Strange Power Wiring Between Indoor and Outdoor Air Conditioning Units

[jbt260]

I have a split system which calls for 208V, 1ph power supply to the outdoor condensing unit. The outdoor unit then provides power to the indoor unit. The install manual calls for three wires between the units, S1, S2, and S3. Here's the part that is bothering me. S1-S2 is 208VAC. S2-S3 is 24VDC. Has anyone came across this before? Is it allowed by NEC? Can S2 be shared by both voltages?

 

[iwire]

It is allowed and it is pretty common with the smaller split systems.

You do need a disconnecting means at the indoor unit.

 

[Little Bill]

I have a split system which calls for 208V, 1ph power supply to the outdoor condensing unit. The outdoor unit then provides power to the indoor unit. The install manual calls for three wires between the units, S1, S2, and S3. Here's the part that is bothering me. S1-S2 is 208VAC. S2-S3 is 24VDC. Has anyone came across this before? Is it allowed by NEC? Can S2 be shared by both voltages?

It is allowed and it is pretty common with the smaller split systems.

You do need a disconnecting means at the indoor unit.

I take it the S2 goes through a transformer and rectifier then to S3. Is that correct Bob?