In Fremer’s case the sub panel could have been wired for 120/240V from the start. All the audio equipment still could be fed from one Line, leg. The electrician would have increased the size of the feeders knowing he would be feeding all the branch circuits from just one Line, leg.
If an isolation transformer was used with dual primary and dual secondary windings and the primary is wired for 240V, and the secondary is wired for 120/240V to feed a 120/240V panel unless the transformer is oversized to feed all the audio equipment branch circuits from just one Hot leg to neutral the audio equipment branch circuits would have to fed from both 120V Hot leg to neutral. Therein the loads some what balanced across both 120V legs.
An example, a 7.5KVA transformer is selected for use. The transformer has dual windings. Each winding is good for half of the 7.5KVA rating. 7500VA / 2 = 3,750VA at 120V each. 3,750VA / 120V = 31.25A.
So if the secondary is wired for 120/240V, (two windings are wired in series), and only one 120V hot leg to neutral was used, (Only one 120V, 3,750VA winding), it’s only rated for 31.25A max... If both legs were used to feed the audio equipment the full 7500KVA rating of the transformer would be available. 31.25A on one leg and 31.25A on the other. But the audio equipment would be fed from both legs. Not good for audio equipment that is connected together by wire interconnects... (One Hot 120V leg to the other Hot 120V leg measures 240V.)
If the two secondary windings are wired in parallel the full 7500VA is available.
31.25A + 31.25A = 62.5A Max available.
So in this case the electrical panel would be wired for 120V only.
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