Electrical – Why are some electrical systems only two phases of a three phase system

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I setup a kiln at our building, and I realized that the electrical system is setup with a three-wire feed – two hots and a neutral. This is fed from a three phase system, though, so the voltage over the two hots is 208v instead of 240v.

Why would something be setup in this way? Doesn't three phase wiring require all phases to be used in order for it to be balanced?

Best Answer

Phase balance is handled upstream from the meter socket

You will see this in two places:

  • Densely developed areas where the low voltage grid is fed from multiple transformers at once (something called "network service" in distribution parlance)
  • Larger multi-tenant buildings that exceed the capacity of a single split-phase service drop from the utility and are not configured to allow for per-tenant utility drops

Either way, what will happen is that the buildings or tenant spaces will be connected to 2 of the 3 phases to get a 3-wire service from the 4-wire wye system, but the phases they get will be rotated/transposed to balance the load:

  • Building/tenant 1 gets A and B phases
  • Building/tenant 2 gets B and C phases
  • Building/tenant 3 gets A and C phases
  • Building/tenant 4 also gets A and B phases
  • ...and so on (there are other ways to arrange them, this is just a demonstration example).

This way, you get an evenly balanced load from several 3-wire services on a 4-wire system. Furthermore, a similar pattern occurs if you go down a 3-phase panel on a 208Y/120 system plugging 2-pole breakers into one side of the panel:

  • Load 1 gets A and B phases
  • Load 2 gets A and C phases (really C and A, but it's irrelevant)
  • Load 3 gets B and C phases
  • Load 4 gets A and B phases again
  • ...and the pattern continues from here

As to implementing this:

  • If these are individual buildings, then the utility will implement this at where the service drop/lateral from the utility splices into their low voltage main (quadplex)
  • If these are tenant spaces in a multi-tenant building, then there will be facilities in the meter bases to control which phase busses the meter sockets connect to (often times, a pair of movable links are used for this, but changing them will require electrician help and a utility power cut to the whole building as they're upstream of the tenant meters)
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