Wiring my Marathon motor

First go to and download the info, including wiring diagram, for the motor. You will need a start contactor with overload protection (OLP). There is no absolute requirement for OLP, but it's highly desirable and worth the cost. Advance Controls Inc makes a line of of contactors with OLP that work nicely for an application like this. The OLP module plugs into the bottom of contactor. You'll have to decide what voltage you want to use to operate the contactor coil before you buy it. Options are 24 VAC, 120 VAC, and 240 VAC. If you plan on the starting switch being close to the motor, 240 VAC will be simplest. If you plan on the starting switch to be some distance away, use 24 VAC (you'll need a door bell transformer to supply the 24 VAC). 24 VAC wire (door bell wire) is cheap and doesn't require fishing through walls or conduit. The pressure switch on the compressor will have to be in series with the starting switch. You can find ACI contactors with OLP for sale in many places. I've bought them from FarmTek in the past. I don't work for or sell products of the companies listed above.
Mike
trg-s338 wrote:

First off, you now have to buy a new mag switch and probably a new mag switch enclosure, to accommodate the larger current. Make sure you pay close attention to the voltage of the coil in the mag switch, if you get the wrong voltage you'll have to install a transformer. New controls can cost several hundred dollars. Then you have to make a bracket for mounting the new controls to the machine, then you have to wire it all up. The wiring is by far the easy part.
GWE

A 5 HP single phase motor, on 240 volts, should be fed with a 20 amp double pole breaker. That size breaker would afford full load current protection for the motor. Check with local code to be sure what is "right" for your location.
Bob Swinney

Good info, but here's a Q: Isn't a contactor w/ OLP just a fused switch, w/ mebbe remote control? In principle, couldn't a simple fused knife switch suffice? -- Mr. P.V.'d formerly Droll Troll

NO! Do it right the first time and save yourself a whole lot of grief. A proper motor starter is critical for all three-phase motor applications, and a smart thing to do for any single phase motors over roughly 3 to 5 HP. A motor starter is much cheaper than the motor going up in smoke repeatedly.
The contactor half of a motor starter has a magnetic coil for fast action, so the contacts spring open and closed fast. They have angled alloy contact points (usually coin silver) that are designed to be operated millions of times under load, and self-clean by a wiping action when they close. They have magnetic arc chutes arranged that any arcing on opening is directed out and away from the contacts to quench quickly. The contacts do get cruddy from wear but still work by design (self cleaning, remember?) but when they wear out and fail to work they are easily replaceable.
Fused switches are simple copper alloy contact points that operate relatively slowly, and Are Not Designed To Be Operated Repeatedly Under Load. They may have some sort of arc chute arrangement, but it's more to keep the switch from self-destructing if opened under load accidentally or in an emergency. Burn the contacts up, and you have to replace the whole thing.
The Overload Protection monitors the starting and running current on all three phases, which is critical if one of the contact sets goes bad or you blow a fuse on one phase.
A simple knife switch doesn't know or care about a single-phasing problem. If the motor blows before the other two fuses, oh well... If the motor is seriously overloaded or in a locked rotor condition, the fuses will react much more slowly than the Starter Overload - again, the motor may pop before the fuses do.
And a simple knife switch won't drop out if there's a power failure as a safety feature like a properly wired Motor Starter does, so when the power is restored the equipment will start right back up again - doesn't matter that you have your arm in there trying to figure out why it stopped... We'll call you "Lefty". If your head was in there, we'll call you 'decedent'.
-->--

Yeah, I agree w/ all of that, but just from a starting/stopping POV, a contactor does seem to be "just" a switch--albeit a fast one. And from a pure wiring pov, the OP *could* just use a switch and still be relatively safe--it seems.
But from an overall motor protection pov, indeed, contactors have a lot going for them. Altho, the "lockout" feature (non-restarting) does have to be wired in by the user, via one of the contactor's auxiliary switches, or the third set of contacts (3 ph contactor), or an external relay. AND, if the coil is 120V, he will need an extra relay to guard against re-starting if *either* leg goes out. Or a 120/220 V transformer so as to be able to use 220 V (both legs) on the coil.
I have a master contactor that locks out the whole shop in the event of a power failure. Esp. important, I'm told, for cnc, rotary converters.
Btw, I found a sort of a cure for noisey/buzzing contactors: I think excessive buzzing means the coil is too weak for the springs. In one of my bigger #3 contactors (an old GE), you can disassemble the whole thing (really elegantly made), and change the springs behind the contacts. I just happened to have a long spring w/ a better force constant (same diam, thinner wire), that I was able to cut up and replace the original springs with. Now, nice normal quiet hummmm. -- Mr. P.V.'d formerly Droll Troll

You should read this old thread:
It sounds like you may have/had a broken shading coil( a little copper loop inset in one face).
Just a thought, you might have missed this thread back in Dec 2005.

Not hardly.
My marathon 5hp is spec'ed at 27.5amps continuous on 230v. It has a starting surge captured by my peak-reading clamp on ammeter at around 100amps. There is NO WAY it is going to run from a functional 20amp breaker. Following NEC rules, since no delayed action breaker was available for my panel, it is currently very happy on a 50amp breaker.
I depend on the motor's own overload protection and let the breaker protect the wiring as that is the way things are designed to work.
That at least is very good advice.
sdb