I could never figure out why the load on the two legs has to be balanced, do your electric devices all turn on and off at the same time? There is always going to be an imbalance.
Yes, 120V loads may be turning on and off at various intervals but there are times the various loads are on at the same time. That is why the known 120V loads should be balanced somewhat within reason on both Lines, legs.
Known 120V loads? Refrigerator, dishwasher, microwave, kitchen two small appliance circuits above the counter tops, chest freezer, central vac, furnace (blower motor. Also used for central air conditioning), cloths washing machine, multiple lighting circuits, sump pump, Bathroom(s) above the counter outlet(s) for hair dryer(s), ect.
Here’s a big problem I see... Occasionally I will read a post on an audio forum where the home owner, audiophile, on his own moved all the 120V motor loads, plus micro wave, and other such loads to one Line, leg, and have his audio equipment on the other Line, leg. This is not good for the home owner’s electrical service or the utility power company’s transformer. Especially the utility power transformer. The power company does not size the transformer by adding up the size, amperage rating, of all the electrical services that are fed from the transformer. The Power Company roughly calculates the average demand load that might be placed on the transformer. A power transformer is most efficient when it is loaded near its rated KVA rating. A Utility Company never adds in a fudge factor when sizing the transformer. Fudge costs money...
The secondary of a typical residential housing development power transformer have what is called a split phase winding. It will have a 240 volt winding that is tapped at its center point. From either outer lead, leg, of the 240 volt winding to the center tap (neutral) it will measure 120 volt, nominal. Basically two 120V windings that are in series are created. With this type of a winding only the unbalanced 120V loads return on the service neutral conductor to the transformer neutral. The balanced Line 1 (L1) to neutral and Line 2 (L2) to neutral 120V loads are in series with one another and are being fed by 240V.
Example, if there is a total combined load connected to L1 to neutral of 50 amps and a total combined load of 40 amps connected to L2 to neutral, only 10 amps will return on the service neutral conductor to the transformer neutral. The balanced 40 amps on each Line, leg, will be in series and will be fed by 240V. The transformer will see a total of 40 amps through the entire 240V winding and an additional 10 amps through L1 to neutral split winding.
Here is an example of a step down split phase secondary winding transformer. Such a transformer might be used to feed an audio system’s equipment. Primary wired 240V secondary 3 wire 120/240V. I will use a 10KVA power transformer with dual voltage 120V/240V secondary windings for the example. The secondary has two windings. Each winding is rated for half of the 10KVA, therein 5KVA each. Each winding has a voltage rating of 120V, nominal.
Some math:
5KVA / 120V = 41.67 amps maximum FLA.
So if the transformer’s secondary is configured, wired, as a 3 wire 120/240V power system from each Line, (L), leg, to the neutral leg the maximum 120V load(s) that could be connected is 41.67 amps.
L1 to neutral 120V load(s) 41.67 amps and L2 to neutral 120V load(s) 41.67 amps.
The two secondary windings are wired in series. The center point of the two windings is the neutral. (Per electrical code the neutral shall be grounded). From either outer lead, leg, to the neutral will measure 120V. From the outer leads, legs, of the entire series winding the voltage measures 240V. (5KVA + 5KVA = 10KVA / 240V = 41.67 amps maximum.)
For a full connected load example:
If L1 to neutral has a connected load of exactly 41.67 amps and L2 to neutral has a connected load of exactly 41.67 amps, zero amps will return on the neutral conductor to the center point (neutral) connection of the two series connected windings. The two 41.67A loads are in series with one another and fed from 240V. Current through the series 240V winding will be 41.67 amps. 240V X 41.67A = 10KVA... 10KVA / 240V = 41.67A
(Note: The neutral connection for each 120V load is still, must be, connected to the transformer neutral leg. The connection maintains a stable 120, nominal, voltage).
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If the above secondary winding configuration is used to feed an electrical panel that will feed audio equipment the two 120V Lines, legs, 120V loads should be balanced somewhat within reason to draw power from both windings of the secondary. Say you have two mono amps that have an FLA of 8 amps each you would not want to put both amps on the same Line to neutral. That would load up one winding of the secondary. The correct way would be to connect one amp to L1 to neutral and the other amp to L2 to neutral. The balanced 120V loads of the two mono amps will be in series and fed by 240V.
How about those that say that all audio equipment that is connected together by wire interconnects should be fed from the same Line, leg, to neutral? Well as you can see from above if all the equipment was fed from just one Line, leg to neutral only one secondary winding of the 10KVA transformer would be used. 5KVA with a maximum FLA connected load rating of 41.67 amps.
If you want to have all your audio equipment fed from one Line, leg, and still have the full 10KVA rating power available wire both secondary windings in parallel, (OBSERVING POLARITY). In parallel the voltage out will be 120V only.
The full 10KVA power rating is available.(5KVA + 5KVA). Maximum amperage rating available for 120V is 83.33 amps.
10KVA / 120V = 83.33 amps.
Primary of transformer wired 240V, secondary wired for 120V.
(One leg of the 120V secondary shall be grounded)...
(An electrical panel is required).
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How a 3 wire single phase split phase transformer winding works:
https://www.youtube.com/watch?v=eVamt9IdQd8
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