Single-phase vs three-phase static transfer switches

Match the switch to the supply the load already uses. The real decision is the pole count: which conductors the switch has to break so that two independent sources stay independent.

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2-polesingle phase, 100–240 V, 12–50 A

3-polethree phase, 200–480 V, 10–600 A

4-polethree phase plus neutral, 200/115–480/277 V

What each delivers

Single-phase power is volts times amps. Three-phase power is 1.732 times line-to-line volts times amps, so the same current delivers far more. A 50 A single-phase switch at 208 V carries 10.4 kVA. A 50 A three-phase switch at 208/120 V carries 18 kVA. A 600 A switch at 480 V carries just under 500 kVA.

Which conductors get switched

A transfer should look like unplugging the load from one outlet and plugging it into a completely independent one. Choose a pole count to see what that means for each conductor.

3-pole or 4-pole?

If the two sources are separately derived, each with its own neutral-to-ground bond, and the load uses the neutral, the neutral has to transfer with the phases. Tying two source neutrals together creates a second ground path, circulating current and a code violation. That is what the fourth pole is for, and it is the normal case between two UPS systems or two transformers feeding 208/120 V or 400/230 V equipment.

If the load is three-wire with no neutral, as many 480 V loads, drives and motors are, a 3-pole switch is correct. The same reasoning is why TwinSource single-phase switches are 2-pole: both conductors transfer, whether the circuit is line-to-neutral or line-to-line.

How to size a switch Three-phase families Single-phase families

Questions engineers ask

Can I use three single-phase switches on a three-phase circuit?

It is not recommended. Each would transfer on its own, which can leave the load with phases from two different sources. Use a three-phase switch that moves all poles together.

My rack is three-phase but the servers are single-phase. Which do I need?

Follow the circuit feeding the rack. If the rack PDU has a three-phase input, use a three-phase switch ahead of it, 4-pole if the PDU uses the neutral.

Do 50 Hz systems change anything?

Only the arithmetic. A quarter cycle is 5 ms at 50 Hz and 4.2 ms at 60 Hz. All TwinSource switches operate at either frequency.

Put it to work

Narrow the range to your voltage, current and mounting, or send us your one-line diagram and talk to an engineer.

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