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I have a dedicated 240V line run to my backyard to power an electric shed heater (240V). I plan on putting an above ground pool in the summer, and would like to get an electric heater for the pool. Does anyone know if I can use the 240V line and install a switch so in the summer it can power the pool heater, and in the winter can power the shed heater. The two will never be used at the same time. Any help is appreciated.

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    The first thing that is needed is what size breaker is on the circuit. Better yet would be what is the wire gauge it will be printed on the cable / wire and how many wires ? Is it in conduit? The answers may be yes but more likely it will need a 4th wire to meet modern code and the size and possibly distance will tell us what can be connected as a load.
    – Ed Beal
    Jan 6 at 16:39
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    If the cable is big enough you could put a small subpanel in the shed. This will allow you to have the appropriate sized breaker for each device.
    – Gil
    Jan 7 at 2:03
  • This is entirely a code question. From a practical perspective such a switch is obviously possible, and equally unnecessary. Just don't use the two things at once and rely on the breaker to stop you! But Code: Question 1) Does code allow two outlets strung together on a 240V breaker in the same way that's allowed on a 120V 15A circuit, relying on users and ultimately on the breaker to prevent them from together drawing too much current? 2) If code does not allow that, (if dedicated breakers are required), does a failsafe DPDT switch comply, where only one outlet at a time is connected?
    – jay613
    Jan 7 at 13:11
  • If the pool is near the shed and its heater is in the shed, you could just install one outlet and only plug one thing into it at a time, avoiding this problem. But presumably if that were the case you wouldn't be asking this. :)
    – jay613
    Jan 7 at 13:17
  • @jay613 Except that likely both devices have instructions "No extension cords", and adding a tripping hazard near a pool is not a great idea. Jan 7 at 13:57
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The right idea is a subpanel.

The reason is, the two different appliances may need different circuit breaker protection.

Every appliance is designed and certified to "fail safe" within a certain current range that corresponds to the breaker it's designed to be on. Put it on too high a breaker and it will burn when it should trip.

The cost of wire favors big feeder anyway.

While you may have heard of troubles small aluminum wire has had, large feeder has proven reliable.

This creates an odd economics where, for the price of 30A wire (#10 Cu), you can have 90A feeder (#2 Al). However, you need adapters to couple large feeder to small wires, and a very economical "adapter" is a subpanel!

So the wind is blowing in your favor for a subpanel, and it will give you a lot of other versatility too for any future loads.

For instance, some are getting better offers for a house that is "EV ready" with 30-60A 240V service.

Note that even though the wire is rated to 90A, you are allowed to put it on a smaller breaker. 60A is as cheap as breakers get, and will fit #2 wire.

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One solution is a double pole double throw (or DPDT) switch that is rated for the current each device pulls. This is assuming they use similar enough power that a single breaker is safe, and the current draw is 30 amps or less. I've seen 30 amp DPDT switches before but none rated higher.

If the power draw is quite different, such as your shed uses 50 amps and the pool heater uses 15 amps, then get a transfer switch. The transfer switch that is on the shelf will likely have matching sized breakers, such as both 50 amps, but they are standard breakers and so swapping one out to match your load should be quite trivial.

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In the absence of details, general guidance regarding the circuit is provided. Safety is the key driver / requirement that drives circuit design and parts selection

  1. A 240V implementation of Double Throw (__DT) switch, can ensure one OR the other heating device is connected.

  2. Wiring, breakers and switches must be rated for the current drawn by the devices or the wiring will overheat and start a fire.

  3. Do not take safety advice from people you do not know on the internet: Double check your circuit design and parts selection with a licensed electrician to ensure it complies with local code: it's not worth the fire risk to DIY the first time.

As this is the starting point, I expect many constructive & helpful comments / edits of this post from my helpful peers.

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  • Also the part linked isn’t a double-throw switch, but single-throw. The needed part (assuming code allows it, not sure personally) would be a DPDT switch.
    – nobody
    Jan 7 at 4:13
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    @manassehkatz-Moving2Codidact Good comment. The diagram has been replaced to allow the reader to select a DPDT for the hot pairs of the two said heaters.
    – gatorback
    Jan 7 at 12:53

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