3 ph Xfmr feeding 1 ph Load

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nhee2

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I came across an existing installation - a 30 KVA 480 Delta - 208/120 Y transformer is installed, supplying a 120/240 single phase panel. I am assuming in this config

- Transformer would be good for 20 KVA
- Required primary / secondary OCP requirements would be the same as if a 3 ph. load were being supplied (250% primary, 125% secondary)

The existing installation has a 60A primary bkr with 400A secondary bkr, so I'll recommend to the owner they change the secondary OCP. Are there any other concerns with running the 3 ph. transformer as single phase supply?
 
So you think they took two legs from the transformer to the 240/120V panel? This means one phase of the transformer is not being used so any loading will be very uneven. This can cause hot spots in the transformer windings and probably premature failure.

You also have increased load on the neutral conductor and bus which would normally carry just the imbalance between phases, but is now carrying roughly the sum of both phases. Similar to if you had high harmonic loads. This could go poorly if your loads max out the rating of the panel. Not sure where the NEC talks about this but could overheat the conductors/bus, melt the insulation, and then arc/short.

Proper secondary OCP size is a good start.
 
The site looks like it was put together with whatever was on the back of the truck when they got there. I am not doing anything with the 120/240 panel, but had noted that the installation was run with SE cable strung off of existing conduits to the transformer then to the load center. Did not look at any terminations as I did not have tools or permission to take off covers.
 
Why would the transformer care if one phase is unused?

As long as it is not overloaded, I just do not see any issue.

Your neutral is still a neutral. The two phases are 120 degrees out of phase so you will never have zero current flow in the neutral but it will still always be less than the max phase current.
 
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Bob that was my thinking - as long as the load on the panel is kept below 20 KVA for the two connected legs, the transformer will not be overloaded. Note ideal but not an issue provided the OCP and conductors are properly sized. Just wanted to throw it out there in case I was missing something.
 
Why would the transformer care if one phase is unused?

As long as it is not overloaded, I just do not see any issue.

Your neutral is still a neutral. The two phases are 120 degrees out of phase so you will never have zero current flow in the neutral but it will still always be less than the max phase current.
With either exactly balanced or completely unbalanced, the neutral current will be equal to the highest phase current, not less than.
 
With either exactly balanced or completely unbalanced, the neutral current will be equal to the highest phase current, not less than.

It seems to me that with the same load current, the neutral current could be somewhere from less than the phase current to more the the phase current, depending on the power factors of the loads.
 
Yea, at a minimum I'd want a neutrals the same size as the phase conductors.
Given just the right power factor you could end up with higher neutral current on the neutral than either phase. This would happen if the two loads were completely in phase with each other. The neutral would then be the negative sum of their combined currents. Not very likely though.

I don't think the zero load on one coil of the transformer is inconsequential. I am not a xfmr engineer but these units are not at all designed for this condition. I would talk to someone at Square D or similar, I think there could be real issues. Normally the magnetic flux from the three coils is roughly balancing each other out, in this condition it's not. I would think there is a lot of vibration, noise, etc. It could also cause a voltage imbalance upstream since only two of three phases are loaded. I don't know what's supply the transformer, but a generator would not be happy with a large imbalance.
 
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