I have a 240VAC garage outlet that will be turned on via a control relay. Is it best practice to switch/relay both lines, or just one like 120V~ loads?
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1Also pay attention to the amperage the relay supports. You will destroy the relay quickly if it is too small for the job.– JPhi1618Commented Oct 24, 2019 at 15:24
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1What are you plugging into this outlet?– ThreePhaseEelCommented Oct 24, 2019 at 23:05
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@ThreePhaseEel this will be switching a portable L2 EV charger.– Jé QueueCommented Oct 25, 2019 at 15:12
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@JéQueue -- do you know how many amps this charger pulls?– ThreePhaseEelCommented Oct 25, 2019 at 23:19
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2 Answers
I'm assuming your wanting to power some 240V appliance with this circuit. In that case, if you switch off only one of the two HOT wires, you will likely cause a problem with the appliance, depending on what it is.
Yes, you would definitely want to use a double-pole 240V rated relay for this application.
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4Even for safety you would want to switch both because they both have voltage compared to ground. In a 120v circuit, neutral is at the same potential as ground so no real reason to switch it off.– JPhi1618Commented Oct 24, 2019 at 15:23
Legally, it is acceptable to switch only one pole of a 2 pole circuit, as it will not allow a 240V device to operate. If you think about it, that is what happens with fuses, they RARELY both clear at the same time.
But that does not apply to circuit breakers, which are required to operate both poles at the same time.
Still, just because you can does not mean you should. If switching a 2 pole circuit, it's best to use a 2 pole contactor. ("Relay" generally means small control type circuits, for power, you call it a "contactor").
When selecting a contactor, you need to assess the nature of the load you will be switching. if it has a motor, the motor HP ratings will not align with the current ratings, because switching of inductive loads like motors is harder on the contacts than switching simpler things like resistors or lamps. So for example if you have a 1HP motor rated at 8A @230V single phase, you would think that a 15A relay would suffice, but most 15A relays are only rated for 1/2HP. Read the fine print.
