Automatic A-B 240 V switch

My house is 150' long. The breaker box is at one end and the AC units are at the other end.

I have a well pump at the same end of the house as the AC units are.

I do not want to buy 8 ga uf-b underground to power the pump, when I already have 6 or 8 run and powering the AC.

I would like to tap into the AC 220 V power.

Is there a switch that would always power the AC unit when it wanted it? The pump could use the power ONLY if the AC wasn't busy cooling my house.

From a comment:

The pump is 3HP, 230V, max amp 17.0. Coming from the 40A breaker is a probable 6/2 cable. The AC nameplate data is 208-230V 1 ph, 60 hz, compressor 208/230V, 1ph, 60 hz, 11.1 RLA, 58.3 LRA, fan motor 208/230V, 1 ph, 60 hz, 1/12 hp, 0.60 FLA. Min Circuit amps 14.5, max fuse 20 amp, max ckt-brkr 20a.

• It depends how much current both the ACU and the pump takes, frequency of operation during say a shower and conflict with minimum cycle time if ACU. But if cycling is OK, the pump relay needs to be changed to double throw so normally closed contacts go to ACU rated for that surge current Aug 18, 2018 at 16:56
• What are the currents for the pump, the AC and the maximum current to the house? Current for a well pump should be much smaller than that for the AC. Current for the AC should be smaller than the maximum current tot the house. Is there some current left for the well pump? A starting current limiter for the well pump might help. Starting current for motors is usally bigger than operating current.
– Uwe
Aug 18, 2018 at 17:18
• How much current do the well pump and A/C pull, and what size is the wire going to the A/C? The simplest approach here might be to convert the existing A/C wire to a feeder powering a small subpanel.... Aug 18, 2018 at 17:39
• @Tony and Uwe, thank your for your insight as well. Aug 18, 2018 at 21:12
• @ThreePhaseEel that was a good Idea. I will need to check to see if the wire is big enough. Aug 18, 2018 at 21:12

There's no need for fancy-pants stuff here, a subpanel will do the job just fine

Since you have fat wire (6/2, although 8/2 would work just as well) running to the existing condensor, what I would do is install a subpanel in place of the existing (presumably fused) A/C disconnect and put breakers for the A/C and well pump in that, instead of rigging up some goofy switching arrangement.

The cheapest option here would be a "spa panel" sized enclosure (while you can put a larger subpanel in here, the restriction to 240VAC only imposed by not having a neutral at this location severely crimps its utility) -- an Eaton BR48L125RP will do the trick here and for not much money, either. You will not need any accessory ground bars -- leave the existing bar bonded to the enclosure (or bond it if it did not ship that way) as it will be your ground bar due to the lack of a neutral here.

Once you've turned off the breaker for the A/C, undone all the wires, removed the existing A/C disconnect box, and mounted the new subpanel with the existing cables fed into it via knockouts, you can then wire up the incoming cable with the white and black to the two incoming hot lugs on the subpanel, and the ground to the big lug on the terminal (ground) bar.

From there, you can then fit a BR220 for the A/C (replacing what I presume must be 20A fuses in the existing disconnect and serving as the air conditioner's disconnecting means) and a BR240 (to handle the starting current as per the pump manufacturer and NEC 430.52(C)) for the well pump circuit, which will need to be wired using 10/2 cable out to the pressure switch and from there to the control box. The hot wires to the existing air conditioner condenser get landed on the lugs of one breaker, while the black and white (hot) wires of the 10/2 cable for the well pump get landed on the lugs of the other breaker, and the bare ground wires go to the terminal bar screws. From there, you can wire up the well pump circuitry (pressure switch, control box, well pump) as per the manufacturer's directions. Don't forget to torque the terminal screws on the breakers and subpanel to the manufacturer's specs using an inch-pound torque wrench or screwdriver, as well!

• Thanks for that alternative solution. As you can tell, the finer details of electrical problems are not my strong suit. I guess you feel that both AC and pump can run simultaneously? I was concerned that the pump would overload the AC system and cause it to have issues. In today's 104 in the shade Austin, my wife would have me by the short hairs if we lost the cooling in the house. Aug 19, 2018 at 20:46
• @Terrence -- given that they don't quite add up to 40A, the only problem I could see is if the two motors try to start at the same time, and even then, that'd be fairly unlikely to be an issue Aug 20, 2018 at 3:58
• Ok, thanks. I am going to start getting my ducks in order with a BR48L125RP. Aug 20, 2018 at 12:19
• ThreePhaseEel, I have my BR48L124RP and am ready to go on the project. >>replacing what I presume must be 20A fuses in the existing disconnect<< As you can see from my pictures there is no 20A fuse. So if the AC says "max ckt-brkr 20a." I wonder why there is a 40A in the Main box? Sep 29, 2018 at 13:29
• "The pump is 3HP, 230V, max amp 17.0." When talking to pump manu, they said you want a 40A breaker due to the surge when the pump starts. Sep 29, 2018 at 13:35

The usual HVAC control is 24VAC. It should be possible to use a simple 24VAC SPST relay to switch power between pump (NC) and HVAC (NO) when your thermostat sends a signal to HVAC unit. Of course, the relay should be rated to the highest current of the two.

I am assuming here that when you say "AC" you mean the outside heat exchange part of the HVAC, not the indoor part of it, because cutting the power to the indoor unit will also disconnect it's transformer.

Also note, that leaving indoor AC part and a pump connected simultaneously may overload your power line if other functions of HVAC kick in, like fan or heating. It might also violate construction code in your area, since they often require dedicated wiring/fusing for HVAC.

• @Maple >> since they often require dedicated wiring/fusing for HVAC. This is probably what I should pay attention to, I am sure I am violating some rule in the universe and will be punished for trying to cut corners. Aug 18, 2018 at 21:11

Water pump interrupts A/C

Your A/C already has a 24V control line going out to the A/C head. You don't want to mess with that, because the furnace has some logic in it to prevent something called a short cycle, which damages the A/C. You'd be better off interrupting the 24V control line from the thermostat, which "calls for cool". It is on the Y terminal and is usually Yellow.

So you need to find your pump's pressure actuation switch. If it's at an inconvenient location, this won't work. Otherwise find the two 240V wires that run down to the pump proper, and attach a small 24V thermostat transformer (\$12) so its 240V wires are wired in parallel to the pump. This transformer must inherently separate the 240V wiring from the 24V wiring, so select one that goes into a 1/2" knockout, or replaces a junction box cover, with all the hot bits on appropriate sides.

Then you run some /2 thermostat cable from there, to anywhere you can intercept the thermostat's "Y" wire. You use this /2 to power the coil of a 24VAC relay. You put the thermostat "Y" wire across the NC contacts of the relay. The pressure switch's "call for water" energizes the transformer, canceling the thermostat's "call for A/C".

Since the relay is low-voltage, we don't need to deal with the firm rules for handling mains voltage.

A/C interrupts water pump

There is a /2 thermostat cable heading out to the A/C condenser unit outside. When the big condenser motor is running, those two wires will have 24VAC on them. Do I need to tell you what to do next? Yes.

Mains electricity must stay inside proper junction boxes and conduit. Low voltage must stay outside. The two can't meet; you can't snake a 24V cable inside a box or conduit containing mains power. As a result, they make relays and such which "sit on the border" - such as RiB relays which screw into a 1/2" knockout hole, and put 240V pigtails inside, and provide outside terminals for the 24V. That's the kind of relay you need to shut off the water pump.

You can also use current-sensing strategies, as I discuss here.

• thank you for that thorough answer. Unfortunately it is way above my pay grade. I believe my safest course of action is running some 8/2 from the breaker to the well pump, keeping the wiring situation simple. Aug 19, 2018 at 12:02
• @Terrence -- can you please simply specify the current and HP rating of the existing well pump, the circuit ampacity of the existing air conditioner condenser, and the size of the existing wire? Aug 19, 2018 at 12:50
• Sorry @ThreePhaseEel. the pump is 3HP, 230V, max amp 17.0 Coming from the breaker is a 40 amp breaker with, well I am not sure of the wire, at least 6/2. AC Power supply 208-230 1 ph, 60 hz, compressor 208/230, 1ph, 60 hz, 11.1 RLA, 58.3 LRA, fan motor 208/230, 1 ph, 60 hz, 1/12 hp, 0.60 FLA. Min Circuit amps 14.5, max fuse 20 amp, max ckt-brkr 20a. Aug 19, 2018 at 15:28