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Usually when I see a 240v device, it is wired to its own double-pole circuit breaker, with each pole connected to a different 120v phase (to provide 240v total). If there is a 120v device in the same location, it is wired to a separate single-pole circuit breaker, which it often shares with other 120v devices.

If a neutral were wired along with the two phases in the 240v circuit described in my first paragraph, either phase could be used along with the neutral to power a 120v device. If there were no other 120v circuits nearby (and with enough capacity), this could potentially cost less than leaving the 240v circuit by itself and adding a separate breaker, hot, and neutral for the 120v device.

Why is the scenario described in my second paragraph uncommon in practice? Are there safety or electrical code concerns that I am unaware of?

Thanks!

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  • To clarify 120V/240V is single phase, not dual phase. Electronically it is like a 240v center tapped transformer. In addition there are several areas where 240 breaker with a neutral is feed to an outlet providing both 240 and 120 for instance your electric range, electric dryer.
    – spicetraders
    Commented Sep 10, 2016 at 14:38

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The problem with doing this is you may have, say, a 30 amp double pole breaker on your 240V line which may be 10g wire, and the you come off it on one phase with a 120V circuit with 14g wire, the 30 amp breaker is to high for 14g wire, which then is not protected from melting.

The US has moved to 4 wire 240V service, to accommodate appliances that have internal 120V electronics.

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    Code disallows using wire too thin for the breaker. But even if you do everything in 10AWG... if you want the common receptacle, those come in 15A and 20A. Not allowed on a 30A breaker. You can still use 3-wire 240V in the USA, but must be hot-hot-ground. Baseboard heaters, compressors, hot water heaters would be wired this way. Neutrals must now be wired to switch locations to support smart switches.
    – Harper - Reinstate Monica
    Commented Mar 12, 2017 at 16:31
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    For some reason, people answering this assume the 240V line has a 30A breaker. What if it is a code legal multi wire branch circuit with 15A dual breakers with handle ties. Can he run both low amp 120V and 220V appliances from this circuit?
    – mcysr
    Commented Oct 15, 2019 at 20:41
  • Because when you see a 2-pole breaker in a panel powering a 240V device, it is usually 30A. Note that the circuit already being 2-pole and powering a 240V device is the context of the question.
    – Harper - Reinstate Monica
    Commented Oct 15, 2019 at 20:47
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    @Harper-ReinstateMonica: Could one use the 30 amp circuit to feed a subpanel with two 15A breakers, each feeding half of an adjacent duplex outlet?
    – supercat
    Commented Oct 13, 2021 at 17:24
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    @supercat as long as the supply delivers both neutral and ground, yes.
    – Harper - Reinstate Monica
    Commented Oct 13, 2021 at 22:46
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The issue is loading

Generally when you see a 2-pole 240V breaker dedicated to an appliance, the appliance is intentionally sized to use the entire circuit.

Take a water heater. Its rating is 23A. Code requires a 125% derate, which puts it at 28.75 amps, just enough to shimmy under the "30 amp" figure. In other words, the appliance uses all of the circuit's capacity, and is designed to do so.

So you try to add a 3A load. Let's say it doesn't require the 125% derate, so now you're putting 28.75 amps on one leg, and 31.75 amps on the other leg. Whoopsydaisy. That would require a 40A breaker, but now you have a problem. Your water heater requires a 30A plug. You can't put a 30A plug on a 40A breaker.

Okay, so hardwire the load. Now you have another problem. If hardwired loads provision more than 50% of circuit capacity, you can't have receptacles on that circuit.

Okay, so hardwire both loads and use the 40A breaker. Nope, the water heater's labeling and instructions require a 30A breaker.

There's no way to do it, besides a subpanel. A subpanel would actually work.

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If I read your question correctly, you want to both have 120V and 240V outlets coming off of the same tandem breaker. The code compliant way to do this is to add a sub-panel at the end of the 240V feed and then have separate breakers feeding the 240V and 120V outlets.

I believe it is against code to have both voltages of outlets directly fed by the same breaker.

Because of the gauge of the exiting wires, you probably cannot increase the amperage of the existing breaker. If your current circuit were 240V 30A, you would feed that into a sub-panel (60 amp will probably be the smallest you'll find). Inside the sub panel you would have another 30 amp tandem breaker feeding the 240V outlet and 15 or 20 amp breakers feeding the 120V outlets.

You will need to be careful not to overload the original 30 amp breaker but if you do, the breaker will flip and no harm will be done, the wiring will be protected.

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  • Note also that the code requires dedicated circuits for most high power drawing devices in a home - this includes most (if not all) 240 v appliances, such as ovens, ranges, dryers, water heaters, water pumps, hot tubs, air conditioners, etc. A second outlet would violate code regardless of which voltage it is wired for.
    – Mark
    Commented Sep 10, 2016 at 13:37
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The way I read your question was if an existing 240/120 was being unused, why is it not common to utilize it for two 120V branch circuits?

Well, in fact it is common practice.

One common scenario is for a residence to have a 240/120 3-wire ( two ungrounded hots + 1 ground) unused range circuit feed two small 120V appliance circuits. This is covered in the 2014 NEC 210.24 under the term tap conductors and it has very specific rules to follow.

Another scenario is to take an unused 240/120 30A #10 4-wire ( two ungrounded hots + neutral + ground ) and turn it into a multi-wire branch circuit, MWBC for short. The installer would remove the 30A breaker and install a double pole 20A breaker and relabel the breaker to its corresponding loads.

Keep in mind there are a lot of Code requirements when adding new circuits. In general for example,

  1. Double pole breakers are required with a MWBC. ( 3 pole too but rarely a residential situation).
  2. Most all new 120V wiring will require GFCI and/or AFCI protection, and while they do make double pole GFCI/AFCI breakers, they can be more expensive.
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What your are describing is ok to do and should comply with NEC code.
In some circumstances a 120V circuit requires a double pole breaker. If 3 wire romex is used as a branch circuit to supply two 120V circuits (Black, Red, and White wire) a two pole breaker must be used so all ungrounded wires in the multi wire branch circuit are disconnected simultaneously.

Keep in mind that the main purpose of a circuit breaker in a US home is to protect the house wiring in the case of an overload or short circuit. Most commonly a home circuit breaker panel will have 15 and 20 Amp circuit breakers for typical 120V convenience outlets. #14 AWG wire can be used for 15 amp circuits and #12 AWG is used for 20 amp circuits. Don't use #14 AWG on a 20 Amp circuit breaker.

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  • No, I don't think that it (your first sentence) would be code-compliant.
    – Dave Tweed
    Commented Mar 16, 2015 at 14:19
  • You can't put 15 or 20A receptacles on a 30A or higher branch circuit...
    – ThreePhaseEel
    Commented Apr 4, 2015 at 19:10
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1) The 120 device would have a common neutral as the 240v device which is fine if the neutral and second phase are not device dependant for continuity(conductors must be spliced not traveling through the device). Circuits depending on devices out side of their branch, to function, is against code as of 2011.

2) A 240 phase requires direct sin oscillation in order to provide an even 60hertz. Any load on one phase of the 240 circuit not shared with the secondary phase will cause unbalanced return on reciprocal phase. This will not be a problem for a resistance coil or load fused device; Motors and sensitive electronics will not like it at all. No code regulation on unbalanced 240/480/720v or 2/3ph loads, just bad electrical work. Only the judgement of the ancient electrical gods will await you for your transgressions.

3)GFCI/AFCI regulations. All breakers not run to appliance loads and under 30A must have an AFCI/GFCI Dual function breaker. (Arc Fault Circuit Interrupter/Ground Fault Circuit Interrupter) this device measures returned feed back voltage recieved at the source of power(the panel) from the neutral and compares it to the voltage leaving the breaker. If ever the variation between hot and neutral exceeds 5mA the devices assumes there is a short to ground somewhere, and the device breaks the circuit. Running a 120 device on a 240 circuit would leave it unprotected and that is against code as of 2014.

4) Dedicated circuits for any appliance loads; excluding dishwasher, garbage disposal, and any appliance load under 600W.

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    Some of the things you say here are right, others are confused, and yet others are not even wrong
    – ThreePhaseEel
    Commented Mar 12, 2017 at 15:47
  • See 2014 NEC Code210.11(B) for requirements for load balancing.
    – Kris
    Commented Mar 12, 2017 at 15:53

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