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

andrewco

Diagrams not included before

SWEI

Need to see existing wiring to those two thermostats, especially where they (or their connected appliances) get their low voltage power. You'd be safest with an interposing relay there.

andrewco

SWEI said:

Need to see existing wiring to those two thermostats, especially where they (or their connected appliances) get their low voltage power. You'd be safest with an interposing relay there.

Thermostat 1) controls the 5kW heater. This is the biggie. It's a NEST using 3 wires (using W1, C & Rh). Using previous guidence I would be breaking Rh with the relay

Thermostat 2) controls the floor heat max draw 1000w but I've noticed it being much less than that. It's a 4 wire lv thermostat.

SWEI

Where do those thermostats get their 24 VAC power?

andrewco

The nest gets it from the RC840T on the C terminal I believe.

The stepwarmfloor (floor heat) I'm still trying to figure that out. It's a 4 wire thermostat.

SWEI

The safest route would be a DPST relay with its own power source.

andrewco

....and control that DPST relay from the current switch I assume?

SWEI

Yes, the current switch would activate the DPST relay coil.

ratio

There might be an issue with that. The diagram you posted up-thread shows the aube relay as having 1.2 VA of power available, 40mA of which is committed to the coil of the relay. The math works out to a surplus of 10mA (unless I screwed it it, haven't done this since skool...), which might not be enough to draw in another relay. Also of concern is the inrush current, which can be an order of magnitude, or more, higher than the listed holding current.

Of course, the easiest way to see if it will work is to try it!

andrewco

aube relay is spoken for. That's the one that controls the 24v thermostat (nest) via the 5kW garage heater. SWEI is suggesting establishing another relay specifically for the purpose of being able to control TWO individual thermostat's call for heat.

ratio

Right, but what powers the new relay? The aube internal power supply might not be stout enough to draw in another relay. (Also some concern re powering the Nest, I have zero experience with 'em, I worry about things I don't know about.) Powering the new relay off of the blue & red would mitigate that.

SWEI

Lacking detailed info on the existing transformers and their loads, I was suggesting either a self-powered relay or a new transformer for the current switch & relay.

andrewco

Sorry - I'm not going to use the existing transformers to power a new relay since all the secondary power is spoken for. As SWEI suggested, I would look at powering the new DPST with a separate power source (most likely the 240v circuit)

andrewco

Just a quick update on this project. I've got all the controls in place and it's working great. I'd like to add a kwh meter to the EVSE circuit. In measuring a 240v home circuit there are two hot leads (L1 & L2). Do I need to measure both leads or can I just measure one leg?

SWEI

If there's no neutral connection, you can measure the current on one leg.

andrewco

Excellent. I'm thinking one of the below units as a good low cost way to monitor it's kWh usage.

one
two

I was looking for a low cost alternative that would allow wireless monitoring or at least logging that can be automatically segmented by month or week but I haven't found it...at least not "low cost".

ratio

ISTR that a NC contact adjustable current switch was wanted for this project. I just came across an ACI part: A/ACSX and A/ASCSX. No clue as to pricing.

For posterity, enjoy!

SWEI

$97-$120 list on those depending on config. Great company, BTW.