Range calculations 220.55

[tortuga]

If you have transformer windings connected A-N-B and A-C, any single-phase load on C-B will flow through and load both transformers; not an optimal configuration. Similarly, a load on C-N will flow through A-C and A-N.

Yeah I agree, there would be no need to use the 'open phase' with the OP's loads.
If we had a 37.5 lighting pot and a 25 power pot and put one of the 15.1 kilns across the open phase
then the lighting pot would reduce to 27.6 and the power pot to 13.8,
so using the open phase would cost ~6kva more than just using the power or lighting transformers.

I would just simply put a 15.1 kiln and the 9.7 kiln on the power pot and one 15.1 on the lighting pot then you'd have 22.4 kva left over for the rest of the loads.
I am sure that a open delta would cost less than a wye as its one less transformer to provide, mount, wire maintain etc.

 

[Elect117]

IA where the phase current is IAB-IAC (depending on vector notation and blah, blah)
IN Grounded leg. Should be close to zero depending on balance.
IB Where the phase current is IAB
IC where the phase current is IAC

The open transformer phase is BC in this case. So there is no current vector for BC to subtract from line IB and IC. Lines IC and IB will function more similar to "single phase lines", and in that regard, are less efficient.

If the split phase transformer (120V) is between A and B then you should not have a load on C-N as that is the 208V leg (measured phase to ground).

The 120V (phase to neutral loads) should flow through the center tapped transformer in the open and closed delta case. That is why that transformer is supposed to be sized larger than the others.

This is starting to get into the weeds on the OP.

The main point being, that having the utility install one transformer bank and running a single phase 120/240V house and a 120/240V 3ph commercial space is most efficient and cost effective way to load up on two pole loads. This would reduce the service size and by relation, make it cheaper to construct. Wire on the service conductors, bonding conductors, etc. will all be able to be reduced in comparison to the single phase service size equivalent for all of those two pole loads.

A 120/208V 3ph service will still reduce your service size and therefore the cost. The only issue is the voltage being lower might impact the equipment that you already own (if sensitive). You will also have to be more vigilant on voltage drop if running 230V loads on a 208V system.

In both cases, 3ph is the cheaper option in material and most likely labor as well. It also allows for the use of 3ph appliances when replacing or expanding.

 

[tortuga]

IA where the phase current is IAB-IAC (depending on vector notation and blah, blah)
IN Grounded leg. Should be close to zero depending on balance.
IB Where the phase current is IAB
IC where the phase current is IAC

The open transformer phase is BC in this case. So there is no current vector for BC to subtract from line IB and IC. Lines IC and IB will function more similar to "single phase lines", and in that regard, are less efficient.

Syncro covered it pretty well in this thread:

Trouble finding straight 240v breaker vs 120/240 breaker

There is no real derating necessary for using the 'open' phase. Typical formula for sizing the unequal transformers are: Power pot = .58T Lighting pot = .58T + S Where T is the total three phase load and S is the total single phase load. Note how the transformer sizing takes into account the...

forums.mikeholt.com

The main point being, that having the utility install one transformer bank and running a single phase 120/240V house and a 120/240V 3ph commercial space is most efficient and cost effective way to load up on two pole loads.

The MH forums is doing its part to bring back 240D services.