hypothetical circuit

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jes25

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Midwest
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Electrician
Let's say we have two SDS systems, 277V line to neutral and 120V line to neutral. Neutral is grounded for both systems. What would happen if one attempted to run a 3 wire circuit with both 277V and 120V loads? Essentially,share a neutral on two different systems. My 1st thought is the current on the neutral would be double, but what about frequency differences?
 
Of course you wouldn't actually do this, (sure its a code violation), but assuming the neutral isn't overloaded, I think it would actually work just fine.

Not sure what you are asking about the frequency. That would be 60 hz for both systems. There might be some phase angle differences, depending on the transformer(s) used.

Steve
 
If the phase angles were appropriately balanced, then the current on the neutral would balance.

Imagine a custom single phase transformer with a 397V secondary, with a tap placed 277V from one end and 120V from the other end. The neutral on such a beastie would behave in the same fashion as the neutral in a 120/240V center tapped secondary.

-Jon
 
The utilities often use a common grounded conductor for their distribution systems. The same conductor is both the primary and the secondary grounded conductor. Of course when you do this you no longer have a SDS as you have a circuit conductor that is common to both systems.
 
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