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I have preparation for an AC connection with 3-phase 220V 3X16A breaker. I want to install a 220V 1-phase unit, which also needs a 16A breaker. The physical connection has screw-on terminals, so there is no worry about plug compatibility.

I have spoken to 3 electricians: one says that I need only change the 3-phase breaker to 3 separate 1-phase breakers, and leave the 2 additional phases disconnected and unused in the AC connection. The other two electricians say that there is no problem leaving the 3-phase breaker and just leaving the unused phases disconnected. All of them agree that there is no worry about the shared neutral of the connections, and say there is no need to re-wire the neutral.

Should I listen to the majority? I tend to trust them more since they are basically telling me that they don't have to work (do nothing = no get paid), so I'm more suspicious of a guy telling me I need to pay him to do the needed changes.

I'm asking both in terms of code-correctness and more importantly, safety for myself, the AC unit and the house (electrocution is bad, blown AC unit is bad, fire is bad).

Edit: to clarify, mains voltage in Israel (where I live) is 220V. Most older homes have 1-phase 40A (or even 25A) breakers, and new homes (like my own) have 3-phase 3X25A breakers. The main usage for the 3-phase is AC units and electric cooktops and ovens. The rest of the electrical wiring in the house is distributed across the three phases to even out the load.

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Both are correct. The 3-phase breaker is just 3 separate 1-phase breakers with a handle physically connected so shutting off one shuts off all.

Code correctness depends on your local jurisdiction. In virtually all USA locations, separate uses of each phase allows either breaker setup, while use of all 3 phases on the same device requires the 3-phase common handle breaker.

The 3 separate breakers would be more convenient with 3 separate circuits since you won't turn so much off if one of them goes off. But there may be a rule that requires this in some location.

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  • I'm not planning to run anything on the other 2 phases, so I guess it doesn't really matter (regarding turning all the phases off if only one needs to be).
    – Eli Iser
    Dec 22, 2011 at 12:18
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I have no idea what your power supply is in Israel, however, the previous answer speaks to 3 phase in the USA. Residential service here does not use 3 phase. Phase is a vector voltage calculation determined by a delta or WYE transformer, used in industrial or service distribution on high voltage lines. In over 40 years in the business, I have never seen a 3 phase panel or breaker in a typical home, unless it is some kind of mansion. If anyone wants to talk code correctness, they better have some reference. The previous answer is inaccurate and misleading. In 120/240 residential service, the two 120VAC lines supplied by the utility are called legs or branches, never phases. A phase is completely different. I am sorry to sound testy here, but answers on this site need to be accurate, especially if someone wants to quote code requirements. I'll apologize in a heartbeat if Skaperen can reference the NEC 3 phase residential service and a 3 phase breaker sections.

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    I wonder how many tv's appliances, etc that have found the joy of a stinger leg?
    – lqlarry
    Dec 23, 2011 at 3:00
  • Must be a special dept at Best Buy that sells 208VAC Bitch leg TV's. lol
    – shirlock homes
    Dec 23, 2011 at 6:41
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    NEC does not prohibit 3-phase for residential. My grandfather did have 3-phase power in his house. There are other NEC rules that limit things in residential, such as voltages serving lights. Three phase to a small residence is rare in USA, though common to large mansions. Three phase is more common in other countries, including Israel.
    – Skaperen
    Dec 23, 2011 at 22:19
  • What is known as a "stinger leg" is the higher voltage end of a 3-phase delta found mostly in North America. It is 240 volts relative to ground. Modern electronic devices using switch mode power supplies with 100-240 volt input, like nearly all computers, would handle it fine if you have it. Most utilities won't provide it anymore, especially to a residence.
    – Skaperen
    Dec 23, 2011 at 22:23
  • I'll update my question to clarify, but basically all new houses in Israel have 3-phase connection, used mostly for AC and electric cooktops. The main breaker is 3X25A, with each separate 3-phase breaker a 3X16A. The voltage is 220V all around.
    – Eli Iser
    Dec 25, 2011 at 6:00
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I found this link by mistake so it's been a while since you asked your question, Eli Iser, so I hope it all worked out for you.

My main reason for responding is shirlock homes answer, tho it appears he enjoys what he does I have been an electrician since 1977 and although we do very little residential and we are commercial - industrial, We do have a service department and am concerned that he may have comments that do not relate to all. Here in Florida we have three phase power to many houses that are not mansion that use it normally for the AC units.

I'm concerned shirlock homes is not an actual electrician when he asked to be shown the code section for the 'three phase residential' section, I'm looking in my 2008 and 2011 NEC code book and I don't see a section that is called "three phase commercial" either.

Skaperen response was 100% correct. We have inspectors that will allow you to use a 3 phase breaker for single phase (240 volt) turning off only two legs and we have some who will not. shirlock homes can reference code section 90-4 in the NEC on that one.

As far as his comment on 'legs' or 'branches' he again shows his inexperience, I have never talked to the power company (which I do on a weekly basis) and been told they where giving me 'legs' or 'branches', it's always been 'single phase' or 'three phase', 3 wire or 4 wire, 120/240v, 120/208v or 277/480v.

Also I have never called the supply house and asked for a 2 leg breaker it's a 1 pole breaker, 2 pole breaker or 3 pole breaker.

I don't mean to be testy here but when your helping someone who is a novice you should actually try to answer the question and not be a smarty pants.

As far as Israel, I do not know how the power is supplied to the customer or distributed thru the panel so I'm sorry, greg 'a good ole redneck electrician from Florida'

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    Welcome Greg. While you make some great points, this site does not work like a forum. This is a Question and Answer site. If you'd like to answer the question origianaly asked, please feel free. If, however, you simply want to add some information (or point out misinformation in others answers) please use the Add Comment button below the other users answers. This will insure the other user is notified of your comment, so they can learn from your many years of experience.
    – Tester101
    Sep 27, 2012 at 17:33
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The electrician that says three separate is most accurate as far as code safety. The other two know it will work but all phases are tied into three ganged breakers. So, if there are separate breakers for each phase it only trips one phase the other two can remain off or unused.

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