Low power on shop electrical circuit.

timsch

Member
I've got a shop that I wired up more than half a year ago. It's got around 100' of #4 wire running to the sub panel from the main. I've been using outlets there without issue that I noticed any number of times, but recently I was running a heavier-duty Skil saw and noted that it spooled up slower than normal. I checked all outlets in the shop and it was the same. I plugged it into my old garage circuit and it ran as it should. Nothing else is being run in the shop when I noticed this. Other equipment like my Stihl leaf blower I compared in both circuits and sure enough, it ran slower on my shop circuit too.

I checked connections at the main and sub breaker panels and everything is tight & clean.

A few weeks ago I was running my 220v stick welder on the shop circuit and I didn't notice any difference at all from it welding on my garage circuit.

I checked the amp draw with my clamp-on amp meter on my garage circuit with the saw and it pulled about 19 amp startup and settled around 9-10amp when running. I did the same in the shop expecting a big difference, but it measured about 17 amp startup and 8 amp running. I checked the current draw on my 5hp Quincy air compressor and it ran steady at 18 amp. That compressor seems to be running sluggish also, but I can't easily check it on another circuit because it's hard wired to the panel.

So, I've got a problem, but I'm not sure how to dig down further. All tips pointing me in the right direction will be much appreciated.
 
I've got a shop that I wired up more than half a year ago. It's got around 100' of #4 wire running to the sub panel from the main. I've been using outlets there without issue that I noticed any number of times, but recently I was running a heavier-duty Skil saw and noted that it spooled up slower than normal. I checked all outlets in the shop and it was the same. I plugged it into my old garage circuit and it ran as it should. Nothing else is being run in the shop when I noticed this. Other equipment like my Stihl leaf blower I compared in both circuits and sure enough, it ran slower on my shop circuit too.

I checked connections at the main and sub breaker panels and everything is tight & clean.

A few weeks ago I was running my 220v stick welder on the shop circuit and I didn't notice any difference at all from it welding on my garage circuit.

I checked the amp draw with my clamp-on amp meter on my garage circuit with the saw and it pulled about 19 amp startup and settled around 9-10amp when running. I did the same in the shop expecting a big difference, but it measured about 17 amp startup and 8 amp running. I checked the current draw on my 5hp Quincy air compressor and it ran steady at 18 amp. That compressor seems to be running sluggish also, but I can't easily check it on another circuit because it's hard wired to the panel.

So, I've got a problem, but I'm not sure how to dig down further. All tips pointing me in the right direction will be much appreciated.
Did you do any voltage checks?
 
I did check voltages. All 110/120 and 220/240 were where they were supposed to be. I didn't check them when anything was running though.
 
I did check voltages. All 110/120 and 220/240 were where they were supposed to be. I didn't check them when anything was running though.
"110/120 and 220/240 were where they were supposed to be." What were the actual voltages. What size breaker/fuse is feeding your #6 cable?

What does the tag on the saw say it is supposed to draw?

You said the saw spooled up better on the old garage circuit. That sounds to me like that is a different location than the shop in question. Is that circuit powered from the same main and what is the distance from the main. Same voltages there?

I would check voltages when the saw is starting/running to see if and how much it drops.
 
I did check voltages. All 110/120 and 220/240 were where they were supposed to be. I didn't check them when anything was running though.
Check a couple 110 and 240 circuits in shop,garage and house, record EXACT voltage at each. Check voltage coming into main panel from meter. If all are within 1 volt, check volts in shop while compressor is running. Report results.
 
Check a couple 110 and 240 circuits in shop,garage and house, record EXACT voltage at each. Check voltage coming into main panel from meter. If all are within 1 volt, check volts in shop while compressor is running. Report results.
The shop is a separate building from the garage. Both are on the same main panel. The garage has a sub-panel a few feet away from the main, and the shop has a sub-panel about 100' away with it's own ground rod.

Voltage readings:

MAIN PANEL:
Mains V = 243.2
Shop breaker = 243.2
Shop breaker leg 1 = 121.7
Shop breaker leg 2 = 121.7
Garage breaker = 243.2
Garage breaker leg 1 = 121.6
Garage breaker leg 1 = 121.7

Garage outlet samples 110/120 = 121.8, 121.4
Garage outlet samples 220/240 = 243.4

Shop outlet samples 110/120 = 120.2, 122.7 (lower voltage read at outlet on other side of shop from sub panel, higher voltage right next to panel)
Shop outlet samples 220/240 = 243.3

Shop outlet samples 110/120 (122.7) w/ compressor running = 120.8

My shop is an old pipe frame structure that was an independent auto shop many decades ago. Almost all of the elecrical wiring and outlets now were run/installed recently, but some of the outlets like the one that read lower @ 120.2V were there already. Some of the breakers in use were salvaged from old stock from a retired neighbor who was an electrician.

The saw I used to test is a Skilsaw SPT 77 WML, Power (Ampere): 15
 
Possible your #4 wire has a break in it. Current flows until you need all the current.
 
The shop is a separate building from the garage. Both are on the same main panel. The garage has a sub-panel a few feet away from the main, and the shop has a sub-panel about 100' away with it's own ground rod.

Voltage readings:

MAIN PANEL:
Mains V = 243.2
Shop breaker = 243.2
Shop breaker leg 1 = 121.7
Shop breaker leg 2 = 121.7
Garage breaker = 243.2
Garage breaker leg 1 = 121.6
Garage breaker leg 1 = 121.7

Garage outlet samples 110/120 = 121.8, 121.4
Garage outlet samples 220/240 = 243.4

Shop outlet samples 110/120 = 120.2, 122.7 (lower voltage read at outlet on other side of shop from sub panel, higher voltage right next to panel)
Shop outlet samples 220/240 = 243.3

Shop outlet samples 110/120 (122.7) w/ compressor running = 120.8

My shop is an old pipe frame structure that was an independent auto shop many decades ago. Almost all of the elecrical wiring and outlets now were run/installed recently, but some of the outlets like the one that read lower @ 120.2V were there already. Some of the breakers in use were salvaged from old stock from a retired neighbor who was an electrician.

The saw I used to test is a Skilsaw SPT 77 WML, Power (Ampere): 15
Testing the voltage with no load will show no problems. Set up a continuous load like a heater or the compressor running thencheck the voltages. unloaded conductors act very different than those at 50% capacity. Also check for heated terminal/connections/breakers. If the wire is aluminum, a faulty connection the copper is a real likely issue. Jim
 
I did check voltages. All 110/120 and 220/240 were where they were supposed to be. I didn't check them when anything was running though.
Checking voltages without the circuit in question loaded is next to meaningless.

ASSUMING ground and neutral aren't bonded check voltage difference between them while saw is accelerating, that will show if there's a NEUTRAL issue.

240 Volt loads "acting" normally and 120 loads acting "funky" points to a neutral issue.
 
Last edited:
I agree with no loads there’s no I x R voltage drop so it don’t tell you much. A pure 240 load doesn’t rely on neutral.

Turn some 120 loads on that are fed from L1 and L2 circuits then monitor both those under load. If one is less than 120 and the other is more, I suspect a loose/open/resistive Neutral connection agreeing with the good advice above.

Check each n every connection but you already did that. Look for evidence of heating or burning at connections as a bad connection causes heating and drops voltage
John T
 
I've got a shop that I wired up more than half a year ago. It's got around 100' of #4 wire running to the sub panel from the main. I've been using outlets there without issue that I noticed any number of times, but recently I was running a heavier-duty Skil saw and noted that it spooled up slower than normal. I checked all outlets in the shop and it was the same. I plugged it into my old garage circuit and it ran as it should. Nothing else is being run in the shop when I noticed this. Other equipment like my Stihl leaf blower I compared in both circuits and sure enough, it ran slower on my shop circuit too.

I checked connections at the main and sub breaker panels and everything is tight & clean.

A few weeks ago I was running my 220v stick welder on the shop circuit and I didn't notice any difference at all from it welding on my garage circuit.

I checked the amp draw with my clamp-on amp meter on my garage circuit with the saw and it pulled about 19 amp startup and settled around 9-10amp when running. I did the same in the shop expecting a big difference, but it measured about 17 amp startup and 8 amp running. I checked the current draw on my 5hp Quincy air compressor and it ran steady at 18 amp. That compressor seems to be running sluggish also, but I can't easily check it on another circuit because it's hard wired to the panel.

So, I've got a problem, but I'm not sure how to dig down further. All tips pointing me in the right direction will be much appreciated.
Tim
You said A few weeks ago I was running my 220v stick welder on the shop circuit and I didn't notice any difference at all from it welding on my garage circuit.
220 volts have no current going through the neutral wire. On the other hand 120v has current in the neutral wire. I would recommend looking at the neutral.
One time a tree fell on the triplex going to my house and broke the neutral wire. My 220 v well pump worked just fine. 120v wasn't working that well.
 
Thanks everyone for the replies.

All wiring is copper. The neutral is bonded to the ground in the shop sub-panel. I wired it up as shown in in Fig. 4 of this link

I hope to get back to testing this after work today.
 
Thanks everyone for the replies.

All wiring is copper. The neutral is bonded to the ground in the shop sub-panel. I wired it up as shown in in Fig. 4 of this link

I hope to get back to testing this after work today.
A sub panel should be wired with 3 insulated and one ground wire.
What size wire did you use for neutral??
If you balance your 120v loads you may have found your problem.
 
You bond your ground at the sub panel, and all equipment enclosures. The ground wire is the return to source.......which trips the breaker.
 
One thing you didn't mention was the amp rating of the sub panel. #4 copper is rated for 70 Amp at 60 degrees C. Max length for 240 volts is 191 feet, over that the resistance of the wire will cause significant volt drop.

So, obviously you need to be wired into a sub panel of 70 amp or less. As others have said, double check the tightness of the terminal lugs, then place a significant load on the circuit and check your voltages. I would check at both ends of the #4 wire to make sure there isn't a volt drop there or the additional load may be causing a volt drop at your main panel for some reason.
 
Checking voltages without the circuit in question loaded is next to meaningless.

ASSUMING ground and neutral aren't bonded check voltage difference between them while saw is accelerating, that will show if there's a NEUTRAL issue.

240 Volt loads "acting" normally and 120 loads acting "funky" points to a neutral issue.
That was my first thought too, wore out! If the 240 v equipment works OK but the 120 v doesn't, it sounds like a bad neutral!
 
To clear up any confusion or possible mis information.

At a true sub panel fed from your main the Neutral Buss and Equipment Ground Buss must be separate insulated and isolated from each other. The sub has a buss
For grounds and another for neutrals they are not connected.

There can be only one neutral ground bond at the main panel so do not do not bond again at the sub

For a 120/240 sub panel you carry 4 wires out from the main. Two hots L1 and L2, one Neutral, one equipment grounding conductor.

If you turn on 120 volt loads on l1 and l2 and measure voltage across them and one is higher than 120 and another less than 120 I suspect a loose or open or corroded or resistive Neutral.

There’s no voltage drop if no current flows so they don’t tell you much

Any pure 240 loads don’t use neutral so they could still work fine even with a bad or non existent neutral. Yet 120 loads csn still be bad if there’s a bad neutral

Check connections and check voltages

This is my professional opinion it may be right or wrong no warranty see what other engineers and electricians have to say and if they agree a bunch of them are on here

John T. BSEE. Retired
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top