Power tools extension cord voltage drop - more than should be? How much is too much

redikillowatt

Member
Location
TX
I'm not sure this is the right forum (area) for this. I took electrical engineering in college, built & wired houses - using some fairly large tools.
Here, I'm talking 120V. I've used 15A tools on 12ga, 50 ft cords w/o problems. Most of the bigger motor tools I used weren't used continuously - that is a factor.

Now, have 2 corded electric - a 10A string trimmer & 12A edger. Both of these would be used for much longer stretches than a circular saw or sawzall. LOTS of people burn edgers / trimmers up.
Because of where several outdoor outlets are, I can reach a lot, but not all of the yard w/ one 50 ft cord. I have (2) 50 ft, 12 ga cords on hand.

Did a test w/ the smaller 10A string trimmer. With no load, there was 122+ V at end of 50 ft, 12 ga. That V likely will drop to 120V or bit less during the day, give or take.
Connecting a multi-meter & running 10A trimmer for a couple min., showed a steady 5 V drop (in 50 ft of 12 ga). So 4%.

I'm wondering how many volts below 120 is too much for the motors they use in these types of tools? A voltage that may not burn it up in a few uses, but is likely to shorten the tool's life?

That's 2x the V drop for 50 ft, 12 ga cords I've seen on several V drop calculators. I'll repeat the test using a 3 outlet mult-tap, for max contact of blades to female outlets - see if it makes a difference. I don't have much experience comparing actual V drops to theoretical calculators.

If the line voltage is 120 (not 122+), that's 115V. I'm not sure if that's wise. And I haven't tested adding 50 ft - or 25' more cord, nor tested the 12 A edger yet.

Mfg's manual shows cords for tools rated (roughly) 7.1 A - 12.0 A, for 50 ft - use a 14 ga cord. If there's (actual) 5V drop for a 10A tool on a 50 ft - 12 ga cord, it'll be more with a 14 ga.

I'll have to measure if 75 ft of cord would reach everything. Ext. cords ARE expensive!
If it would, not sure if 50 ft of 12 ga + 25 ft of 10 ga would work for 12A edger. Don't know I've ever seen a (good) 25 ft, 10 ga ext. cord. Might have to make it.

For the same amp range 7.1 A to 12.0 or 13 A, for 100 ft cord the manual says use 10 ga. Use 14 ga for 50 ft, but 10ga for 100 ft?
I'm not sure if 50 ft of 14ga is big enough for 12A edger w/ heavier load at times than a string trimmer.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Keeping in mind that this is explicitly not a DIY forum, but your questions involve electrical theory and not how to do something:

You are correct to be concerned about voltage drop and motors. Unfortunately it is impossible to give a 'this much voltage drop will be a problem. '

Motors draw different amounts of current depending upon operating state. Generally it is heat that kills motors, so the hotter a motor runs the shorter its life. Transient excessive current can also damage the brushes in brush commutated motors.

My guess is that 'too much voltage drop' is defined by the motor sagging under what would normally be an acceptable mechanical load, and burning out the brushes. With the same extension cord and gentle use there would be no problem.

But that is just a guess.

Jon
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
I remember that Milwaukee, back in the day when the only option was corded, stated that their tools were designed to work with long extension cords and lower than normal voltages encountered on construction sites. It was a selling point.

-Hal
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Engineer
It depends on what kind of motor it is.

A series-wound brush-type motor -- by far the most-common type in corded portable power tools -- will not be harmed by low voltage or high source impedance. It will simply slow down. (and draw less current, and generate less heat) The only criteria for "how much voltage drop is too much" is your personal preference for a slower-running motor.

When doing voltage-drop calculations, be attentive about whether the impedance/distance chart is for a single conductor or a pair.
 

redikillowatt

Member
Location
TX
Thanks for the replies.
Another thing I thought about, is the wiring in the added garage circuit. I put it in myself - w/ GFCI - yrs ago when 20A resid. circuits still used 12ga.
It's a home run from the load center to the 1st outlet on that circuit + 2 outlets off of it. Don't know the exact distance, but I'd guess 45 ft to 1st outlet ( I tested from last nite). The other outlets on the circuit are ~ 15' & 20' farther. Nothing else runs on that circuit - unless I plug it in.

But that means there's already a 45' to 60' - 65' "cord" in the equation (for V drop), before plugging in cords. IF... I run it off the farthest outlet, then another 75' (min.) of ext. cord, to reach most of the yard. (may have to set a sub-panel by the mail box - LOL).
I can easily measure V drop using 1- 50', 12ga cord, then on 100' of 12 ga. I'm guessing on 100', 12ga cord, I'll see about 112 V (if no load V=120). Even less if it's really hot outside.
Whether I need to (get a loan) for a 25' - 50', 10ga cord, not sure.

I read one reviewer of this trimmer say "it runs fine on 100' 12ga cord." Maybe, but it may be slowly shortening the tool's life.

@drcampbell (not the same Campbell that taught ground water remediation?) -
"series-wound brush-type motor... will not be harmed by low voltage or high source impedance. ...and draw less current, and generate less heat"
If that's true, why do all quality mfg's make a very big deal about not running tools (outdoors, attended to) on lighter extension cords or circuits than they or some authority recommends?
Why do more expensive (some inexpensive) motors have thermal protection (or circuit breakers) built in, if all that running them at lower voltage will do, is (mostly?) cause a performance decrease? I don't mean to be sarcastic - just whether I understand you correctly?

@hbiss - "...a range of acceptable under/over rated voltages of +/- 10% over motor rating.
I could ask to speak to their engineering dept - see what they say. -10% would be 108V on low side, which is fairly low. IFF... motors are specifically designed for that, it might be OK. I don't know I've ever seen (anywhere) a power tool mfg state an acceptable voltage range. It may be in design specs, but I've never seen those specs hinted at,

Winnie, I appreciate your comment - it's not a DIY site. I'm a bit past diy'er when it comes to electricity. As you hinted, I'm trying to compare theoretical vs. real world effects of "too much V drop." Based on research, I was surprised at the measured V drop vs. tables & calculators show.

I'm not (totally) sure about part of your statement. The motor on this trimmer or on my 240V table saw can't violate laws of physics or of electricity.
 

gar

Senior Member
210628-2224 EDT

redikillowatt:

You need a general education on mechanics, thermal, and electrical theory.

All devices no matter what they are have thermal limits, or considerations.

For example: a bar of steel heated sufficiently will turn orange. and become soft and easily bent. Watch a blacksmith work a piece of metal.

With enamel insulated copper wire you will generally have the insulation electrically breakdown before the copper gets real soft.

So whatever you do with an electrical device temperature rise (actually absolute temperature) of the components need too be considered. So I can do about anything I want with an electrical device so long as I don't continuously exceeds its ratings. Some aspects may allow for different overload periods, some are almost instantaneous limits.

I will only discuss series wound brush type motors.

Output speed is a function of motor design, applied voltage, and mechanical load on the motor. Increasing load increases needed supply current, and this increased current causes more internal voltage drop in the motor, and therefore speed. At a particular applied voltage stalling the motor by locking its rotor produces the maximum current for that applied voltage. As source voltage is lowered that locked rotor current drops.

If you use a long extension cord, then the motor current may produce a substantial voltage drop in the cord. This does not matter if you can get sufficient torque, and speed without overheating the motor. By reducing the mechanical load you can avoid overheating the motor. Can you do the work you need to by lowering the mechanical load I can not know. You have to experiment.

If you want a nice working yard tool, then consider a battery powered EGO machine.

.
 

paulengr

Senior Member
NEMA and UL general service conditions are +10% / -15% from the nominal utilization voltage, 110 V. For motors NEMA reduces this to 10%.

There are charts showing you maximum extension cord lengths and gauge based on amp draw.

That being said, total waste of time. EGO makes good stuff. I have actually retired my gas landscaping tools. There are limits. I get about 20 minutes on the leaf blower and half a day with the hedge trimmer. With two batteries I can go almost all day. Edger: really? Use edging. Once and done.

I used to have a corded leaf blower. It was cheap at the time. What a pain. I’d go back to a gas backpack blower any day over that thing. Underpowered and the cord was nothing but a nuisance.

I know Stihl also makes decent cordless. Unless it has drastically improved stay away from Greenworks. Most tools of this type sold in the US are made by Stihl Virginia Beach anyway. MTD, Husky, Stihl, many others all by the same plant with over a dozen labels.
 

mikeames

Senior Member
Location
Germantown MD
Occupation
Teacher - Master Electrician - 2017 NEC
I remember that Milwaukee, back in the day when the only option was corded, stated that their tools were designed to work with long extension cords and lower than normal voltages encountered on construction sites. It was a selling point.

-Hal

What Hal Said above.

Yea I have a airless paint sprayer that is designed to be used on long cords. Its has a DC motor. It has an inverter and power supply that always gives the motor what it needs regardless of extension cord length. It needs this because the motor responds to pressure drop. So when the trigger is pulled momentarily or continually it has to respond appropriately with no lag or sag in response or the paint wont spray consistently.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
We have seen VD issues with large pipe benders on construction sites. The combination of the temporary wiring length and the cord make the benders unable to bend 4" EMT.

Recently I was asked to troubleshoot a bender that couldn't bend the 4" EMT and when I tested the voltage at the machine it had only 70 volts. We generally run a separate #12 or #10 homerun just for the bender.
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Engineer
Gotta love construction sites. My personal favorite is seeing a snake's nest of ratty old extension cords with the jackets damaged and the conductors exposed, strung hither & yon through the mud & puddles, being walked on and driven over, and being asked to get rid of the damn gfcis because they keep tripping.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Gotta love construction sites. My personal favorite is seeing a snake's nest of ratty old extension cords with the jackets damaged and the conductors exposed, strung hither & yon through the mud & puddles, being walked on and driven over, and being asked to get rid of the damn gfcis because they keep tripping.
Yes, for years we used GFCI circuit breakers on 120 volt temp power drops in new construction. But the things you mentioned created a logistical nightmare of having to reset tripped circuit breakers especially when it rained. For that reason we no longer use GFCI circuit breakers for temp power. Cords are supposed to be inspected but no one seems to bother doing that.
 

Strathead

Senior Member
I hope others will correct me if they find I am missing something...
First, it is the combination of the extension cord and all the wire length between the plug and the utility. Second, a motor will use more amperage to try and do the same amount of work when the voltage drops. Because of this, the wires will get warmer with more amperage. So my advise to the layman is, first as others stated, when the motor can be heard to be bogging down, probably too long or too small a wire gauge. Second, occasionally feel the cord and the connector. If it feels warm/hot to the point where it is just starting to feel uncomfortable, probably too long or too small a wire gauge .
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Engineer
... a motor will use more amperage to try and do the same amount of work when the voltage drops ...
That's true of "fixed" speed induction motors. Not all motors.
 

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
Yes, for years we used GFCI circuit breakers on 120 volt temp power drops in new construction. But the things you mentioned created a logistical nightmare of having to reset tripped circuit breakers especially when it rained. For that reason we no longer use GFCI circuit breakers for temp power. Cords are supposed to be inspected but no one seems to bother doing that.
That is until an OSHA inspector shows up on site.
 

paulengr

Senior Member
I'm under the impression that OSHA requires the grounded conductor to be opened. Portable GFCI have this capability. I believe I read it from ECM or something similar. I'll look for it

It’s called assured grounding. See OSHA 1910.3xx. Three options:
1. Use doubly insulated tools. These have the square within a square symbol and will be intentionally 2 prong.
2. Use a GFCI. I hear you on rain. Use a portable plug in one. Problem solved. Used to be outrageous. Now they’re like $30.
3. Assured grounding. Requires quarterly inspection of all the cords. They don’t tell you what to do but generally a Megger test between conductors is sufficient. When we did it we just used colored tape and had a chart of what colors were for which quarter.

Most opt for option 2 because it’s the “easiest” but not the only option.
 
Top