Understanding Delta 3 phase 4 wire

[zcanyonboltz]

I know that the B phase of a Delta system is the high leg. Does the high leg only get used on a single pole breaker to supply 208V to that circuit? I am thinking about what voltage you would have or if this is even how its done if you had a two pole breaker with A and B phase or C and B phase? From what I've read a 3 pole breaker with ABC would be 240V, is this correct?

 

[kwired]

First the way load can be distributed within a delta source depends on a few factors.

Whether or not you have a full delta or if you have an open delta

Whether or not you have different size "lighting transfomer" (the one that has the mid point grounded) compared to the other transformer(s) in a bank of transformers that make up the delta system.

High leg is more less a by product of making a system that does have the ability to supply both 240 volts (single and three phase) as well as 120 volt single phase loads. If you want to have 120 volts on all three phases you need a wye connected source - but by doing this you give up 240 volts and have to settle for 208 volts phase to phase.

The 208 volts to neutral of the high leg are not intended to supply single phase 208 volt loads. Though it may work you introduce other complications both with compatibility of some equipment (you won't find single pole breakers rated for 208 volts unless you get into the 277/480 type breakers or 600 volt breakers which will not fit your typical 120/240 only panel) or potentially overloading conditions within the transformer should you put too much 208 volt load on it.

 

[ActionDave]

I know that the B phase of a Delta system is the high leg. Does the high leg only get used on a single pole breaker to supply 208V to that circuit? I am thinking about what voltage you would have or if this is even how its done if you had a two pole breaker with A and B phase or C and B phase? From what I've read a 3 pole breaker with ABC would be 240V, is this correct?

You have 240V line to line on all three phases of a Delta system. You can use two pole breakers on A,B and B,C to feed straight 240V loads. If there are any 120V loads you stay away from the high leg.

 

[JFletcher]

208V->N could be used in very limited capacity to serve, say, lighting loads with a xfmr with a 208V tap, but iirc 2 transformer bank open delta is limited to 5% of its capacity with wild leg (B phase) to neutral loads. You have to use a straight rated 240V breaker for this, not a slash rated 120/240V.

eta: and yes, you can run 3ph 240V loads with an open delta configuration.

 

[zcanyonboltz]

Thanks for the info! Makes sense now.

 

[Smart $]

... You have to use a straight rated 240V breaker for this, not a slash rated 120/240V.
...

To set this completely correct, any breaker connected to the B high leg cannot be 120/240V [slash] rated... must be 240V [straight] rated. Doesn't matter how many poles.