Oh right LOL I do better with diagrams
Establishing 240/120V service with 480V Split Phase
- Thread starter Sean.Day72
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wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Hmm, in terms of load VA supported for minimum coil currents, we'd be better off taking our 4 coil series autotransformer and connecting it to the 480V N and L1 only. So it would be an unbalanced load on the 480V supply, and we'd only be using 3 of the 4 coils. But then a 10A 120V load does show up as 5A on the 480V line and neutral, giving us a 2:1 ratio rather than 4/3 : 1.
Cheers, Wayne
Cheers, Wayne
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I was thinking that code requires a connection to the grounded conductor.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
210.9 says "Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer." 215.11 is similar.
So I don't think that actually says that the grounded conductor has to be connected to the autotransformer, just that the circuit supplied must be connected to the grounded conductor.
You agree that only 2 out of 3 of the 240/480V system conductors can be connected to the autotransformer, because of the circulating current issue?
Cheers, Wayne
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Good point on the code requirement; I'd read it that the autotransformer must be connected to the supply neutral, but I believe you are correct.
I also agree that if you connect the neutral on the supply side, the transformer will try to 'balance' the neutral relative to the hots, so you might have circulating current issues. On the other hand if you _don't_ have the supply neutral connected to the neutral point of the 4 coils, you have some weird current flow paths for L-N loads; the N essentially bypasses the autotransformer entirely.
On the plus side, if the neutral bypasses the autotransformer entirely, then the bizarro system we are discussing could be build with a standard 240/480 : 120/240 transformer; you would arrange the 240V coils in parallel, and then arrange the series coils 120-240-120. The 480V would be connected to the far ends of the 120V coils, and the 120/240V tapped from the ends of the 240V coil and the supply neutral.
And the complexity of the above is all the more reason to support the OP's decision to use a bog standard 480V:120/240V isolated transformer rather than any sort of autotransformer connection.