# Couple of questions on RPC build



## Barncat (Nov 3, 2018)

I am going to be building my first RPC soon, and I have a stupid question. I have a 3 pole fuse block for power to go through on the way out to the machines. If the equipment motor is ten amps, do I use 3 ten amps fuses, or does the ten amps get divided between the 3 lines?

Also, I bought a lot of used electrical boxes at auction, and one of them had some volt meters on it. Would there be any benefit to wiring these in to see real time volt readings while the equipment is in use?


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## JimDawson (Nov 3, 2018)

If the equipment is 10 amp, I would use fuses that match the wire sizes.  #12 wire = 20 amps, #10 wire = 30 amps on each leg.  The fuses are really to protect the wiring, not the equipment.  The motor should be protected by a thermal overload after the contactor.

The voltmeters really add to the ''cool factor'' of the RPC, and are helpful for adjusting the balance.  Go for it.


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## Barncat (Nov 3, 2018)

I am not actually sure on the amperage of the equipment motors, I haven't gotten that far into it yet. But I understand what you are saying and that helps. Thanks


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## Eddyde (Nov 3, 2018)

I am almost finished with my RPC build, or rebuild is more like it. Here is a link: https://www.hobby-machinist.com/threads/rotary-phase-converter-noise.73303/page-2#post-620047 there are links on it to other builds and schematics, as well.

I would totally use those meters!


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## markba633csi (Nov 4, 2018)

Looks like two of those meters are missing front covers- you wouldn't be able to set the zero on those two 
mark


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## JimDawson (Nov 4, 2018)

markba633csi said:


> Looks like two of those meters are missing front covers- you wouldn't be able to set the zero on those two
> mark




I'm going to have to disagree with you here.   The little yoke hanging below the movement bearing support bar is what adjusts the zero.  That yoke normally engages with an eccentric pin on the meter cover adjusting screw. But it is held in place by friction at the support bar.  So by just moving the yoke with a small screwdriver it is possible to adjust the zero without the meter cover in place.


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## markba633csi (Nov 4, 2018)

Ah yes you're right Jim;  I guess the real problem would be metal chips getting into the movements
M


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## JimDawson (Nov 4, 2018)

markba633csi said:


> Ah yes you're right Jim;  I guess the real problem would be metal chips getting into the movements
> M



Yup, that would be a problem.


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## Barncat (Nov 4, 2018)

markba633csi said:


> Ah yes you're right Jim;  I guess the real problem would be metal chips getting into the movements
> M


Hopefully I will have this far enough away from any chips, but haven't finalized location yet.


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## BaronJ (Nov 4, 2018)

Hi Guys,

Spring the cover off the bottom one, adjust them all for zero and make a Plexiglas cover that fits over the top of them all. 

One point on some meters the adjusting tab can be at mains voltage, so do not touch the adjuster with the power on !


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## Blackjackjacques (Nov 8, 2018)

Barncat said:


> I am going to be building my first RPC soon, and I have a stupid question. I have a 3 pole fuse block for power to go through on the way out to the machines. If the equipment motor is ten amps, do I use 3 ten amps fuses, or does the ten amps get divided between the 3 lines?
> 
> Also, I bought a lot of used electrical boxes at auction, and one of them had some volt meters on it. Would there be any benefit to wiring these in to see real time volt readings while the equipment is in use?
> 
> ...



If you believe NEC Table 430.52, you need 175% of the FLA with time-delay fuses or 300% of the FLA with non-delay fuses, or for a 10A 3 phase motor, either (17.5A) 20A, or 30A.  Start-up current for small AC motors is 6X FLA, and that is unloaded.  10A fuses are likely to blow.  You do not say what your voltage is, but the meters shown appear to be 500 V full scale and you probably would not be able to tease out important differences between the 3 phases.   Current would probably be the more useful parameter to monitor. However, if you are going for the Mr. Carlson's Lab look, it does look slick. The conductor to the motor should be 125% of the motor FLA, or 12.5 A, so you could use 14AWG.


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## Eddyde (Nov 8, 2018)

Blackjackjacques said:


> The conductor to the motor should be 125% of the motor FLA, or 12.5 A, so you could use 14AWG.


Technically you may be correct. However, I am pretty sure the NEC requires minimum #12 AWG for any motor over ¼ hp.


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## Blackjackjacques (Nov 8, 2018)

Eddyde said:


> Technically you may be correct. However, I am pretty sure the NEC requires minimum #12 AWG for any motor over ¼ hp.



Since I'm invoking the NEC, then it appears prudent to continue in that fashion and go with the 12 AWG if that is what they say.  I have no heartburn with that and 12 AWG and would prefer that to 14 AWG in any case.  Thanks


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## Barncat (Nov 8, 2018)

I hadn't looked at the meters much before I posted, but you are right, they won't work with my application. The lights are also for 480, so I won't be using those either


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## BaronJ (Nov 8, 2018)

Hi Barncat,

I'm curious !  Why wont the meters work for you, or the lights for that matter ?


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## Barncat (Nov 8, 2018)

The scale on the meters was pretty large, and inside of the box they were hooked to some sort of multipliers. I don't have the know how on how to get them to work for me. I put power to the lights to test them, and they didn't come on, either the bulbs we're burned out, or they actually need something closer to 480 as the sticker on them says.


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## BaronJ (Nov 8, 2018)

Hi Barncat,

You could always test incandescent bulbs with a cheap multimeter, though many indicator lamps like that are actually neon bulbs with a series resistor.

As far as the meters are concerned a close up picture of the front and any wiring at the back would help Identify exactly what they are, because they would supply useful information as to how well your RPC was functioning.


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## JimDawson (Nov 8, 2018)

If you want to use panel meters, I bought these for my RPC.  Seem to work well and are quite accurate, they agree with my Fluke meters.

https://www.ebay.com/itm/100A-AC-LC...9:g:pDEAAOSwldRaAvwb:rk:2:pf:1&frcectupt=true


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## Blackjackjacques (Nov 8, 2018)

Eddyde said:


> Technically you may be correct. However, I am pretty sure the NEC requires minimum #12 AWG for any motor over ¼ hp.



Just had a few minutes to check. 2017 NEC Article 240.4 (D) permits 14 AWG, so 14 AWG is correct.  Also, 430.22 (G) explicitly permits 14 AWG. Do you have another cite in the NEC that says something else?


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## Blackjackjacques (Nov 8, 2018)

Barncat said:


> The scale on the meters was pretty large, and inside of the box they were hooked to some sort of multipliers. I don't have the know how on how to get them to work for me. I put power to the lights to test them, and they didn't come on, either the bulbs we're burned out, or they actually need something closer to 480 as the sticker on them says.



The things you identify as a multiplier is likely a voltage divider generally comprised of resistors. You can use the existing meters if you know the resistance of the meters, current is usually given at full scale and labeled on the meter, in which case all you need to do is build a new divider network, however, your meter scale at full deflection is labeled "500" I have seen folks write their own scale and past over the old scale, and adjust the divider network accordingly.  But they always look a bit cheesy, and new meters are relatively cheap. Frankly, I would scrap the meters or give to someone who can use them, and proceed building your RPC without them.


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## Barncat (Nov 8, 2018)

Here is a not great picture of the inside of the box. I can get a better picture later.


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## Barncat (Nov 8, 2018)

JimDawson said:


> If you want to use panel meters, I bought these for my RPC.  Seem to work well and are quite accurate, they agree with my Fluke meters.
> 
> https://www.ebay.com/itm/100A-AC-LC...9:g:pDEAAOSwldRaAvwb:rk:2:pf:1&frcectupt=true


I think for now I will not have any meters, just get it built. I do like the look of those, however. Was it your RPC build thread with a air compressor motor that used those?


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## Barncat (Nov 8, 2018)

Blackjackjacques said:


> The things you identify as a multiplier is likely a voltage divider generally comprised of resistors. You can use the existing meters if you know the resistance of the meters, current is usually given at full scale and labeled on the meter, in which case all you need to do is build a new divider network, however, your meter scale at full deflection is labeled "500" I have seen folks write their own scale and past over the old scale, and adjust the divider network accordingly.  But they always look a bit cheesy, and new meters are relatively cheap. Frankly, I would scrap the meters or give to someone who can use them, and proceed building your RPC without them.


Yes, they appear to have 4 resistors in them, wired in series. And I don't plan on using them right now, they may end up on eBay to offset the cost of the box I am using in my build.


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## JimDawson (Nov 8, 2018)

Barncat said:


> I think for now I will not have any meters, just get it built. I do like the look of those, however. Was it your RPC build thread with a air compressor motor that used those?



Yup, that's the one.  https://www.hobby-machinist.com/threads/another-rotary-phase-converter.63204/


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## Eddyde (Nov 8, 2018)

Blackjackjacques said:


> Just had a few minutes to check. 2017 NEC Article 240.4 (D) permits 14 AWG, so 14 AWG is correct.  Also, 430.22 (G) explicitly permits 14 AWG. Do you have another cite in the NEC that says something else?


Back in the late 90's I was contractor on a remodel job, there was a 14 gauge branch circuit I wanted to use for a ½ hp fan motor, The electrician said it couldn't be done, it had to be a minimum 12 gauge for more than a ¼ hp motor. We changed it. I even asked the inspector if it was necessary (just to keep sparky honest) he said yes. Perhaps the code changed since then but it seems prudent to use a slightly heavier gauge anyway.


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## BaronJ (Nov 9, 2018)

Hi Barncat,

I suspect that they are ammeters !  Need a close up of the front.  

If they are, then they are probably moving iron type and would be very useful in monitoring the phase currents.


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## Blackjackjacques (Nov 9, 2018)

Barncat said:


> Yes, they appear to have 4 resistors in them, wired in series. And I don't plan on using them right now, they may end up on eBay to offset the cost of the box I am using in my build.



That particular meter panel seems to be an oddity even for a 3 phase 440 V system. Having three lights to indicate the presence of all three phase voltages is useful, but I don't know why you would also need to see all three phase voltages.  Typically all one sees out there are perhaps three lights at the source and a single "power available" light at the equipment.


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