Replies

1. 
   Jamie_Buxton | Oct 26, 2003 06:20pm | #1

   The breakers in the breaker box protect the wiring from the box to the outlet, and are sized accordingly.   10 gauge copper wire can be protected by a 30 amp breaker.

2. 
   JohnWW | Oct 26, 2003 06:24pm | #2

   Mitch,

   The breaker is sized to protect the wiring in the wall from overheating and potentially causing a fire.  The length of the run and what is plugged into the circuit are irrelevant.  I don't have a reference book handy but I think that 10 gauge wire can have up to a 30 amp breaker.  If you need more than that for the machine, you will have to run a larger gauge circuit.  On a machine like the Robland the actual protection of the motors is typically handled by the magnetic switches.

   John W.

3. 
   watkinsjd | Oct 26, 2003 06:30pm | #3

   Mitch

   You should size the breaker to fit the wire.  In this case a 30 Amp should be safe for 10 ga. copper wire.  Each motor should draw around 15 amps or less and assuming you use only one at a time you shouldn't overload the circuit.

   BTW, a woodworkers forum is not necessarily the best place for electrical advice.  I have seen some really off the wall theories posted here.

   John

4. 
   AngeloD3 | Oct 26, 2003 06:35pm | #4

   Hi Mitch, long time ago I could give you a more exact answer, but, here goes; 1hp is 74x watts using 120 volts, which is about 6.5amps. Three horse power should be 18-20 amps @ 120v. Using 220v cuts the amperage in half so you would be very safe in using a 30 amp breaker with no.10 wire with more than one motor running. I would assume that you might occasionally have two motors running. You might endanger yourself with three motors running at the same time. Hope that helps. There should be some electrician reading the threads to give you more help. If you can't figure it out, ship the unit to me for a three or four year test and I will write you a performance report. :)

   Benny

1. 
   mitch | Oct 26, 2003 07:39pm | #5

   thanks to everybody- this particular machine can only run one motor at a time so i'll assume 30a will be fine.

   m

5. 
   TKanzler | Oct 27, 2003 03:47pm | #6

   "i'm finding conflicting advice in the literature ..."

   What literature are you referring to, and what is the conflicting advice? 

   Be seeing you...

1. 
   mitch | Oct 27, 2003 04:23pm | #7

   actually not so much conflicting as confusing.  like some of the posts here, for example, make a point of saying that the breaker is for protecting the wiring and that the motors and switches should provide ample protection to the machine itself, but what i really wanted to know, in a nutshell,- and should have been more specific about- was:

   "what is the minimum/maximum/correct size breaker that will protect the entire system in the event of a legitimate overload situation without tripping unnecessarily during start-up or heavy service load like ripping thick hardwoods (assuming proper blade, feed rate, etc)?"  

   in other words, you got your basic 220v, 1ph, 3hp motor in a medium/heavy load application with a more or less typical wiring distance and gauge- what's the usually recommended breaker size?

   thanks-

   m

1. 
   TKanzler | Oct 27, 2003 06:29pm | #8

   The reason I asked is that the manual, and sometimes the UL label (if it has one) will often have conductor size minimums, and/or fuse or breaker requirements, especially maximums, and I thought that might be where the confusion is coming from. 

   Maximums are given because the NEC allows fuses or breakers on motor driven equipment much larger than the usual 15/20/30A limitations on 14/12/10 gauge conductors used in general receptacle and lighting applications (to allow for start-up current, especially on hard to start equipment; see NEC 430), but at the risk of damaging the motor control in the event of short-circuit or ground-fault after the control.  The manufacturer will therfore often give maximum fuse or CB ratings, which the NEC requires that you observe.

   "what is the minimum/maximum/correct size breaker that will protect the entire system in the event of a legitimate overload situation without tripping unnecessarily during start-up or heavy service load like ripping thick hardwoods (assuming proper blade, feed rate, etc)?"  

   In motor circuits, it's the motor control that normally protects the entire system from overloads and overcurrent, not the CB's or fuses.  The breaker is there to protect against short-circuit and ground-faults.  That's why the NEC allows much larger breakers/fuses on motor circuit conductors than would normally be allowed for general lighting and receptacle circuits.  So, in a nutshell, if you machine came equipped with a motor starter with overload protection, you only need a circuit of high enough ampacity to meet the Code requirements for 3 hp at 230V single-phase (which is 17A * 1.25 = 22A), but not higher than any restrictions given in the manual and/or UL label.  Your 30A circuit using 10 gauge conductors meets and exceeds the requirements of Article 430, as long as the breaker or fuses isn't larger than what the manufacturer allows. 

   Perhaps your confusion comes from the difference between wiring your motor-driven equipment per NEC Article 430, as many inspectors will require, even in your house, and following the (more familiar) Articles like 210, which apply to almost everything else you'd find in your home.  For small motors like these, in most cases you end up in the same place anyway.

   By the way, if I were you, I'd ask the manufacturer what, if anything, should be done differently since the machine was designed to operate at 220V 50Hz and you're giving it 240V 60Hz.  It's worth an email, at least.

   Be seeing you...

   Edited 10/27/2003 11:36:29 AM ET by Tom Kanzler