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Old 02-17-2009, 01:28 AM   #16
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Understanding 120V branch loads in 100A 240V system


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Originally Posted by GoyimPersuasion View Post

I need to research this more, but for now, for a multiwire circuit, why would the load become 240v if the neutral was cut, if the receptacles were designed for 120v? Wouldn't the equipment on the 120v just not work?
Thanks for all replies.
It's the same as putting some of the equipment in series with other equipment.

This can cause the voltage on a particular load to appear to be anywhere from 0 to 240v on a MWBC with a broken neutral. It would appear to be 240v if there were a short circuit on what was the other leg, and it would appear to be 0v if there was nothing plugged into the other leg.

If the legs were perfectly balanced, then all the 120v stuff would work normally, at 120v... until the load changed on one leg, that is.

This usually manifests itself as "lights get brighter when I turn an unrelated appliance on" type problems.

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Old 02-17-2009, 07:49 AM   #17
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Understanding 120V branch loads in 100A 240V system


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Originally Posted by GoyimPersuasion View Post
This implies that all 240v loads are balanced no matter where one places them on the service panel, since each 240v load, first doesn't use the neutral, second they draw the same amperage from both legs?

In addition, just made the connections from the replies, that since 240v draw from each 120v, and each 120v is 180 degrees apart, and also since each supply leg is also a return leg (both are supply and load side), therefore, there is cancellation on each side of a 240v load.
Actually not all 240 volt appliances will be perfectly balanced. There are some appliances that are 240/120 volt loads where the neutral is used for some 120 volt applications (usually control circuit or lights, etc). The 240 volt portion of the appliance will draw equally from both legs, but the 120 volt portion will draw an additional load on only one of the legs to neutral. Things like electric dryers and stoves come to mind. Water heaters and baseboard heaters are almost always 240 volt loads with no 120 volt application in them.
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Old 02-17-2009, 09:15 AM   #18
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Understanding 120V branch loads in 100A 240V system


>>> cancellation on each side of a 240 volt load

It is actually augmentation (summation, aggregation) on the legs and cancellation on the neutral when we have both 120 volt loads and 240 volt loads.

Suppose we have A side 120 volt loads, B side 120 volt loads, and pure 240 volt loads all turned on at the same time.

Current comes on the A leg to serve the A side loads at the same time current comes on the A leg to serve the 240 volt loads so the total current is the sum of what these loads draw.

Current goes on the neutral for the A side loads and comes on the neutral to serve the B side loads, at the same time resulting in some or possibly full cancellation.

Current goes on the B side for the B side loads at the same time current goes on the B side for the 240 volt loads.

(For 60 Hz alternating current all the "comes" and "goes" switch places every 1/120'th second.)

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Last edited by AllanJ; 02-17-2009 at 09:23 AM.
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