3-pole Load Center

[Bill Adamsen]

I have several 5.0 - 5.5 (16.2 amp max) hp loads on a rotary phase converter (RPC - Phasemaster MA1-B) whose nameplate states that the "largest motor" hp is 7.5, with total of 22.5hp. I have the RPC powered (single phase) by a disconnect switch with a 40 amp two-pole breaker in the main panel, and 35 amp two-pole time delay fuses in the disconnect used to start the RPC. The RPC is wired with 8 awg on the single-phase input and 10 awg 3-phase output. The machines are not run concurrently - though nothing prevents that from happening. Technically, any of these motors could be run on a 12 awg wire, but then that wire would be protected by a 35 amp time delay fuse which ... is likely not safe.

I'd like to extend the 3-phase to some smaller machines - tapping off the disconnect box with conduit and running to several outlets. The first outlet should support that 16.2 amp max load, and then the second outlet should support a much smaller motor in the 2hp range (5.6FLA at 220volts and another 2.4 at 440volts - yes there is a transformer).

My gut tells me I should have a 3-phase breaker panel between the RPC and the machines with at least two 3-pole breakers. A 30 amp 3-pole breaker for the 5.5 hp with 12 maybe 10 awg, and 15 amp 3-pole breaker with 14 awg. Since a 3/4 conduit can hold up to 11/10 awg thhn, I should be ok putting the four 10 awg wires and four 14 awg wires in the same conduit for the less than 40 foot run? I'd also be ok using time-delay fuses instead (probably a 12amp and 25amp respectively)

Is this a rational approach, and if so, is there a small 3-phase load center type panel box with at lease space for 2 3-pole breakers as described that someone can recommend? Alternatively, I would consider some sort of in-line approach if someone has a recommendation.

[David Kumm]

Bill, I use a load center but also have a main run around the shop sized for the heaviest load but then I branch off to fused disconnects for some smaller loads like you are proposing. Three pole fusedd disconnects are cheap on the bay and allow you to fuse down to an appropriate size and reduce wire size accordingly. Dave

[Bill Adamsen]

Thanks David, I like the idea of the main run, but according to the Manufacturer's spec (see attached), I'd be required to run 8 AWG. Is that what NEC would require if I have a 35 amp time delay? Conduit fill recommendations allow 4 8 gauge conductors in a 3/4" conduit - so I could do it with the 35 feet or so of conduit that's already installed (and empty) but pulling 8 AWG sure seems cost prohibitive and overkill.

Correction
Performed the derating and it doesn't appear I can put 4/8AWG in the 3/4 emt conduit, especially since I have two 90 degree bends. According to my calculations I could get as many as 8/10AWG.

[David Kumm]

If you are running the rpc with 40 amps input you can only get approximately 20-25 amps output. THHN is rated at 35 amps for #10 wire. To get the 35 amp output on your rpc you would need 60 amp input and yes I'd put 1" conduit in for the #8 although it is unlikely you could ever run the max 35 amp output for an extended period of time. Dave

[Rick Fisher]

I run a 3 phase panel off my converter. Its an expensive set up, but it should last forever. The breakers are really expensive.