06:31PM | 08/11/99
Member Since: 02/12/99
9 lifetime posts
can anyone give me some basic voltage principals of

220 three phase electric circuits? can i wire single phase 220 saftely off the high leg of 3 phase?


02:46PM | 08/13/99
Member Since: 10/10/98
34 lifetime posts
I suspect you mean 2-phase. 220 volt.

For residential properties, it’s somewhat basic. Two main cables come from the street to the house; each is a single phase, 110-volt line. One is the opposite phase from the other, which is a feature of alternating current. The details are probably not important to you, but basically each phase is different, hence, two phases. Since the phases are different, when both phases are used the voltage doubles from 110v to 220v.

In your main circuit panel there are two buses, each one connected to one of the 110v phases. If you looked at all the breakers, you would see typically two rows, and probably draw the conclusion that each row represents one of the 110v lines. That is not true. The buses are staggered in such a way that each breaker slot in a row is on the opposite bus from the one on either side of it.

A single-phase breaker occupies one slot on the bus, and therefore only uses one of the phases and is 110v. A two-phase breaker will occupy 2 adjacent slots, and thusly uses both phases and is 220v.

Well, there’s your basic lesson. As far as your question on running a 220v single-phase, well, based on the above discussion you can see that you can’t get 220v from only one phase of a two-phase since each “leg”, as you call it, only rates at 110v. Besides, running wires off a single leg of a two-phase breaker can be dangerous, and would probably not be sanctioned by your local code. You are also not allowed to clamp more than one wire under each screw of a breaker. If you need a new branch circuit for a single-phase application, you need to add a new breaker.

If your question comes from the fact that you no longer have any open slots to add a new breaker, most manufactures of circuit panels and breakers offer what is known locally as “half-height” breakers, which are simply two single-phase breakers, molded together, which fit in the slot of one. Using one of them would provide you with the extra room you need. If your needs are somewhat different, post more details and maybe I or someone else can assist further.

Good luck.


05:14PM | 08/15/99
Member Since: 02/12/99
9 lifetime posts
thanks tom for the reply. your right i was referring to two phase 220. it was atype error on my part. however i still am a little shy on 3 phase information. this is for an industrial application.[20 horse frequency converters and various wood duplicating machines and high speed routers
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