Control which Leg gets 0 load

[dtiemeyer]

My engineer showed me a 240v 3 phase panelboard which uses 3 pole breakers but one of the legs (always the center B leg) is 0 load, and the load is distributed always across A-C legs. How can I control the legs that a load lands on in this case. In trying to modify the single-phase subpanel to control this, I realize I do not know a way to separate the load of what is being powered from what it looks like being powered by an upstream panel; essentially I want to take all 3 legs of connected load and distribute them to 2 legs outgoing.  Currently the only way I see to get the loads on the desired legs A-C, is to move the breaker up or down in the panelboard, which is undesirable. 

[HVAC-Novice]

I assume B-leg to be unused was chosen by Revit when balancing the load. If you add other loads to A, or C, and re-balance, it might jump tp use B. 

What type of system is it? Is it 240V between legs, or from leg to neutral? Do you mean the European 230/400V system? What device would only use 2 phases? Why a single phase panel with a family that seems to use 3 poles? Why a 3-pole breaker on a 2-pole device? 

[dtiemeyer]

@HVAC-Novice Please see responses from my engineer:

What type of system is it?  240V 3-Phase High Leg Delta

Is it 240V between legs, or from leg to neutral? This is fun, 3-Phase High Leg has a 240 leg to leg. The A leg and the C leg are 120 V from leg to neutral, the B leg is 208V from leg to neutral.

Do you mean the European 230/400V system? No, see above.

What device would only use 2 phases? The distribution panelboard feeding the panel we are working on is 1 Phase (2 legs) 240/120V. To not have the B leg going to the panelboard, this would create a 208V leg to neutral which is not something most equipment is rated for, a three-pole breaker is connected to the panelboard because the buss goes A>B>C>A>B>C… as you go down the circuits on a panelboard. This means that to get only the A and C phases, you would have to pick a specific breaker number like 5,7 to get a situation where it is C>A between two poles in sequence.  

Why a single-phase panel with a family that seems to use 3 poles? Answered above

Why a 3-pole breaker on a 2-pole device? To avoid the B leg, a three-pole breaker is attached but no conductors are connected to the B leg only the A and C, neutral and ground.  

[sragan · Accepted solution]

Revit 2025 has the ability to model high leg systems, which wasn't available in previous systems.

But I still assume you probably have to move the breakers around to get them on the right phases.   Or maybe you could assign "Space" to all the B phases before circuiting loads.  Then they will be forced to use A and C.

[HVAC-Novice]

I have nothing further to contribute, but this explanation of high-delta may be useful.

[dtiemeyer]

@sragan thanks for pointing this out, I hadn't noticed that yet, since we haven't started on a project in 2025 yet, the project for this question is in 2024. Cheers. And thanks @HVAC-Novice for digging deeper with the questions and engaging the convo to get us to the answer.

[sragan]

See attached for the Revit 2025 Distribution settings, with the high leg options.