1ph vs 3ph disconnect

[mannyb]

Anyone ever install a 3phase disconnect and use it as a single phase disconnect. Basically use the 3phsse as 1phase

 

[jim dungar]

Yes.

 

[Carultch]

Anyone ever install a 3phase disconnect and use it as a single phase disconnect. Basically use the 3phsse as 1phase

Certainly OK to do that. You simply abandon the poles you don't need. Or repurpose them for another circuit, if you'd like to switch multiple circuits at the same time.

It's common that unfused units may only be available in 3-pole options, while the fused counterparts are built for both 2-pole and 3-pole options.

 

[ptonsparky]

That 3 phase unit may have a different HP rating and may not be suitable for 1 phase. It will say on the label

 

[mannyb]

Certainly OK to do that. You simply abandon the poles you don't need. Or repurpose them for another circuit, if you'd like to switch multiple circuits at the same time.

It's common that unfused units may only be available in 3-pole options, while the fused counterparts are built for both 2-pole and 3-pole options.

What about as service equipment? If so what would be a good argument to make to inspector

 

[Carultch]

That 3 phase unit may have a different HP rating and may not be suitable for 1 phase. It will say on the label

I'd expect that the HP rating of a disconnect would correspond to the same Amps per pole at the same voltage. This means I'd expect the single phase HP rating to be 57.7% the 3-phase HP rating. I.e. 1/sqrt(3).

I looked up an example, where the 3-phase rating is 25 HP and the single phase HP rating of the same product is 15 HP. The theoretical single phase HP rating, assuming the same FLC as the 25 HP motor (and all other factors equal), is 14.4 HP, which is consistent with 15 HP. The 15HP rating on the datasheet is slightly more permissive than theory, but it's what I'd expect within typical rounding to standard values.

 

[Carultch]

What about as service equipment? If so what would be a good argument to make to inspector

Service equipment requires that you have a fused disconnect anyway, so you aren't put in a situation where the only viable product would require the abandoned pole. You can get disconnects that are purpose-built to the required number of poles, and usually the 2-pole unit is slightly cheaper than the equivalent 3-pole unit. It still has mostly the same components, just with a blade and a fuse holder omitted from the assembly.

I'm not aware of any code rule that either permits or prohibits abandoned poles on service disconnects. I recommend either clearing it with the AHJ in advance in writing, or plan on using a 2-pole unit. What would be restricted, is what you do with the otherwise-vacant pole. I wouldn't count on using the otherwise-vacant pole for anything, if you were using it as a service disconnect. Keep it vacant and keep the spare fuse elsewhere (if applicable).

 

[jim dungar]

I'd expect that the HP rating of a disconnect would correspond to the same Amps per pole at the same voltage...

The horsepower rating is applicable for the making and breaking of a single motor locked rotor/starting current. If there are multiple motors fed by this switch its HP rating is immaterial.

 

[jim dungar]

I'm not aware of any code rule that either permits or prohibits abandoned poles on service disconnects.

Listed multi-pole overcurrent protective devices have been tested to protect all of the various pole configurations possible; a 3-pole device has been tested with 1, 2, and 3 pole configurations.