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The open bank: two pots, full three-phase

Two pots on the pole, and a full three-phase secondary coming off them. Nothing is missing and nothing is broken. That is an open bank, and once you know what you're looking at, you'll see them everywhere.

The Open Bank. Two transformers in the XFMR Lab sandbox producing a full three-phase secondary.

The full breakdown. More on the channel ↗

There are only two ways to build a normal open bank

Open wye / open delta, and open delta / open delta. You'll likely run into the open wye variant more often.

On the open wye / open delta, both pots' H1 bushings go to their own phase, and both H2s tie to the primary neutral. Two phases and a neutral is all you need, and that is the whole reason this bank exists: the utility only had two phases on site and didn't want to build a third one in.

Open wye open delta in the XFMR Lab sandbox: two transformers with H1s on phases A and B, H2s tied to the neutral, and a wye vector diagram with two windings meeting at a grounded neutral point.

The other two-pot bank is the open delta / open delta. Two pots again, but now each pot connects phase to phase on the primary, and the two transformers are each supplied a different pair of phases. So the open delta needs all three phases up top: two cans on the pole, three phases feeding them.

This bank is much less likely, because if you have three phases present, the main reason for building an open bank disappears.

Open delta open delta in the XFMR Lab sandbox: two transformers wired phase to phase across different pairs of all three primary phases, and a delta vector diagram with two windings and one dashed open side.

📌 Open bank: a three-phase transformer bank built from two pots instead of three. The two builds differ only on the primary side: open wye / open delta takes two phases plus the neutral, open delta / open delta takes all three phases. Going open saves a transformer, not a phase wire.

So how do two pots make three phases?

It's the same answer for both banks. Both primaries are fed off voltages that are already 120 degrees apart: two legs of the wye, or two different pairs of the delta.

Each pot copies its own primary's angle onto its secondary, so the two banked secondaries come out 120 degrees apart too. You only need two primary voltages offset by 120 degrees to make a three-phase secondary.

Put your leads across the open side and you don't read zero. You read the same 240 you'd read across either pot, sitting 120 degrees off both of them. That third leg is real. It just doesn't have another transformer's core behind it.

The open bank secondary drawn as a delta triangle with two coil sides built and the third side dashed, showing 240 volts across every side including the open one.

Erase one side of the delta

A two-pot bank, an open bank, is always a delta secondary. There are some oddball exceptions, but we'll cover those another time.

So here's the delta triangle most of you know by now, and we're going to erase one side. Two transformers, two sides, but all three corners are still present, so all three secondary phases are still created.

The closed three-pot delta secondary above the sandbox bank: three coil sides, the grounded X2 center tap on the lighting pot, and 208 volts on the wild leg corner.

The same triangle with one side erased to a dashed line: two coil sides remain, all three corners survive, and the wild leg corner still reads 208 volts to the ground reference.

Everything else is just the same delta secondary we already know and love: 120 volts phase to ground on the coils with the ground reference, and 208 on the opposite corner. Move the ground reference, move the wild leg.

? You roll up on a pole with two cans and a three-phase secondary. How do you tell which open bank it is?

Answer: Look up. Count what's feeding it. Two cutouts and a primary neutral connection is the open wye / open delta. Three cutouts, one per phase and no primary neutral on the bank, is the open delta / open delta. The secondary looks identical either way, so the primary is where the answer lives.

What does an open bank cost you?

Versus a closed bank, open banks just aren't as good, and here's the simple reason: you can't get as much three-phase out of the same transformers.

The pots are every bit as efficient as they always were. Efficiency doesn't change. It's a kVA capacity issue: you can't use everything you paid for on an open bank, so the pots have to run bigger to make up for it.

The breakdown and the numbers behind all of this are coming soon in another video.

Why one pot is almost always bigger

We already know the lighting pot carries all of the single-phase load. But on an open bank, the lighting pot also carries two thirds of the three-phase load.

So it's almost always going to be a bigger pot than the power pot, which carries nothing but one third of the three-phase load. That's the one big can and one little can you see on these poles.

The XFMR Lab open wye open delta bank with a 50 kVA lighting pot labeled LIGHTING and a smaller 25 kVA power pot labeled POWER.

You can never make three-phase out of single-phase

To button this up: you can never make a three-phase secondary out of a single-phase primary.

A transformer secondary only comes out in phase with its primary, or 180 degrees out. There is no 120-degree setting on a transformer, or anything like that.

So take two pots off the same phase, and their secondaries end up 0 or 180 degrees apart from each other. The vector goes flat, and that's just a single-phase service.

Two transformers fed from the same phase, with a phasor wheel showing both vectors on the same spoke and the empty 120 and 240 degree positions dashed, collapsing the triangle into a flat line.

✅ The short version

Two ways to build a normal open bank. Open wye / open delta: two primary phases plus the neutral, and the one you'll see most. Open delta / open delta: all three primary phases, and much rarer, because with three phases present the reason to go open disappears.
Both work the same way: the two primaries sit 120 degrees apart, each pot copies its own primary's angle onto its secondary, and the open side reads a full 240 even with no core behind it.
An open bank is always a delta secondary. Erase one side of the triangle and all three corners survive, so all three phases survive. The wild leg still reads 208 to the ground reference.
The cost is capacity, not efficiency. You can't use everything you paid for, so open bank pots run bigger.
The lighting pot carries all the single-phase plus two thirds of the three-phase, which is why one can is almost always bigger.
No transformer makes three-phase from a single-phase primary. In phase or 180 out are the only options, so the vector goes flat.

▶ Build it and break it.

You just read the why. You can wire up both open banks yourself in XFMR Lab, the open wye / open delta and the open delta / open delta, then meter the secondary and watch two pots hand you full three-phase in real time. Build it, break it, and see the voltages come to life.

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XFMR Lab is $19.99, one time. All lessons, 16 labs, the Sandbox, and Death Traps included. No subscription.