# Running 15 or 20A mill on 50A circuit breaker



## Skowinski (Jun 22, 2020)

Been searching around, but apparently missing this - what is a simple solution for powering a milling machine that needs 15 or 20 amp protection, when the 240V circuit to the wall outlets is on a 50A breaker?  I don't want to change the breaker to a lower amperage one, because this breaker powers 3 different 240V outlets, and one of them will occassionally be used to run a welder.  (no, these two pieces of equipment will not be used at the same time...)

Thanks for any advice.


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## mickri (Jun 22, 2020)

Run the cord from the 50 amp outlet to a 20 amp breaker in a small sub panel,  https://www.lowes.com/pd/Square-D-70-Amp-2-Spaces-4-Circuit-Main-Lug-Load-Center/3135851, and then to the mill


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## tq60 (Jun 22, 2020)

No worries...

The breaker protects the wiring and not the device.

The electrics in the mill are self protected and do not care what is other side of outlet if large enough.

A picky person would insist on 50 amp cord etc but a suitable cord to carry the needed current from outlet to machine can be made ad a transition extension cord.

50 amp male at one end and whatever the mill male is a female to match.

Many appliance cords are not rated for the 15 amp outlet they are plugged into.

Sent from my SAMSUNG-SM-G930A using Tapatalk


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## Reddinr (Jun 22, 2020)

> Many appliance cords are not rated for the 15 amp outlet they are plugged into.



This is true...but.  The cord usually has a large enough wire gauge and is short enough/low enough resistance to pop the 15A breaker quickly if shorted.  So, the smaller wire is somewhat protected, to a point.   

A 15A protected short is less dramatic than a 50 Amp one.   My advice is to do it right and put in a small, inexpensive sub panel with an appropriate breaker and appropriate wire gauge.  It is not that difficult or costly to DIY.  Maybe put in a couple of other convenience outlets while you are at it.

_A very long time ago a former boss insisted that a ~40 Foot, ~18 Gauge SJT cord off of the output of an isolation transformer, protected by a 15A breaker was a good cost saving idea.  I disagreed but he didn't change his mind until I ran the cord on the concrete floor, shorted it at the end and turned on the breaker.  After a few seconds, before the breaker popped it was hot enough to bubble the outer jacket and so we went up to 14 Gauge.  (I'm not sure the numbers in my example are exact because it was 35 years ago.)_


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## pontiac428 (Jun 22, 2020)

You could install an appropriately rated breaker in your mill's electrical box, at the opposite end of the power cord, and leave your existing house wiring alone.


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## Dabbler (Jun 23, 2020)

I've been staying away from this one, but I just have to intervene.  Step away from your current situation for a minute and consider your wife's 1/6 HP sewing machine with the 18 gauge line cord.  You cannot carry 15 amps for even 10 minutes on that cord, but until sewing machines became computerized there was NEVER a breaker or fuse in them.  that's because it only has to trip a 15 amp breaker, which needs about 40-100 milliseconds to do.  It won't even get hot.

If you have extra breaker positions in your panel, by all means get a 20A breaker and make a new circuit. That would be just fine.  but you aren't actually protecting anything.  Even if you have a 12 ga lead to your machine and you somehow short it, it will still throw the breaker - if you use a 50 amp plug.

Remember:  this does *nothing *to protect your equipment, and certainly not your motor. If you overwork your motor, and it overheats, they you lose the motor. If you stall the motor - you will burn it out on a 20 am circuit just as badly as on a 50 amp circuit. Each circuit will deliver the exact same knockout punch to your motor. I don't know how many table saws I've seen with burnt out motors that never tripped the breaker.

So plug in your 15A mill.  you are fine.  You don't need a 20A breaker.  Just use a 50A approved plug and reasonable lead in cable.  All my machines except my welders use 12 ga 19 strand rubberized cable.  My welders all use 4 gauge 19 strand ultraflexible cables, because I hate cable failures, but I only need to make the lead in once for my lifetime.

@Reddinr  has the right of it.  breakers are there to prevent fires in case of a short.  

Oh @Reddinr a comment on the 18 gauge cord.  it should have tripped the breaker in under 100 milliseconds.  Thre are two possibilities as to why that didn't happen:  Firstly is that if the breaker was a Federal Stab-Loc and manufactured in the right time period (30 years ago is about right) - they went through some serious quality control issues that burnt down a bunch of houses and a few factories (the breakers wouldn't trip).  That's why Federal is out of business (shame - I liked Stab-Loc until then)  The other possible reason is that when you install breakers in an industrial panel, you usually choose the slower blowing breakers due to transients in factory settings.  In Canada, they allow 2 seconds to trip.  In that case I would 100% agree with your thinking - then 14 or 12 ga wire is the only option in that setting.


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## projectnut (Jun 23, 2020)

As mentioned a circuit breaker upstream of the machine protects the infrastructure wiring, not the machine.  I have my Bridgeport running on a dedicated 50 amp circuit just because it was available in the load center.  It doesn't need nearly that much power, just an outlet with that configuration to meet code.  I could replace the 50 amp breaker with a 20 or 30 amp one, but that still wouldn't protect the machine.  

You haven't mentioned what type of mill you have, but most commercial 3 phase models have overloads, sometimes referred to as "heaters" in each line.  They are essentially internal circuit breakers.  If too much current passes through the heater it will open the contacts and stop the motor.  Heaters come in various amp ratings.  If I remember correctly the ones in my mill are 4 amps.  

When I first got the mill I was having trouble with the overloads tripping when doing anything other than light cuts.  I looked in the box and found the high school I bought it from had replaced the original 4 amp overloads with 2 amp overloads.  Apparently they didn't want the kids to over work the machine.


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## Bi11Hudson (Jun 23, 2020)

The above responses about cover it all, although I would do things my own way as much from habit as anything. *TRUE*: The breaker protects the circuit, not the load. The load will have an "over-current" device somewhere on board. Consider not just the sewing machine above, but any lamp or kitcher counter load plugged into a 15A(or 20A) circuit. The cord alone should be a give-away. Many such appliances, fans, blenders, etc. have what is called "impedence protection" for light loads. If you really want to know what that means, look into http://www.hudsontelcom.com/uploads/ShopElex.pdf. But that's a loonnngggg lesson.

The reason for this post is to cover specifically motor protection, usually a motor starter VS a contactor. A contactor is nothing more than a high current relay. Sometimes they are "definate purpose contactors" but still just a large relay. Currents of several hundreds of Amps, perhaps even a few thousand. My industrial background coming into play here. . . 

A motor "Starter" on the other hand is a "contactor" with an overload device. That is likely the protection for a mill larger than the small "bench-top" machines. It most  likely be a "solder pot" device, although there are other methods. 

The solder pot provides a time rating because motors draw enormous currnnt for a few milliseconds when starting. Sometimes 6 or more times run current. The functional element is not really solder but a similar "eutectic" alloy that allows the motor to draw that enormous current for a couple of seconds before it melts. When it does melt, a spring loaded device opens a contact in the motor circuit, dropping out the "contactor" portion of the starter.

Depending on manufacturer, and age, and a few other variables, the solder pot may be directly a part of the heater or may be an external part. The heater is specified very specifically for the motor. Look to the motor nameplate for "Full Load Amps" or "FLA". The heater *must* match that number quite closely, just installing "any old heater" will cause trouble. It must be fairly accurate.

There is one exception to this. A motor rated for 10 Amps FLA (for example) might have a heater rated for 8.5 amps. This would be more common for home shop machinists where the maximum load might never be reached. It is advised that the proper heaters be kept close by "just in case" they are needed. The other way, where the heater is larger than the motor, must *never* be allowed. That would allow the majic smoke to get out. And we all know how difficult that smoke is to get re-inserted.

The "line cord" must be large enough for for all motors and control circuits. But it is not necessary for it to be large enough for starting current. That lasts only a few seconds and the starter overloads will shut down the motor. Repeated attempts to start the motor in a short time span may well overheat the heaters. The only solution to that is to have a cup of coffee (or adult beverage) and let things cool down. The line cord may get warm but supposedly is rated for that. I'm "old school" and don't trust plastics but the cord should have a temperature rating embrossed on it.


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## Skowinski (Jun 23, 2020)

Thanks everyone, I'm learning.  I understand the most basic things about electricity and electronics, but breakers and wire gauges seem to escape me, lol.

 I guess I think of breakers as being much the same as fuses.  Thinking about it, fuses are there to protect the wiring, to prevent melt downs (and potential fires).  So, if the milling machine I'm planning to buy specifies 240V and 9A full load current, as long as I don't have a <9A circuit breaker, just buy the correct plug for the 50A outlet on the wall, connect it to the wire from the machine, and start playing.

That's an easy solution.


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## Dabbler (Jun 23, 2020)

@Skowinski just one more minor point:  Consumer breakers are only designed to carry 80% of that value in a long or continuous load situation, which is 12A  for a 15A breaker in long term use.  So your 9A motor needs a breaker greater than 11.25A, because of design constraints, or, worded better, for a 15A circuit, never use a motor rated at 12A or greater.  use a circuit with greater capacity.


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## matthewsx (Jun 23, 2020)

Skowinski said:


> Thanks everyone, I'm learning.  I understand the most basic things about electricity and electronics, but breakers and wire gauges seem to escape me, lol.
> 
> I guess I think of breakers as being much the same as fuses.  Thinking about it, fuses are there to protect the wiring, to prevent melt downs (and potential fires).  So, if the milling machine I'm planning to buy specifies 240V and 9A full load current, as long as I don't have a <9A circuit breaker, just buy the correct plug for the 50A outlet on the wall, connect it to the wire from the machine, and start playing.
> 
> That's an easy solution.



Correct as above, this is another thing I like about this place. You can discuss electricity (which most of our machines need to function) and people with experience will respond. Any possible bad advice will be pointed out almost immediately thus keeping you safe unless you do something silly. Electricity is dangerous, but like anything dangerous it's better to have knowledge than be in the dark....  

John


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## Skowinski (Jun 23, 2020)

matthewsx said:


> Correct as above, this is another thing I like about this place. You can discuss electricity (which most of our machines need to function) and people with experience will respond. Any possible bad advice will be pointed out almost immediately thus keeping you safe unless you do something silly. Electricity is dangerous, but like anything dangerous it's better to have knowledge than be in the dark....
> 
> John



Yep, I hang out here because I've learned a lot from people who have gone down these roads before me, and are willing to share their knowledge.  There are other areas (e.g. motorcycles) where the roles are reversed, but here I'm the beginner, and that's fine.  I like to research everything before I take any action.  Look before you leap.  With electronics and electrical stuff, that seems to be an especially important rule to live by.


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## Reddinr (Jun 23, 2020)

> it should have tripped the breaker in under 100 milliseconds


Dabbler- I left a couple of details out of my "back in the day" story so it didn't go on and on.  The isolation transformer was a fairly small one and the breaker was a "long delay" one.  The impedance of the transformer in combination with the long, small cord is what caused my concern.  I was surprised how fast it happened though.    

Short circuits are only part of the story as people have mentioned above.  Overloads can also happen and could potentially draw current that is below the current the breaker is rated to trip at but higher than the cord that connects the breaker to the machine can handle without overheating.  In those cases, having a breaker that is more closely sized to the load is a good thing.  It will trip in more cases than not.  Chances of this happening without a machine fuse popping before causing any real harm are probably low.  For me though, the cost of doing it right is low enough so you can avoid the risk.

cheers everyone!


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## hman (Jun 23, 2020)

matthewsx said:


> Electricity ... it's better to have knowledge than be in the dark....


Profoundly punny!  I love it!


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