Limiting capacitor-start motor inrush

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
228
Just a quick question regarding my available options for limiting motor inrush. I've got a 40's or 50's vintage Century capacitor start motor on my lathe - 1 horsepower, 1.25 service factor with a nameplate FLA of 12 as connected for 120V. Dual voltage, reversible. It is being supplied by a 15A general purpose branch circuit and controlled by a drum switch. Overload protection by Edison-base fuse.

There is approximately a 1 in 30 or 1 in 40 chance of said motor tripping the magnetic element in the circuit breaker protecting the branch circuit each time I start the motor. Being that it is in a lathe, it sees dozens of starts in an hour, not to mention occasional bursts of jogging.

My question is what sort of options are available on the market for limiting motor inrush on the line side of the controller? Due to the nature of this installation, trying to fit e.g. a starting torque limiter on the load side of the drum switch would be a pain right in the butt. I could easily end up being over on pipe and/or box fill if I were to try that.

Would a suitably sized choke in series with the incoming lines to the drum switch be the most elegant solution for clamping just a handful of amps without negatively impacting motor performance? Or are current-sensing torque limiters available which are designed to operate on the line side of the controller?

Thanks.

EDIT:

I just saw mention of Square D's High Magnetic line of QO circuit breakers. These seem to be exactly the right solution for this situation.
 
Last edited:

LowQCab

Joined Nov 6, 2012
1,343
I would re-wire your Motor for 240V, then, with a little creative switching,
you can start it on 120V, ( at half the Current ) then switch to 240V.
If the Start-Capacitor is still in good condition, this scheme shouldn't be a problem.

A "3-Way" Light-Switch, ( in a proper box of course ), will do the trick.
You will need to make a dedicated run back to your Panel,
with a Ground, a Neutral, ( White ) and 2-Phase-Legs, ( Red-Black ), and a 15-Amp Double-Breaker.
( Do not use 2 individual, Single, 120V- 15-Amp Breakers, this can create a dangerous condition ).
4-Conductor, 14-ga., "Romex-Style" Cable is available everywhere if you don't have Pipe.

Actually, if all you want to do is solve your Breaker-Tripping problem,
and you don't need an actual low-speed mode,
just convert over to 240V and you'll have no further problems with Breaker Tripping.
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Thread Starter

Just Another Sparky

Joined Dec 8, 2019
228
Yeah... that's a little hokey for my liking. Not to mention it would still put me over on pipe and box fill. I'd end up having to jump to 1" and #10s to make that work. I've got 1/2" and #14s in there. I try to keep things to the book in my garage for the most part.

It wouldn't even be a matter of running 240V down there. Starting the motor with 120V in a 240V connection will result in a quarter of the power. But like I said... that's hokey. I start, stop, jog, reverse and bump this machine dozens of times in an hour. Dealing with such a silly arrangement every time would be impractical in more ways than one. It would even become a question of whether the motor would then develop *enough* torque to break the static friction on the bronze spindle bearings along with belt drag and static friction in the gear train. The last time I tried that way back when I was evaluating the machine before taking it home, it wasn't even enough to turn the motor on it's own. Granted that was before I cleaned 70 years' worth of crap out of it and changed the bearings, but even so... it turned freely by hand.

I ordered a 15 amp high-mag. I'll lock out the feeders to my panel and swap out the existing 15 amp standard-mag. I have a feeling that's all I need now that I know they exist.
 
Last edited:

LowQCab

Joined Nov 6, 2012
1,343
10-gauge wire ???
I don't understand.
And what is a high-mag ? or a standard-mag ?
Are you referring to a standard Magnetic-Circuit-Breaker, vs a "Motor-Rated" Circuit-Breaker ?
( Motor-Rated Breakers are common in Commercial work, but very unusual in Residential work ).

If you re-wire your Motor for 240V, 2- 14-gauge wires on a Tandem 15-Amp Breaker
would be more than adequate, and completely eliminate Nuisance-Tripping.

Going to a 240V Motor connection cuts the Amperage required by the Motor in half.

As long as you have a proper Ground Wire, you can color-mark, (tape), the
Neutral-Wire, and make it into a Hot-Wire, for creating a 240V Circuit,
but only if it's not connected to any other Circuit, of course,
and is clearly marked everywhere it is exposed.

There is another factor that could be causing you problems ........
Circuit-Breakers tend to "wear-out" and/or, get weak, when they are routinely tripped,
or, if they are used in "Switch-Service", ( used routinely to turn on or off a Circuit ).
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Thread Starter

Just Another Sparky

Joined Dec 8, 2019
228
10-gauge wire ???
I don't understand.
And what is a high-mag ? or a standard-mag ?
Are you referring to a standard Magnetic-Circuit-Breaker, vs a "Motor-Rated" Circuit-Breaker ?
( Motor-Rated Breakers are common in Commercial work, but very unusual in Residential work ).

If you re-wire your Motor for 240V, 2- 14-gauge wires on a Tandem 15-Amp Breaker
would be more than adequate, and completely eliminate Nuisance-Tripping.

Going to a 240V Motor connection cuts the Amperage required by the Motor in half.

As long as you have a proper Ground Wire, you can color-mark, (tape), the
Neutral-Wire, and make it into a Hot-Wire, for creating a 240V Circuit,
but only if it's not connected to any other Circuit, of course,
and is clearly marked everywhere it is exposed.

There is another factor that could be causing you problems ........
Circuit-Breakers tend to "wear-out" and/or, get weak, when they are routinely tripped,
or, if they are used in "Switch-Service", ( used routinely to turn on or off a Circuit ).
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Correction, 3/4" and #12s. I'm used to dealing with 20A branch circuits. As I add more conductors to a pipe, I have to de-rate the ampacity of those wires. In order to handle the same current, larger wires are needed. As larger wires are needed, so is larger pipe.

To expose all 6 current carrying conductors from the motor and 4 from the drum switch to a junction box in order to select between 120V and 240V motor connections - plus line and neutral feeding through the same conduit - I can only use 50% of the wire's ampacity. For a 15A circuit, that means I need #12 conductors rated 90*C. For 12 #12 current carrying conductors plus one equipment ground, I would need 3/4" conduit to fit them all.

As stated, I've got 1/2" conduit installed.

High Magnetic breakers are standard QO form factor thermal-mag breakers with an increased magnetic element trip point to accomodate high inrush loads. 1000% instead of 700% if the internet is to be believed.

I knew that they make adjustable trip breakers in larger form factors, but didn't know of any options for QO form factor at the time I submitted my OP. Hence the edit.

I'm not ripping out drywall and hammer-drilling to run a permanent 240V circuit down to the garage. Nor am I stealing power from my range every time I want to turn something. Some day when I own a house with a 3 stall garage or a pole barn I'll install a 100 amp sub-panel in said garage and/or pole barn. But not in this house which I don't intend to stay in forever.

For now, I have a strong feeling that an $8.00 high-mag will do the trick.
 
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LowQCab

Joined Nov 6, 2012
1,343
Square-D is the best,
I just never heard "Motor-Duty" Breakers referred to as "High-Mag",
but I guess the name fits, maybe I'm just too old.

I wonder if Square-D still makes Thermal-Only Breakers ??
Back in the day you could Special-Order them.

BTW,
You don't have to follow the same Pipe-Fill-Rules inside of a Machine. or Motor.
Your Motor probably has 16-Gauge Wire inside the Junction-Box.
When a Motor-Reversing-Switch is incorporated into a Machine,
the normal NEC-Rules may not apply to the Wiring between the Motor and the Switch.

And, as long as you are not doing it professionally,
you can personally do whatever you feel is practical and safe.
The NEC-Rule-Book can be extremely conservative at times.

The chances that you're going to have a packed-Pipe with every Conductor running
at maximum Amperage for long periods is slim to none,
but I have seen Pipes, in an old Commercial-Warehouse, that you couldn't hold your hand on.
That's what those Rules are designed to prevent.

There's also the factor of the length of the run, but in Residential, it's virtually never a factor.
The only time Wire size should be increased is on runs approaching ~100-feet,
or for certain dedicated Motor-Circuits that have extreme start-up demands, and so
require a larger Breaker size to prevent Nuisance-Tripping, this is generally only done
when a dedicated Motor-Starter is also being used.

2- Wires supplying a 240V Load at 5-Amps,
will run substantially cooler than,
2- Wires supplying a 120V Load at 10-Amps.

"" I can only use 50% of the wire's ampacity ""
This is not good way to operate.
Nowhere in the NEC Code-Book are you allowed to
"claim" that, "it's OK 'cause I'm only going to be using 50% of the Circuit's Ampacity".

The Breaker sets the Maximum-Current, and dictates the minimum Wire-Gauge,
the Pipe fill Rules are simply there to insure, that in a worst-case situation,
the Wiring in the Pipe is not likely to exceed the
Temperature-Rating of the Wire-Insulation type being used.
If the Pipe is not over-filled, you can safely use every Circuit at it's expected 80% capacity,
which with 14-gauge Wire is ~12-Amps, and it is very unlikely that You have
more than 4 Circuits in that Pipe all running at ~12-Amps,
if you did, you'd have one heck of an Electrical-Bill !!!
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Thread Starter

Just Another Sparky

Joined Dec 8, 2019
228
"" I can only use 50% of the wire's ampacity ""
This is not good way to operate.
Nowhere in the NEC Code-Book are you allowed to
"claim" that, "it's OK 'cause I'm only going to be using 50% of the Circuit's Ampacity".
Pipe fill derating my dude! If you've never derated for pipe fill then what have you been doing your whole career? Guesstimating? Shooting from the hip?

More wires generating heat, more insulation trapping that heat inside the pipe, less allowable ampacity.

Formerly Table 310.15(b)(3)(a).

12 current-carrying conductors in a conduit -> 50% reduction in wire ampacity. That's the law.

By the by, the motor has got #12s in the pickerhead.

And I wouldn't have to follow NEC rules if I were a manufacturer or professional engineer installing wiring integral to the machine conformant to industry standards. Otherwise all field wiring between the service and motor leads has to conform to the NEC. For perspective we're talking about a field-fitted conduit run from a junction box on the back of the machine - that's 100% NEC jurisdiction.

And forgetting all that, why bother with such a convoluted, hacky solution when I can fix the nuisance tripping at it's source properly by swapping the breaker for $8?
 
Last edited:

LowQCab

Joined Nov 6, 2012
1,343
"" Guesstimating? Shooting from the hip? ""
You might actually say that and be fairly accurate ........

Most of my Electrical-Construction experience was working as the sole employee,
in 2 separate circumstances,
with 2 totally different, but extremely experienced, employers,
as a Commercial/Industrial-"Trouble-Shooter",
with only occasional "Ground-Up" "New-Work" jobs,
and occasional new-heavy-equipment installs.
On the "New-Work" jobs, the simple rule was always start-out with 3/4-Pipe as a minimum,
including using 4-and-11/16" J-Boxes almost exclusively.
This makes Life so much easier when you're pulling Wire by yourself.

I basically could do things anyway I wanted, with either employer,
which was always leaning towards over-kill,
including pulling in extra un-required Circuits so that when I came back 3-Months later
for the next install, I wouldn't need to re-pull any new Wire,
and I always insisted on using Stranded-Wire.
Planning ahead is great when you have a Boss that's hardly ever working on a tight-budget,
and a loyal Corporate-Customer-Base that has the Money,
and the intention, of maintaining Top-Standards.

I never had to change a thing I installed, even with the fussiest of Inspectors,
(which I would normally pump for info on what they "liked to see" ).

I've done very little Commercial "Production-Work", and when I did I hated it.
I especially don't like having to wear Steel-Toed-Boots and a Hard-Hat.

Same with 2-years of Residential-Service-Work,
nothing but dealing with somebody else's Hackery,
and too many burned-up "Zinsco" Panels, hated it too.
But, luckily, I wound-up spending half of my time
trouble-shooting over the Phone for the rest of the guys on other jobs.
I was also the one who got assigned to the Big-Box-Truck,
which always had anything you could ever possibly want or need in the back.

So anyway, yeah,
I've probably only calculated Pipe-Fill around 3 or 4 times total, in my entire life,
I never really had the need to do it exactly "by the book".
Ya got me.
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