Breaker Sizing - 3-phase Load Center

[Bill Adamsen]

I have a mid-80s Italian 28 inch Centauro Bandsaw which runs fine and has been forever. It has what looks like the "factory" power cord - a twelve foot long 12/4 SJO. The saw is powered by a 3-phase 5.5hp 16.2 amp motor (Motori Bonoro Cento).

This brings up a more fundamental question related to an overall redesign of the circuit for this machine. It is currently on (plugged into) what is effectively a 35 amp circuit (40 amp breaker and 35 amp time delay fuses) since it is powered by an RPC which is powering numerous 3-phase machines in the shop. The saw has never tripped that breaker or blown the fuses. Testing load is a bit problematic, with the fuse/breaker ahead of the RPC, I'm not sure which I'm actually testing if I reduced the size of the initial breaker. Clearly, a 12/4 cord is not rated for 35 amps, but that's not the question here. I'm looking to add a 3-phase load center between the RPC and tools which would provide a solution to that problem. What size breaker is this machine going to require to run? I know that selection of that breaker is going to have some impact on the THHN I was going to run for that circuit. For instance #12 is "rated" at 25 amps, but limited to a breaker no larger than 20 amps (per NEC 310-16). Theoretically, the 12/4 cord is sized just fine for a 20 amp circuit, I'm just not sure the machine will start on that, and I don't have a good way (I'm not experienced in this - see above issue with testing circuit) to test before installing the load center. Also, does the cord of that gauge have any impact (voltage drop or resistance?) on the breaker sizing?

Looking for experienced views. Thanks!

[David Kumm]

Bill, 16 amps sounds more like a 7.5 hp motor ( most 5 hp are 12-13 ) and a bandsaw pulls some extra while starting up. I tend to run wiring of #8 and #10 primarily has my Phase perfect output maxes at 30 amps continuous. I'd run #10 with a 25 or 30 amp breaker. Cost difference isn't that great although #8 gets you into bigger conduit or flex. I tend to oversize as I change machines around and what serves a 3 hp motor one year might serve a 7.5 next year. Use L15-30 for most everything 3 hp and over so I can change my mind and move stuff around. #12 in a circuit just limits too much with not much cost benefit. Dave

[Bill Adamsen]

That mimics my thinking. I too use L15-30 ... and that brings up an interesting question. What would the inspector (dare I say insurance company?) say if there is an L15-30 outlet on a 25 amp circuit? I guess I need the L15-20 and a jumper or, put in #10 and 30 amp breaker? Another question, let's say I choose #10, and I'm planning on running this to a junction box with mixed 3-phase and single. Can I leverage that #10 ground as my "common" ground for both the single and 3-phase assuming it is the heaviest gauge?

[Jeff Duncan]

As far as "will it work" part of the equation, almost surely. I've run 7-1/2 and 8 hp motors (roughly 21amps) on 20 amp circuits somewhat successfully at my last shop…(this refers to 3 phase equipment only!) It was for short term temporary use, but they did run on the circuits. Motors draw a lot of power at startup but don't run at that rated amperage under use. If the motor draws too much amperage, like on an extra heavy cut, you'll blow a fuse….or pop the breaker. Start up shouldn't be a problem as slow blow fuses and breakers can handle it. At 16 amps your right at the line where you would go from a 20 amp to a 30 amp circuit. I think you'd be fine on a 20 amp fuse, however unless your running a long distance there's probably not a big difference in cost going to the larger circuit. In my new shop I ran 10 gauge to everything except the really large machines as it was just easier to have the flexibility down the road. I will say 10 gauge was a PITA to run as it's a LOT heavier and a LOT less flexible than 12 gauge. But I just ran 3 gauge for my wide-belt Monday and that makes the 10 seem like spaghetti

A couple little nit picks I'll add, to make sure Dave is paying attention In 3 phase a 7-1/2 hp motor would be roughly 20-22 amps. A 5 hp would be right in the neighborhood of your saw at 16 amps. Also while wire upsizing isn't going to hurt anything, #8 would be for 40 amp circuits! So unless you see yourself getting into 10 hp and up motors, probably not worth the expense. I do agree though that if your "growing" a shop #12 can easily be outgrown.

As far as the plugs….my rule of thumb is to use the rated cap. of the circuit or less. So 30 amp plug on a 25 amp circuit would be a no-no for me, not sure about your local inspector?

good luck,
JeffD

[David Kumm]

Jeff, how you been? My three phase motor plates run 12-13 for 5 hp, 16-18 for 7.5 and 21-23 for 9-10 hp. A single phase 5 hp will go about 19 amps. Dave

[Jeff Duncan]

Hi Dave, doing better now that the shop is mostly operational! Still a few machines running off extension cords and no air lines yet, but making progress!

There's always a bit of difference between different motors, but my dust collector and shapers are all in the 19 - 22 amp range. At 16/17 amps I'd think your closer functionality-wise to a 6 hp motor than 7.5? Only thing I have bigger, (except for the wide-belt), is the planer at 9hp, not sure what that pulls though as it's a PITA to get to! I thought my compressor motor was 15-16 amps at 5hp, but I'll have to double check.

JeffD

[Bill Adamsen]

Thanks folks. I made room for the new 3-phase load center and ordered several 30 amp and a 20 amp breaker(s). Based on the run lengths, I am planning to use #10 for the 30 amp circuits which based on your comments and my own analysis, makes me feel pretty confident should start and run that bandsaw. That was really the big question. It is the only machine where I'm running those lines where I just didn't have a good feel for whether there might be a problem with less than the 35 amp time-delay fuses currently used. I'll be back if it turns out there is an issue. It wouldn't be the first time nor likely the last.

[Bill Adamsen]

At the risk of "beating a dead horse," here is what I plan to do.
One photo shows the "blank spot" I've identified where I plan to put the 3-Phase Load Center, second photo shows a mock up of where it would be installed, and where the 3-phase output would be directed. One simple solution would be to run circuits through my single-phase load center to existing conduit both up and down. Are there issues with this?

[Bill Adamsen]

Project finished awaiting inspection. Based on comments I received at the PM Forum, I separated out the raceways ... didn't run 3-phase through the single-phase panel. One head-scratcher, the RPC is supplied with single-phase from the disconnect, and then returns the third leg in the same conduit. From where should that originate and return ... disconnect or load center? We decided to send that load through the panel and directly to the lugs since it comes back in the same conduit as the "feed to the load center." This appears to be in compliance with NEC 312.8, and logically (definitely logistically) seems preferable to running the 3-phase back into the disconnect. It's not a "main breaker" load center so anyone working on it would (should) understand that the feed is from the disconnect switch. If that's off, power to the panel is off. It also keeps the 3-phase out of the disconnect.

I'm happy with the end result. Came out looking pretty much like the drawings and mockup.

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