cut off the car charger's power. That is complicated and needs electronic know how. A simpler method would tap into the dryer's circuitry downstream of its on off switch (e.g. of the maijn control knob). You need to open up the top rear part of the machine to access this circuitry so better to do only on an older dryer whose warranty has expired. Then run wires to send power supplied via the dryer innards out to power the coil (solenoid) of the contactor that in turn controls the car charger. Or have a manual toggle switch outside the dryer to activate the contactor,coil and powered by a third (15 amp) circuit in the subpanel..

 

As a forum newbie I can't see how to quote stuff and reply to individual replies, so here we go.

Astuff: Yeah, I don't know what they were thinking when they wired this house. It's a mess. But a 40A breaker is what I've got to work with for now. I have a strong desire to completely replace my baaaadly oversubscribed breaker panel with a brand new one. That's my ultimate plan for this: Rip it out and do it right! BUT yes, that share-your-circuit device is pretty much what I'm trying to replicate, apparently, but for less than $300. Guess I wasn't the only person with this bright idea.

Tolyn Ironhand: LOVE your name & avatar! I think I saw you at the rennfest last weekend :-) But on to business: First, my mistake, it was 12-3, I grabbed it from where it was sitting on top of my coil of 14. But despite the EV 240V charger coming with a 50A plug, it's a device that won't charge past 24A. WHY did they do this? I wish I knew!

PJmax: I'd love to have a big cutover switch (and not just for the Mad Scientist aesthetic), but I have to make this automatic so as to be child/idiot/pothead-proof. I live in an exciting household.

AllanJ: I was considering running a subpanel for the dryer and EV, but I think I'm better off ripping out my existing 20-slot breaker panel (which already has 32 circuits in it) AND the 6-circuit subpanel (which already has 7 circuits in it) and putting in a shiny new 42-circuit panel and making things a little bit less of a mess. Because, wow. Plus that'll make it SUPER easy to install a new breaker and run a new dedicated line for the EV charger - right now I'm using a plug-in one but the hardwired one I'm going to have installed soon will use I think a slightly higher amperage.
I used to do little electronics projects like this all the time - sense one thing and react based on it - but usually I was involving clever things like transistor amplifier circuits, TTL gates, etc. That might be the way to go. This dryer is newfangled enough that tapping off of the panel isn't going to be helpful - it doesn't turn the panel off when the dryer is done running, it'll stay on for eons. Very frustrating.

Thank you all for your input, at least I know what not to do :-)

 

The wire(s) needs to sized to the breaker or fuse that is protecting it, not by the expected load. If it is a 30 amp circuit it requires #10 cable. If it is 40 amp circuit it requires #8 cable. If it is a 50 amp circuit it requires #6 cable.