Simple electrical question

[Darren Zisser]

I'm getting a 15hp rotary phase converter. How big of a 220v single phase circuit breaker do I need?

Thanks.

[Barry O'Mahony]

See Article 455 of the NEC.

[Steve Stube]

If that's a simple question, here is a simple answer. Determine the downstream total load (sum of the motor/s amps) multiply by 1.73 to choose the wire size needed upstream (plus a fudge factor to allow for long runs/voltage drops) and select the circuit breaker to protect the wire.

Of coarse the Mfg. of the rotary Phase Convertor will no doubt give you a reccommendation.

[Barry O'Mahony]

If that's a simple question, here is a simple answer. Determine the downstream total load (sum of the motor/s amps) multiply by 1.73 to choose the wire size needed upstream (plus a fudge factor to allow for long runs/voltage drops) and select the circuit breaker to protect the wire.The answer may be simple, but it is incorrect. NEC 455-7 specifies OCP for phase converters.

[Steve Ash]

NEC 455-7 would be a correct answer, because OCP (over current protection) covers breakers and fuses. But the simplest answer is to check out the MFG's name plate rating on the phase convertor.

However did you double check to see that you are getting a wye phase convertor and not a delta phase convertor.(These are wiring configuration's of phase convertors and not manufacturer's, just making sure to inform you of the differences.) My son ( an electrician by trade) has run into problems of trying to get 3phase machines to run off delta phase convertors based on the second leg (high leg) being 208 volts versus the other two at 120 volt.

[Mac McAtee]

What ever happened to getting a three phase motor of the same or larger HP than what you want to run, hooking 220V to two legs and hooking 3 legs up to the machine you want to run? Throw the power to the slave motor, take your tennis shoe and spin the shaft of the slave motor and get it wound up and running. From then on you can run that 3 ph machine simply by starting it up.

Thousands of shops all over the world have been doing that since electricity was invented.

Any phase converter that is more complex than that is simply putting perfume on a pig.

If the slave motor is rated to draw X amps at full load, choose a breaker that is that + 10% and you are done.

I can feel the slings and arrows being loaded as I type.

[Steve Stube]

Well by all means Barry, keep us in suspense.

[Jim Becker]

What ever happened to getting a three phase motor of the same or larger HP than what you want to run, hooking 220V to two legs and hooking 3 legs up to the machine you want to run? Throw the power to the slave motor, take your tennis shoe and spin the shaft of the slave motor and get it wound up and running. From then on you can run that 3 ph machine simply by starting it up.

Thousands of shops all over the world have been doing that since electricity was invented.
Yes, that will work, Mac...but isn't necessarily the best idea for many of the 3 phase machines that folks use today that include bunches of electronics. You need a much more stable and controlled power source to avoid issues with said electronics. Someone recently posted a picture of the innards of their sliding saw...food for thought when choosing a 3-phase power source.

[Dev Emch]

See Article 455 of the NEC.


Yah, what he said!

The answer is not that simple. The 1.73 constant converts loads between three wire three phase and two wire single phase (220 differential loads).

NEC 455 bumps that massively for the wire sizing between the three phase system (BUSS) and the breakers. This takes into account the multiplier effect of the converter as it has a real affect on the single phase draw.

More important however is this. The breakers sized according to 455 will not protect your converter. If your using a SINGLE breaker to feed everything downstream of your conversion circuits, then the converter taps into the L1 and L2 lines comming off this breaker and it needs additional protection.

Furthermore, 455 mandates that you have a means to disconnect **ALL** hot conductors from the converter using an approved switch within visiable range of the converter. I suggest you use a two leg disconnect box with the handle and fuse bays.

455 also mandates that *NO* single phase loads be attached to the wild leg. Sounds simple enough as the wild leg to neutral is about the sqaure root of three times your leg to neutral voltage. This will fry an elelctric toothbrush in a hurry and turn a night light into a flood light! DONT DO IT. But more importantly, this section carries an implicit requirement. YOU WILL NEED TO LABEL AND MARK THE HIGH LEG OR WILD LEG. Looking at the 1996 NEC, art 384-3(e), they go into marking the high leg to neutral line with an orange wire or with orange phase tape. Now the reason behind 384-3(e) is not because of phase converters; many inspectors none the less will require this. This is both a good idea and a bad idea. Its good to be consistant; however, in true three phase systems, single phase loads are attached leg to neutral all the time and this orange leg informs you which one has the higher leg to neutral voltage. If your used to hooking up organge to white lines and you do this on a phase converter system, you have just violated 455 and fried your toothbrush!

In terms of the phase converter itself. 455 requires a nameplate to be attached to the converter. You home brew converter guys take note! On it will be the max input amps. Now this amperage has nothing to do with the overall three phase supply driving L1 and L2. This is only for the converter and should not be exceeded. Thus, you need to provide protection for your converter and not just the hunkin wire feeding L1 and L2. Personally, I use a dedicated two pole breaker to feed the converter. The converter has two input lines and one output line. That output line is your wild leg. Of course, there is the green wire. The point is, those two main input lines need to be protected inorder to protect the converter from killing itself. I use a second breaker to supply power to the main L1 and L2 lines. These two are sized differently because they are application different.

The bump up factor which I reall is 2.5 times the three phase load instead of 1.73 times the three phase load has to do with the fact that you can run much more than 15 HP of combined load on your converter. This is unique to rotary phase converters. The largest motor I can start would be a 15 HP motor but I can run combined loads of up 30 to 45 HP on this converter.

But your installing a 15 HP converter. So did I. Mine is a 15 HP Kay converter. That means it has a 30 HP idler motor. This is another Kay-ism. Most makers dont do this and you may have only a 15 HP idler. THis will matter when it comes time to start up heavy duty or hard start loads. A 24 inch jointer with a 5 inch cutter is a hard load. A table saw is an easy load. Hard loads is where the converter earns its keep in a hurry. For my Kay, I have a 60 amp dedicated breaker supplying power only to the Kay. The supply lines are #6 gage THHN.

The wild leg from the converter feeds a 225 amp three phase service panel. L1 and L2 are massive lines sized for 225 amps. The L3 is a punny little #6 gage wire. WHY? Well, the L3 line can only supply so much current before the converter trips. So its pointless to use the massive wire here. But more importantly, you recall the multiplier effect I talked about? The service panel has a 225 amp back plane and this back plane acts as distribution central for incomming and outgoing current flows. As each machine is brought online, it consumes current so current flows into the plane from the converter wild leg and out of the plane to start up the current machine tool. Once the machine is running, it runs either at idle or at full load or somewhere in bewteen. And the load goes up and down. A drill press is only loaded when your drilling downwards. On the upstroke or while getting ready for the next hole, its at idle. At idle, it is actually assisting the converter and current is flowing out of the plane on the L1 and L2 leads and comming back into the plane on the drill press connection to the L3 backplane. This means the idle drill press is acting as a mini phase converter. This is how you can run upwards of 45 HP on a 15 HP converter. But to do this right, you really should install a three phase service center. Remember, breakers are BI-DIRECTIONAL.

But more importantly, there is no free lunch. If your running upwards of 45 horse power, you will need to size the single phase load to handle this. This is why NEC 455 does not endorse the 1.73 sizing constant.

Hope this helps.

[Rob Russell]

Dev,

FYI, the requirement to mark the wild leg with orange only applies to delta 3-phase where one of the legs is grounded. That's not the open delta 3-phase that a RPC generates.

That doesn't mean one can't mark the wild leg as being orange (I am), but it's definitely not an NEC requirement.

Rob