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Thread: Rotary phase converter amp draw
08-22-2011, 01:24 AM #1
Rotary phase converter amp draw
I have only 8 gauge wire run to our shop. I am told we can pull 50 amps through the 8 gauge. If we run 2 machines that are 2hp and use a 10hp rotary converter, how many amps will the converter draw and will it be ok with the 8 gauge wire? Would there still be room for an air compressor?
Thank you for any and all info you can provide.
08-22-2011, 01:52 AM #2
Rob, a lot depends on what the machines are/what their startup loads are. Big lathes without clutches are in "startup" draw for longer than some other tools. Plus, code requirements for RPCs are a worst-case scenario calculation. For my 7.5 hp RPC, I think 60A was the breaker size required, with wiring to match -- even though it doesn't draw anywhere near that. (If you have to adhere to the codes...)
You can do some motor starter/relay logic that will prevent the compressor from starting if certain other machines are running. RV shops have pre-built devices that do that for air conditioners -- you could use one of those if your air compressor voltage/phase is compatible. Otherwise, the logic isn't too tough.
08-22-2011, 02:01 AM #3
Thanks Chip. Pardon my ignorance, but is it safe to say that running 2 machines at 2hp and a small compressor will be ok with 8 gauge wire and a 50 amp breaker?
08-22-2011, 11:12 AM #4
Inrush current is your biggest problem, hence the suggestion for some relay logic to prevent the compressor from starting when the other machines are running.
I am certainly no electrical wizard, but from a 15HP rotary I run a 5HP geared head mill on 8AWG. It draws 35amp on the high speed side of the motor at start up (Spindle ON) this drops off radically once running. I regularly run the mill and my 1HP drill grinder at the same time with no problem off a 50amp breaker on the single phase side. Both machines are 3 phase. Now at times my compressor kicks on (220v 5HP Single Phase) but it does not affect the machines. If all three were to start at the same time, highly unlikely, I am sure I would have a problem with the cnc mill, very voltage sensitive, but not the grinder.
The run to all of the machines is short (2 car garage) and that helps, if your runs are long it can cause additional problems.
Hope this makes you feel more secure, I was like a cat on a hot tin roof when I set my shop up 5 years ago, but so far so good.
08-22-2011, 02:31 PM #5
Thank you for the great replies!
08-22-2011, 11:51 PM #6
One horsepower is equal to 746 watts of electricity, regardless if you run 2,4,6, or 8hp on your 10hp rotary converter your rotary converter is still 10hp or 7460 watts. Assuming that you’re running 240v single phase, amperage draw would be 7460w / 240v, or 31 amps. You need to add in a safety factor (typically 1.5 times the require amperage) to figure the gauge wire needed to accommodate the specified load (1.5 x 31 = 46.5). So, at the very minimum you would need a 50 amp breaker and #8 wire to handle your load (I would probably go up one size and use #6 which is rated for 65 amps). Try doing a google search on "Ampacities of conductors" and you'll get a wealth of information to assist you on selecting wire sizes.
08-23-2011, 12:08 AM #7
Rob -- didn't mean to ignore you, but got tied up at work. Looks like Steve and Chris have got you covered, though.
I bumped my wiring up from 8 to 6, primarily because I found a good deal at the local scrapyard -- new 60' flex conduit pigtails for computer servers. Sounds like your 8 will work if you monitor loads, and if your run is not excessively long.
(Regarding the "safe" question -- your on-site electrician will be the best judge of that. But do a survey of your RPC loads, and other shop loads, so you'll know just where you stand. And by survey, I mean not just "small compressor", but "2 hp 220V compressor" or "7.5 hp 3-phase compressor". Both may be "small" to this crowd, but they straddle the "safe" line for your RPC sizing discussion.)
08-23-2011, 12:19 AM #8
All the power comes in from the single phase line.
SOME of the power passes thru the converter to the third wire output (manufactured phase). About 33% of the total power out is passed thru the converter,
SOME of teh power is lost in converter losses.
Current-wise, you have the draw for each motor connected, plus the draw for the converter motor, with the latter based on about 50% power maximum.
FLA for the 2 HP is about 7A, so 14 for the two.
A added current comes through the converter, another 14 plus losses.
The converter draws around 10 or 12+ amps on idle
You will have something around 40+ amps draw.
75C cable (use that rating based on allowed terminal temps) is 45 A, so you are squeaking by at full draw.
You are supposed to protect at 125% of max nameplate, requiring over 45A conductors so it looks like you should go to 6 ga.
But if you wire the phase converter with its own protection, and bring its output (manufactured phase only) over and combine with direct wired single phase, you do not have to have the phase converter input current included with the load motor circuit.
That is a recommended wiring method, and is fairly common. You can find details at the phase converter maker's sited... check Arco and Phase-A-matic
You would be fine with the 8 then.