240/120V systems

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Rick 0920

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My office has a 240/120V service with "B" phase as a high leg. I understand that line to line 240V loads are fine for use on "B" phase. But what about equipment that has subdivided 120V loads? For example, the gym has an electric dryer and the break room has an electric range. Can these appliances be damaged if wired using "A" and "B" phases or "B" and "C" phases?
 
Anything that has a neutral needs to make certain you don't get that high leg on any 120 volt loads. With common cord and plug connected single phase items is probably bad idea in general to even attempt to use the high leg on such circuits. Swap appliances and not know there is a high leg there might find out the hard way there is a high leg there. Straight 240 volt receptacle- a little more appropriate as there shouldn't be neutral connection in those appliances.

The other thing that factors in here is to connect such circuits to a 2 pole breaker you are supposed to use a straight 240 volt breaker instead of a 120/240 rated breaker. Not that they aren't available but they often are not as readily available and when you can get them they are kind of pricey in comparison to a 120/240 rated breaker. You don't have this problem with three pole breakers because they are rated straight 240.
The straight 240 two pole likely cost as much or even more than a three pole though.
 
Rick 0920 said:
My office has a 240/120V service with "B" phase as a high leg. I understand that line to line 240V loads are fine for use on "B" phase. But what about equipment that has subdivided 120V loads? For example, the gym has an electric dryer and the break room has an electric range. Can these appliances be damaged if wired using "A" and "B" phases or "B" and "C" phases?
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Yes, they certainly can. One line would see 208v to the neutral.
 
You might also note that if you are using any 2 pole breakers that will connect to "B" phase, they must be "straight rated 240" as opposed to "slash rated 240/120" (see 240.85)
 
A high-leg delta is a standard 1ph 120/240v source AND a standard 3ph delta source, superimposed on each other. In fact, the high-leg, open-delta service originated as a modification to existing 1ph services for new equipment.
 
Rick 0920 said:
My office has a 240/120V service with "B" phase as a high leg. I understand that line to line 240V loads are fine for use on "B" phase. But what about equipment that has subdivided 120V loads? For example, the gym has an electric dryer and the break room has an electric range. Can these appliances be damaged if wired using "A" and "B" phases or "B" and "C" phases?
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It sounds like you're asking specifically about appliances that need two hots and a neutral. Yes, they could still be damaged if the internal parts that use 120V are connected to the B phase and see 208V. You might get lucky and end up with those parts connected to the other phase on the two pole breaker. But it's a stupid risk to take, basically like flipping a coin to decide if you'll damage the appliance. Also some appliances use both 120V sources for different parts internally, to balance the load.

The B phase should only be used for equipment that doesn't need a neutral.
 
Cooking appliances that use nichrome heating elements sometimes don’t like the high leg. (Straight 240, no neutral, such as a cooktop) I had one at a church where the element popped. Could have been bad manufacturing, but changed the phase anyway. The only thing I could figure was the higher voltage to ground on that leg, but then again, it may have just been a fluke.
 
These services can be build differently, and often the B is supplied by a smaller transformer, and often in an "open delta" wiring. If the feed is built with a center-tapped transformer as A-N-C and a second transformer as A-B, you could use A-B for 240v loads (and not put a load on the A-C xformer), but do not connect that load to B-C as now you're loading both transformers. (Look at some wiring diagrams.)

IMHO, better to leave the high-leg for 3-phase loads only and not have to worry about it.

See also https://forums.mikeholt.com/threads/120-240v-3-phase-4-wire.2571605/
 
Hoshizaki 208-230V 1Ø ice machines require a neutral because of the pump, condenser fan, water valves are 120V, the control board will shut the machine down if it senses any leg is over 140V, a machine was installed at a farm shop & would not work when it had worked at it's previous location, turned out the electrician had used the high leg & another phase for it, as soon as they no longer used the high leg it started working. That one was a fun one to troubleshoot over the phone, and is a good example why the high leg should not be used for L-N loads

As to why a farm shop needs a ice machine, regs require that farm workers have access to cool water, having a ice machine helps keep them hydrated in the heat of working in the field.
 
zbang said:
Or that if it's an open-delta and you use the wrong leg, you end up loading both transformers, not just the one making the open-delta.
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I think there is only two ways to do that, one is with high leg to neutral connected loads

Other is , assuming C is the high leg and B is the common leg to both transformers, that if you connected a single phase load between A and C.

Ok a third way is when supplying a true three phase load where you also will have some current between A and C.

A load between B and C would only load the high leg transformer in an open delta source.
 
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