Voltage drop on long runs inside shop

[Shovelhead]

Probably overthinking it but still curious about voltage drop on long runs inside a large shop, or even a big azz house.

Wiring a 16x30 wood framed room inside a 42x54 metal building.
Panel is located in front corner outside of the framed room.

I pulled four 120v circuits from the panel in conduit under the slab to the back wall where I’ll locate a junction box. About 70’ of wire. THWN #12.

Due to the layout I’m now heading back where I came from. LOL
From the j box another ~8’ along a wall girt and then entering the framed room. I’m feeding 8 outlets total on both sides of the wall between the framed room and the shop.
By the time I reach the last outlet inside the room that’s another 30’. So we have a circuit with ~110’ of #12.

When I look at voltage drop tables it appears a problem could arise, depending on load.

How or when might there be a problem using typical 120v power tools? Miter saw, band saw, sanders, routers, etc.
Very unlikely more than one tool would be in use at the same time and I’m also staggering the outlets between two circuits on that wall.
This is all dependent on load isn’t it?

Thanks

 

[mike93lx]

You will be fine.

A 120v welder might be a problem, or trying to run a 20a motor.

I have run everything you listed on 100' 12/3 extension cords many, many times, and it's nothing unique

 

[Shiftless]

You will be fine.

A 120v welder might be a problem, or trying to run a 20a motor.

I have run everything you listed on 100' 12/3 extension cords many, many times, and it's nothing unique

to that ^^^^^^

The only time I’ve ever seen long runs of wire cause damaging voltage drop was when I watched a guy plugging a table saw into a 100 ft. long 16 ga. extension cord. I said in a loud voice that I couldn’t do that and it would probably damage the motor. He replied that he has done it before with no problem. Guess what happened?

 

[Shovelhead]

I really wasn’t worried about heating up the wire so much but got to thinking about initial start up of a motor and under voltage damage which I suppose would have to remain longer than instantaneous.

 

[Shovelhead]

It’s hard to find specs on power tools for “running” amps. They all advertise this saw has a 10 or 15 amp motor which really don’t tell the whole story.

Also made me wonder about people with 3500 square feet two story houses and circuits running all over hells half acre.
But, I reckon them folks don’t have sanders in their bathrooms.

 

[mike93lx]

Also made me wonder about people with 3500 square feet two story houses and circuits running all over hells half acre.
But, I reckon them folks don’t have sanders in their bathrooms.

It's the 1.75hp table saw in the 3rd floor guest room that'll get you

 

[Shovelhead]

Yeah, or maybe Suzy Q plugging in her 10 inch, I mean 10 amp, out on the balcony

 

[sparky 1971]

I've only seen voltage drop a real issue one time. A 120 volt air compressor wouldn't start. When I got there and started checking things out, I discovered it was on the end of about 350' of #10 wire. We moved it to another receptacle so that it was being temporarily fed from about 250 feet while I pulled new wire and it worked, but I'm sure there was still considerable drop on start up. You don't have anything to worry about.

 

[wyliesdiesels]

Ive seen portable air compressors struggle to start on 100' extension cords... the saws wont have loads on them so i doubt it will be much of an issue.... if you really start biting hard into the wood it may trip a breaker as it draws more amps and the voltage drops...

 

[Shiftless]

I really wasn’t worried about heating up the wire so much but got to thinking about initial start up of a motor and under voltage damage which I suppose would have to remain longer than instantaneous.

The sustained under voltage caused by a 16 ga. 100 foot cord (my post #3 above) was what killed the motor on that table saw. Not much load on it of course, just the inertia from starting up a 10 inch blade. It was a fine motor until that. In less than half a minute, it went from fine to completely dead.
The guy let me part it out. I took a nice Diablo blade and a dado set. Then I helped him toss it into a nearby dumpster. It was a direct drive Craftsman so not worth trying to replace the motor. The built in circuit breaker didn’t pop.
Hopefully he will remember the lesson about long undersized cords.

 

[RPH]

Repeated low voltage starting creates the issue of heat build up. This will cause the varnish on the magnet wire to soften creating potential shorts in the windings, the end has begun.

 

[mike93lx]

Repeated low voltage starting creates the issue of heat build up. This will cause the varnish on the magnet wire to soften creating potential shorts in the windings, the end has begun.

Voltage drop can definitely cause issues and I don't see anyone arguing that.

It just seems like consensus is that for this application, it won't be a problem

 

[Innovate1]

Long ago I tried to run a radial arm saw on a long small cord. The motor didn't come up to speed and I shut it off after only a few seconds. Got a bigger cord and it worked fine. If you shut them off quickly it shouldn't kill it but not good for it either. Was just shopping for a retractable cord reel and noticed most were 16 gauge or 12 gauge. Might have gone for a 14 gauge if I found one that looked decent but went for the 12. Left me wondering why there aren't many 14 gauge ones. Seems like that would cover most things.

 

[nh_yota]

Large homes and shops have a main panel and one or more sub panels to cut down on circuit length.

 

[rooster59]

You have 4 grounds, 4 hots, and 4 neutrals 12 ga in conduit?

 

[sparky 1971]

You have 4 grounds, 4 hots, and 4 neutrals 12 ga in conduit?

Single phase would be four hots, two, three, or four neutrals, and one ground.

 

[mike93lx]

Single phase would be four hots, two, three, or four neutrals, and one ground.

*could* be. Diy'rs do wierd stuff sometimes...

Really does sound like a small sub would have been prudent in this case. I bet a single 240v 40a circuit with #8 would have covered it all. Probably even a 30

 

[sparky 1971]

*could* be. Diy'rs do wierd stuff sometimes...

Really does sound like a small sub would have been prudent in this case. I bet a single 240v 40a circuit with #8 would have covered it all. Probably even a 30

I guess I should have said "should be". If someone pulls multiple grounds in one conduit, they have no business changing a light bulb. Subs are never a bad thing, but in this case I think he's more than ok and overthinking things.

 

[mike93lx]

I guess I should have said "should be". If someone pulls multiple grounds in one conduit, they have no business changing a light bulb. Subs are never a bad thing, but in this case I think he's more than ok and overthinking things.

Is that worse than using too big of a breaker?

 

[theoldwizard1]

... I watched a guy plugging a table saw into a 100 ft. long 16 ga. extension cord. I said in a loud voice that I wouldn’t do that ...

Where my Dad live "Up North", the transformer that feed his house was undersized (4 or 5 more house were built in the 50 years after it was installed). He had to use a stick to flick the blade on his table saw to get it to start and it took 30+ seconds to get up to speed !

 

[sparky 1971]

Is that worse than using too big of a breaker?

Way worse. Way, way, way worse.

 

[Shovelhead]

Well now I’m scared to f’n death to change a light bulb.

I pulled 2 grounds.

3 circuits are #12
3 hots, 3 neutrals, 1 ground

1 circuit is #10 for HVAC
2 hots, 1 ground

3/4 pvc to a couple feet of 3/4 liquid tight to a plastic j box.

I did not plan to tie the #12 and #10 ground together in the j box.

Something wrong with my plan?

 

[mike93lx]

Well now I’m scared to f’n death to change a light bulb.

I pulled 2 grounds.

3 circuits are #12
3 hots, 3 neutrals, 1 ground

1 circuit is #10 for HVAC
2 hots, 1 ground

3/4 pvc to a couple feet of 3/4 liquid tight to a plastic j box.

I did not plan to tie the #12 and #10 ground together in the j box.

Something wrong with my plan?

No. You are good. An extra ground isn't hurting anything besides the cost

 

[wyliesdiesels]

Well now I’m scared to f’n death to change a light bulb.

I pulled 2 grounds.

3 circuits are #12
3 hots, 3 neutrals, 1 ground

1 circuit is #10 for HVAC
2 hots, 1 ground

3/4 pvc to a couple feet of 3/4 liquid tight to a plastic j box.

I did not plan to tie the #12 and #10 ground together in the j box.

Something wrong with my plan?

you can share an EGC between multiple circuits. EGC needs to be sized for the largest breaker. in your case it needs to be #10

 

[Shovelhead]

What I'm uncertain of is something saying you have to size the ground to the largest conductor in a "raceway".
I have different sized conductors so at first I thought I'd just use the #10 ground and tie all the #12 circuits grounds to it.

But I decided to keep the #10 separate and only tie it to the ground going to the HVAC disconnect box. Why, I don't know, just seemed like a good idea to not have that HVAC ground tied to the ground wires for the other circuits.

All that said, do I still need to pigtail the #12 ground / grounds, to the #10?

 

[nh_yota]

What I'm uncertain of is something saying you have to size the ground to the largest conductor in a "raceway".
I have different sized conductors so at first I thought I'd just use the #10 ground and tie all the #12 circuits grounds to it.

But I decided to keep the #10 separate and only tie it to the ground going to the HVAC disconnect box. Why, I don't know, just seemed like a good idea to not have that HVAC ground tied to the ground wires for the other circuits.

All that said, do I still need to pigtail the #12 ground / grounds, to the #10?

When sharing a ground the idea is that the ground would only carry current during a fault condition unlike hots and neutrals so it only needs to be sized to carry the fault current of the largest circuit, assuming that only one circuit would have a fault condition at any one time.

 

[Shovelhead]

Would the fault have to occur in the conduit?
If all circuits in that conduit are properly grounded from the panel to each device, I don't see how a fault on my HVAC circuit (#10 wire) would affect the other 3 circuits (#12 wire) in that same pvc conduit.

But. I'm not an electrical engineer, or an electrician by trade.

 

[sparky 1971]

Would the fault have to occur in the conduit?
If all circuits in that conduit are properly grounded from the panel to each device, I don't see how a fault on my HVAC circuit (#10 wire) would affect the other 3 circuits (#12 wire) in that same pvc conduit.

But. I'm not an electrical engineer, or an electrician by trade.

A fault can occur anywhere in the circuit(s). A fault in the HVAC won't affect anything else but if there was one ground wire it would have to be large enough to handle the fault. You're fine as is. All you needed was the #10 ground to the panel, but since you also have a #12, its fine. Not necessary, but it won't hurt anything. Maybe in the future you will need another circuit. You can tie a hot and neutral to the #12 ground and pull them in with it.

 

[Shovelhead]

10-4
Thanks 71

Do I have to tie the #12 grds to the #10 grds? Or can I have them separate tied to their respective wire size in the j box as was my plan?

 

[alfredeneuman]

All of the grounds need to be connected together regardless of the size
You can leave the #12 grounds connected to the boxes as long as it feeds a 20A circuit.

 

[Shovelhead]

I really wasn’t worried about heating up the wire so much but got to thinking about initial start up of a motor and under voltage damage which I suppose would have to remain longer than instantaneous.

Would like to talk about this a bit more. I got 3 circuits finished and made up inside the panel and did some testing yesterday. Sorry for the long winded post but want to splain it best I can.

My electrical math is non existent, and would appreciate some explanation of what I’m seeing.
This may all be old hat to some of y’all but I am just curious to learn more about what I have and what I’m seeing.

These circuits are #12 THWN, from main panel about 60-65’ to a J box, from J box they are 12/2 romex, combined total of about 100’ - 110’ at most to the furthest receptacle from main panel.

I used a shop vac and a 12” miter saw.
Shop vac = 5.25 hp , ~11-12 A
Miter saw = 15A

Plugged both in to the same circuit, different receptacles.
Miter saw at approximately 80’ , and shop vac 90’.
Wire Measurements are guesstimates but fairly close.

Using a Fluke set to record minimum voltage and a cheapo clamp on meter.

Line voltage was ~124.
Fired up miter saw and meter dropped to 106v at same receptacle. By myself so wasn’t able to check amps at panel.

Later on I had my old lady operate the miter saw while I checked amps at the panel.
Had shop vac running seeing 11A on the clamp meter.
She turned on the miter saw and pretty dang sure my meter showed 40A at start up!
With both running my clamp on read 20A.
GE “half size” thin 20A single pole Bkr , did not trip.

First off, that saw and shop vac will never ever be running at the same time. But.
Couple things here that concern me and I ain’t smart enough to figure out.

1) yesterday was 70 degrees, likely very little load on my Ckt from the substation. Strong line voltage was 124-125.
But what about August, 95 degrees, and maybe line voltage is closer to 117-120.

2) Why didn’t the 20A Bkr trip when the saw started and I seen 40A.
Same reason a tree limb that rubs a line sometimes dips the system voltage rather than tripping the Bkr at the Sub. Must see a sustained ”fault” for a determined time, depending on relay settings.

3) I have got mixed answers on the wire I used, the distance, the derating, and what temperature table to use for derating, and the actual conductor rating, prior to derating.
In my uneducated mind, #12 is good for 20A. Add in the above and it gets real cloudy to me.

Today I’m going to repeat the same thing to make sure I wasn’t seeing things.
Also do the same on a circuit/receptacle that’s 6’ from the main panel and record the readings.

Thanks for your time and thoughts.

 

[b-boy]

Ive seen portable air compressors struggle to start on 100' extension cords... the saws wont have loads on them so i doubt it will be much of an issue.... if you really start biting hard into the wood it may trip a breaker as it draws more amps and the voltage drops...

I can verify this.

When my barn was under construction, I tried running a portable air compressor on 150ft 14ga extension cord from my house. It did not go well.

 

[dcg9381]

I recently ran 12/3. Breakers are 20A. Basically I alternate outlets down a wall (different breakers) that's about 80' in length, probably 90' of wire total.

We've had no issues in a wood shop with standard 120V tools. But my POC runs a little hot, probably 127V at the panel.

 

[mike93lx]

Would like to talk about this a bit more. I got 3 circuits finished and made up inside the panel and did some testing yesterday. Sorry for the long winded post but want to splain it best I can.

My electrical math is non existent, and would appreciate some explanation of what I’m seeing.
This may all be old hat to some of y’all but I am just curious to learn more about what I have and what I’m seeing.

These circuits are #12 THWN, from main panel about 60-65’ to a J box, from J box they are 12/2 romex, combined total of about 100’ - 110’ at most to the furthest receptacle from main panel.

I used a shop vac and a 12” miter saw.
Shop vac = 5.25 hp , ~11-12 A
Miter saw = 15A

Plugged both in to the same circuit, different receptacles.
Miter saw at approximately 80’ , and shop vac 90’.
Wire Measurements are guesstimates but fairly close.

Using a Fluke set to record minimum voltage and a cheapo clamp on meter.

Line voltage was ~124.
Fired up miter saw and meter dropped to 106v at same receptacle. By myself so wasn’t able to check amps at panel.

Later on I had my old lady operate the miter saw while I checked amps at the panel.
Had shop vac running seeing 11A on the clamp meter.
She turned on the miter saw and pretty dang sure my meter showed 40A at start up!
With both running my clamp on read 20A.
GE “half size” thin 20A single pole Bkr , did not trip.

First off, that saw and shop vac will never ever be running at the same time. But.
Couple things here that concern me and I ain’t smart enough to figure out.

1) yesterday was 70 degrees, likely very little load on my Ckt from the substation. Strong line voltage was 124-125.
But what about August, 95 degrees, and maybe line voltage is closer to 117-120.

2) Why didn’t the 20A Bkr trip when the saw started and I seen 40A.
Same reason a tree limb that rubs a line sometimes dips the system voltage rather than tripping the Bkr at the Sub. Must see a sustained ”fault” for a determined time, depending on relay settings.

3) I have got mixed answers on the wire I used, the distance, the derating, and what temperature table to use for derating, and the actual conductor rating, prior to derating.
In my uneducated mind, #12 is good for 20A. Add in the above and it gets real cloudy to me.

Today I’m going to repeat the same thing to make sure I wasn’t seeing things.
Also do the same on a circuit/receptacle that’s 6’ from the main panel and record the readings.

Thanks for your time and thoughts.

All motors pull higher amperage when starting. It can be from 2-6x the running load. Breakers don't instantly trip when they exceed their rating. It's a combination of time and current (ultimately heat) that would cause them to trip, so a momentary higher load of just 2x is not enough

 

[Shovelhead]

Yes I’m well aware of start up currents.

Well, today was pretty much same deal.
Actually, I seen 57A on start up of miter saw, with shop vac running. That still surprises me.

When the shop vac and miter saw were plugged in to a receptacle ~ 8’ from the panel, my numbers were basically the same. Voltage was 124.8 , clamp on meter said a constant 20A with both running.

I do wonder how long you could maintain a constant 20A of load before that 20A Bkr tripped.
I learned some things, and I don’t think there’s any worry to my wiring. It was interesting to do the checks.

Thanks

 

[RPH]

Look up and learn about circuit breaker curves. These tests are and documented.
Here is an example of one. All you ask in answers is in there.

 

[mike93lx]

I do wonder how long you could maintain a constant 20A of load before that 20A Bkr tripped.
I learned some things, and I don’t think there’s any worry to my wiring. It was interesting to do the checks.

Forever, excluding environmental conditions or voltage drops that would increase current