Intelligent transfer switch for two loads on a 30A 240v circuit

andrewco

Ok - this is a bit convoluted but here's the deal.

This is kind of a heating issue but it's complicated by having to charge an EV off of the same circuit (no options otherwise).

I have a 5kw heater that is controlled by a LV thermostat (Nest). It's plugged into a L6-30 outlet off of a 30A 240v circuit.
Enter an EV requiring the same circuit. So, I'm looking for an automatic transfer switch that would "pull" the circuit over to EV when needed and then return to the garage heater when not in use.

It's the function of a transfer switch....I'm just not sure where to find such a thing or even if it exists.

Zman

As far as the transfer goes, A DPDT switch will handle it. The question is how do you trigger it?
Could you just use a timer?
Do you have the model of the charger?

andrewco

It would be great to be triggered by the vehicle itself (Nissan leaf). Otherwise, trigger it on power demand would be the logical option.

SWEI

The Leaf has both SAE J1772 and CHAdeMO charging receptacles. Ideally, the changeover should be coordinated by the charging station -- since it communicates with the vehicle to manage a number of things -- including charge current.

If the charging station doesn't have a suitable output to control this, I would install a current sensor switch set for 3A on one of the hot legs feeding the charger. Use that to fire a DPDT contactor, which will allow the heater to run anytime the charger is not drawing enough to prevent both the charger and a 5 kW resistive load from peacefully coexisting on that 30A breaker.

JUGHNE

Is your garage insulated and attached to your house?
Mine is and unheated. It seldom drops below 45 degrees and that is a with a dog door.

I think that in the spirit of being green, you should not have to heat your garage and charge the Leaf at the same time.

Isn't the Nest a smart control that knows when you are coming home and could turn the heat up?? Could it not turn the heat down? If time programming is capable by the Nest then if could shut the heat down when needed.

Actually when you get out of the car and have to plug in the charger cord could you just switch the heater off? Maybe I am just thinking too simple

Zman

SWEI said:

The Leaf has both SAE J1772 and CHAdeMO charging receptacles. Ideally, the changeover should be coordinated by the charging station -- since it communicates with the vehicle to manage a number of things -- including charge current.

If the charging station doesn't have a suitable output to control this, I would install a current sensor switch set for 3A on one of the hot legs feeding the charger. Use that to fire a DPDT contactor, which will allow the heater to run anytime the charger is drawing enough to prevent both the charger and a 5 kW resistive load from peacefully coexisting on that 30A breaker.

That is a clever solution. You could actually leave both devices wired to the 240 volt and just use the current sensor to break the low voltage call for heat coming from the nest.

andrewco

I really like this idea as well. As it solves another problem which I was wondering about. The thermostat that controls the heater is actually powered off of the heater circuit. If I used a true transfer switch, then I would essentially not be powering the thermostat. Not a huge issue but I'm guessing after constantly being switched on and off it would not have a great sense of humor.

So, controlling the low voltage thermostat, which line would I break so as just to stop the call for heat but not remove power from it?

njtommy

Zman is 100% currect.

andrewco

JUGHNE said:

Is your garage insulated and attached to your house?
Mine is and unheated. It seldom drops below 45 degrees and that is a with a dog door.

I think that in the spirit of being green, you should not have to heat your garage and charge the Leaf at the same time.

Isn't the Nest a smart control that knows when you are coming home and could turn the heat up?? Could it not turn the heat down? If time programming is capable by the Nest then if could shut the heat down when needed.

Actually when you get out of the car and have to plug in the charger cord could you just switch the heater off? Maybe I am just thinking too simple

Yes, it's insulated and it's my workshop as well. The thermostat is set to 45F (safety temp since the stupid nest can't have a program temperature below 50f). Typically, the heater will only run when it get's nasty a$$ cold out and it does that so my paint and other temperature sensitive items in the garage don't foul.

All-in-all, it's pretty minimal. It hasn't run at all for the last 14 days - granted it's been warmer, but still.

The problem is the most likely time that the heater will run will also probably be a time when the leaf is charging (middle of the night).

Yes, you are thinking I'm going to remember to turn the heater on. The best controls are those that run with logic that doesn't require intervention of a human...hence this request :-)

JUGHNE

Are we talking about the current sensing device used in furnaces to energize the humidifier or electronic air filter? I have not used them and don't know how much current draw is needed to activate the device. That has to be a simple and economical method. But where do you gracefully install it? They usually hang inside a furnace and are unseen. A simple 6X6X6X4 J-box could contain it. It could break the W wire going to the heater.

ratio

Adjustable current switches are available. This model is adjustable from .3 to 150 amps.

I'm not a shill for RIB, really! They're just what the supply house carries.

andrewco

ratio said:

Adjustable current switches are available. This model is adjustable from .3 to 150 amps.

I'm not a shill for RIB, really! They're just what the supply house carries.

That's funny...you sure you don't have a controlling interest in RIB...especially for a $15 part :-)

Seriously though, this control circuit needs to absolutely work since if the heater comes on and the leaf draws anything significant, then the circuit will trip.

Thoughts?

SWEI

In the unlikely event that something goes wrong with the current switch, the breaker will still trip. The most likely thing to go wrong would be someone mis-adjusting the switch IMO.

Zman

Your heater should only be drawing about 20 amps.
If you get an adjustable sensor, you could run both the heater and the charger at the same time unless you need to change more quickly. You might determine that 5 amps is all you need for an overnight charge.

andrewco

I'm powering the lv thermostat off of the heater as well. What wire do I need to break to the thermostat from the current switch? (S/W/R)

Zman

You don't want to break the power or the common to the t- stat. You want to break the switched leg going to the heater relay. Basically switches in series.

andrewco

Maybe I'm confused. I'm switching the heater with the thermostat. I just want to prevent the thermostat calling for heat when the current switch detects current above X. Attached is the diagram with the relay & thermostat. Can I just break the W (heat) wire with the current switch?

andrewco

ooops. It just hit me why switching at the thermostat won't work. The thermostat (maybe it's integral to the heater actually) when switching from on to off, takes a period of time to actually turn the heater off. This could cause a condition where when the heater is running and then the car demands 24A, the current switch detects this and shuts the heater off. But there is a delay and thus the heater could run for another 30s or so still drawing the full 5kW.

Thoughts on ways around this?

REVISION: I think I'm confusing a fan delay with the heater coil being turned on and off. I'm going to verify with a meter, but I actually think that the thermostat does instantly turn the heater coil on or off based on call-for-heat. I'm probably being "tricked" by the fan delay which doesn't mean the heating coil (most of the load) is still powered.

SWEI

I don't think I see a current switch there?

andrewco

SWEI said:

I don't think I see a current switch there?

Correct. Not yet - as I'm not sure (hence the question) of where to put the output of the current switch on the thermostat.

ratio

That's close, but you've got the red & black backwards. Power goes in on the black (& blue), & comes out through the red.

Keep in mind that (assuming the schematic is the correct one) you'll be switching the complete load through the relay pack. The one you've shown seems to be rated high enough, it should work, but my preference is to switch high voltage via cheap contactors when possible.

You are correct, the fan delay keeps the fan on after the heat itself drops out. This is pretty important, make sure you don't break the fan circuit with your relay or you'll be resetting the high limit pretty often.

Just break the W to hold off the heat, bear in mind that the RIB part I linked to is normally open, close on current sense, you'd need a relay to invert that, maybe a NO or form C contact is available.

andrewco

Yea - my bad. I forgot that was an older picture that I was using before I switch the black lead for the contactor. The relay pack is intended to handle this exact kind of load and switching frequency.

Good point about the NO of the RIB part. I don't suppose there are any similar (cost effective :-) current switches on the market that are NC. Otherwise, do you have a suggestion on a specific relay to invert the NO of the current switch?

SWEI

http://www.crmagnetics.com/switches/ac-current?shopby=85

ratio

Non-adjustable setpoint, won't quite meet our needs. Maybe this?

andrewco

ratio said:

Non-adjustable setpoint, won't quite meet our needs. Maybe this?

Yep - need an adjustable set point. Not that I'm going to be moving it around but I'm not 100% sure what my set point will be. That being said - the CR4395 looks like a good option. A bit more pricey, but probably by the time you invert the logic of a NO current sensor with a relay its....well, still more :-)

Pursuing the CR4395, there are a bunch of options. trip status, output options and transformer.

My guess for options are:

Trip Status: EH (energized on high)
Supply voltage: 240 VAC
Trip Range: 1.0-10AAC
Trip on Delay: X (no delay)
Output Options: ELR (mechanical relay)
Transformer: Internal

This sound right?

ratio

That was my take on it as well, but maybe the 3-30 A range? I dunno what exactly your looking at current-wise. With the form C contact, it shouldn't matter EH or EL, they're equivalent, but that might confuse people of you get too fancy.

I book marked that place, now if I can just find a use for some of those current indicators...

andrewco

Good point on the range. Since this is all driven (pun intended) by the charging of the EV then it would stand to reason the charging profile of the EV should guide this choice. Someone posted a charging profile for a leaf that showed after doing most of the charging it then cycled between 0 and 2A. This would lead me to believe that I should be able to turn the heater back on when it is below 2A and thus need something that goes down below 3A more preferably in the 1-10A range.

This is a charging profile of a much lower power charging station and I'm hopefully going to be charging in the 20-24A range when pulling full power.

See attached chart.

ratio

I think at this point you need to know what your loads are like. Got an amp clamp? According to the schematic you posted, that heater will draw at least 20.9 amps tapped at high, possibly excluding the fan. Maximum load on a breaker is 80% of the rating, that gives us 24 amps to work with. Two amps seems like a safe bet, although the question of the fan remained open. You may need to tap the heater at a lower wattage to get enough headroom to be able to heat when you're down to a trickle charge.

andrewco

Yea - but I'm confident that I'm going to need to adjust within the range of 1-10A. That being said, when I pull everything apart I'll throw an clamp ammeter on everything before I finalize my set point and even with that start off on the low side. I really need to get an accurate profile of how MY Leaf will charge off of a 240v circuit. The EVSE (charging cable) I have is also configurable to draw anywhere from 6A-24A (actually higher but that's just plain stupid :-)

I need to get all the parts together and then I'll start messing around with things rather than pull it all apart and having stuff hanging all over.

ratio

You mean plan on completely finishing the --ooh! shiny!

andrewco

ratio - you know me better than most.

ratio

Heh. My house is full of half-started projects. Some of which I'm actually going to have to complete. SWMBO actually laughed when I told her what I read on a sign last summer:

Ladies, when a man tells you he's going to do something, he's going to do it. You don't have to remind him every six months.

Let us know how it works out.

andrewco

Okay - finally got around to start the wiring of this. I'm doing this in phases - first is to get the line voltage part of it all set. Being this is all 240v and I have two hot lines, I have a question about wiring. In this diagram you will see that I need to continue L1 & L2 past J2. This is pretty simple with a 110v circuit but with 240v AND using 10Ga things are a little more challenging.

So two parts to this: 1) does this look reasonable and 2) is there some standard way to continue L1 & L2 past a switch or should I just pull a separate set of wires from J1?

ratio

Wire nut little piggy tails on L1 & L2, take that to the switch. It must be the same gauge as the feed wire, #10 for a 30A circuit. Make it stranded if you can. Hopefully there'll be enough room to jam everything into the box.

andrewco

The more I think about it, the more I'm thinking I'm going to just run wire directly from J1 to J3 rather than add another set of wirenuts to J2. I assume that's ok as well.

ratio

Technically (which is the best kind of correct!), the heater itself shouldn't be used for splices. Is J1 part of the actual heater? I'd look for a jbox before I brought it out of the heater.

That said, electrically speaking, it'd work just fine.

andrewco

Yea - bad diagramming on my part. These are all jboxes - none of this is done in the heater itself. The only thing that is located in the heater is the relay for the lv thermostat. And it's in a separate part that has other wiring connections.

Ok - so the next delimma is that I ultimately want to open two thermostat circuits from a single control (the current switch). I obviously can't make them part of the same circuit but how do I control two thermostat contacts with a single control?

As a note I'm using this to sense the current from the EV and control the lv thermostat: CR4395

ratio

Two separate thermostats, as in two separate heaters? There isn't a simple way to do that with the relay pack you've specified. A line voltage relay off the load side of the relay pack would do it, but that seems inelegant. Maybe redesign with 24vac contactors and an xfrmr?

andrewco

Hmmm...I almost think I'm just about there. With this setup, I plan to implement turning off the heat for the garage heater via the thermostat by using the current sensor switch. I'm just wondering if I can use the current sensor switch to trigger TWO relays on two different (garage heater & floor heater) thermostats.

SWEI

andrewco said:

wondering if I can use the current sensor switch to trigger TWO relays on two different (garage heater & floor heater) thermostats.

As long as they're both fed from the same transformer, yes. If they're fed by different transformers, things get a bit tricky -- especially for the next guy that has to work on the system.

andrewco

SWEI said:

As long as they're both fed from the same transformer, yes. If they're fed by different transformers, things get a bit tricky -- especially for the next guy that has to work on the system.

Hmmm, not sure I understand. I've updated the diagram to relfect what I'm trying to do. I've also included the output options of the current sensor. It can be ordered as only one of the output options. I'm trying to open two separate thermostats (24v) based on the trigger of a single current switch.