Voltage Drop Residential

luvflyin

luvflyin

Touchdown! Greaser!
Joined
May 8, 2015
Messages
15,856
Location
Santa Barbara, CA
Display Name
Display name:
Luvflyin
I’m about 270 feet from the transformer. Not as the crow flys, but the run of the wire. That’s to the Main panel. I’m going to add an outlet in the back yard about another 50 feet from the house. There’s probably about 30 feet from the main to the point where I’ll be tapping of from to get that last 50 feet of run. It’s a 15 amp circuit wired with 14 gauge. Question is, is there anything to be gained by doing that last 50 feet in 12 gauge to mitigate voltage drop.
 
That is not too far from the transformer.

If you may have a need to operate a high current device, up around 15 amps, the lower drop of 12 would be cheap insurance, plus, put a 20 amp breaker, in case you want more some day.

Cheap to go bigger on wire size at installation, expensive later.

I put 10 in to my garage, 3 wire, 240 volt. Overkill when built, but a friend gave me his old 45 amp 240 volt generator, I put it in the detached garage. The 30 amp 240 breaker in the house will carry enough load to take care of my critical needs.

Again, the larger wire is cheap right now.
 
Last edited:
$126 vs $84 12 vs 14/3 romex at homedepot now.
Or is it buriable?

(Plus difference in breaker price)

What do the charts say the voltage drop is?
 
Let'sgoflying! said:
$126 vs $84 12 vs 14/3 romex at homedepot now.
Or is it buriable?

(Plus difference in breaker price)

What do the charts say the voltage drop is?
Click to expand...
It won't be direct burial. I dunno what the charts say. I've done some googling and can't find it. Probably there and I just can't see it. The breaker is already there. This isn't about Amps. It's about Voltage. There is a drop in voltage over distance. I'm wondering if using bigger gauge wire will decrease the voltage drop.
 
Capt. Geoffrey Thorpe said:
Yes. But not a lot.

Voltage Drop Calculator

The purpose of this calculator is to determine voltage drop, percent voltage drop, and voltage at load based on the length of the circuit.
wiresizecalculator.net
Click to expand...
Thx. I was waiting for you. As I remember you're an electrical engineer. If it makes a difference, even a little, I'll probably go with the big stuff.
 
MauleSkinner said:
I don’t want to go to the link…is a mechanical engineer 1%-2%ish more than an electrical engineer, or less? ;)
Click to expand...
If he has the data, it doesn't matter that he's a mechanical engineer instead of electrical. I'm sure he knows Ohm's law, just as I, as an electrical engineer, know the laws that apply to simple mechanical stuff.
 
What are you wanting to run on this circuit? Typically voltage drop isn't a major factor for house wiring - run the appropriate gauge romex for the breaker and you're fine. In this instance, difference between 14 and 12 is roughly 3 and 2 ohms/1kft, respectively. At steady state of 80% load on a 15a breaker over 80ft that's 0.9V difference. For lighting or other typical resistive loads not going to change anything noticeably.
More important could be flicker. If you're running a 1/2hp motor and an incandescent light, you'll definitely see it every time the motor starts. Slightly less with 12AWG than 14AWG. LED lighting tends to ride through flicker much better.
 
MauleSkinner said:
Ohms law deals with volts, amps, and resistance. It doesn’t quantify mechanical engineers and electrical engineers.
Click to expand...
I had a mechanical engineering student that tried to explain to me that mechatronics was "outside his discipline" - I managed to not laugh at him.
 
Mechanical engineers build weapons.
Civil engineers build targets.
Thread drift.
 
DreadPirateWill said:
What are you wanting to run on this circuit? Typically voltage drop isn't a major factor for house wiring - run the appropriate gauge romex for the breaker and you're fine. In this instance, difference between 14 and 12 is roughly 3 and 2 ohms/1kft, respectively. At steady state of 80% load on a 15a breaker over 80ft that's 0.9V difference. For lighting or other typical resistive loads not going to change anything noticeably.
More important could be flicker. If you're running a 1/2hp motor and an incandescent light, you'll definitely see it every time the motor starts. Slightly less with 12AWG than 14AWG. LED lighting tends to ride through flicker much better.
Click to expand...
There's a small sump pump at the end of the existing 14 gage run. The extension I'm going to run out from there will be lighting. Maybe a power tool every now and then. No big motors though.
 
I did not understand this was an addition to an existing 14 ga run. Extend with 14 ga will be fine.

I thought that you were putting in an all new run from a 15 amp breaker.
 
DreadPirateWill said:
What are you wanting to run on this circuit? Typically voltage drop isn't a major factor for house wiring - run the appropriate gauge romex for the breaker and you're fine. In this instance, difference between 14 and 12 is roughly 3 and 2 ohms/1kft, respectively. At steady state of 80% load on a 15a breaker over 80ft that's 0.9V difference. For lighting or other typical resistive loads not going to change anything noticeably.
More important could be flicker. If you're running a 1/2hp motor and an incandescent light, you'll definitely see it every time the motor starts. Slightly less with 12AWG than 14AWG. LED lighting tends to ride through flicker much better.
Click to expand...
Here's what I got. I paced it off and came up with about 270 feet from the transformer to the meter box. There is a 100 amp main breaker there. It's a single whatever ya call em. Not like the twofer ones that are barred together for typical 220 circuits. Looks like about maybe 6 gage running from there to the breaker box inside. Two blacks and a white. The run from the main at the meter to the breakers is I'd say about 50 feet. The run from there, 15 amp 14 gage, to the box outside is about 50 feet. Nothing in between. I put that circuit in a couple weeks ago. Wish I'd thought about this then before I buried it behind new drywall. Oh well. That's where I'm going continue another about 50 feet into the yard. The sump pump at the box on the house is 1/3 hp, 9.5 amp.
 
luvflyin said:
Here's what I got. I paced it off and came up with about 270 feet from the transformer to the meter box. There is a 100 amp main breaker there. It's a single whatever ya call em. Not like the twofer ones that are barred together for typical 220 circuits. Looks like about maybe 6 gage running from there to the breaker box inside. Two blacks and a white. The run from the main at the meter to the breakers is I'd say about 50 feet. The run from there, 15 amp 14 gage, to the box outside is about 50 feet. Nothing in between. I put that circuit in a couple weeks ago. Wish I'd thought about this then before I buried it behind new drywall. Oh well. That's where I'm going continue another about 50 feet into the yard. The sump pump at the box on the house is 1/3 hp, 9.5 amp.
Click to expand...
*Typically* the run from the transformer to meter panel is owned by the power company. That, and what you've got beyond all sounds pretty normal. Tagging on an extension for some lights and the occasional power tool is likely no problem. You could run into an issue if sump pump is running and you try to start a table saw at the end(or vice versa), likely to trip breaker. Had you run 12AWG from a 20A breaker, probably sump pump and tool no problem.
Caveat to this is I'm not versed in CA interpretation of NEC nor am I your county inspector, so landing is at own risk so to speak.
 
DreadPirateWill said:
*Typically* the run from the transformer to meter panel is owned by the power company. That, and what you've got beyond all sounds pretty normal. Tagging on an extension for some lights and the occasional power tool is likely no problem. You could run into an issue if sump pump is running and you try to start a table saw at the end(or vice versa), likely to trip breaker. Had you run 12AWG from a 20A breaker, probably sump pump and tool no problem.
Caveat to this is I'm not versed in CA interpretation of NEC nor am I your county inspector, so landing is at own risk so to speak.
Click to expand...
Thanks. What do you figure is the minimum voltage I should have at the end of the run? I built a new house in Washington a few years ago. The guy from the power company said a new transformer had to be put in because it would be to far from the nearest existing one. Said they have to guarantee delivery a certain minimum voltage to the house. Don't remember the number. But I'd figure it would be pretty much universal.
 
luvflyin said:
Thanks. What do you figure is the minimum voltage I should have at the end of the run? I built a new house in Washington a few years ago. The guy from the power company said a new transformer had to be put in because it would be to far from the nearest existing one. Said they have to guarantee delivery a certain minimum voltage to the house. Don't remember the number. But I'd figure it would be pretty much universal.
Click to expand...
Typically the range is +/- 5% of nominal for the utility at the meter base - so at the mains you should have 114-126V. This can change throughout the day, or seasonally dependent as well. At the end of a run from that, could have as low as 110 around the house without a problem. Usually you'll be much closer to 120, but lower is not necessarily cause for alarm. You'll often hear of motors "rated" as 110V, this is to allow for normal voltage drop.
If you really need help sleeping you can do some reading on the mechanisms behind conservation voltage reduction - the utility might actually be lowering voltage towards the bottom of that range to reduce load. Side benefit is it lowers your bills too, though it does cause resistive heaters to run longer.
 
luvflyin said:
This isn't about Amps. It's about Voltage. There is a drop in voltage over distance.
Click to expand...

There is NOT a voltage drop over distance. It's ALL about amps, and the cable cross sectional area and material, and the distance.

V = IR — — Voltage = Current * Resistance

The voltage drop is the product of the resistance (for a given cable, proportional to the length) and the current. The AMPS.

If there is no current the voltage drop is zero.
 
The distance isn't a big deal. The voltage drop calculation starts at the main panel, 80' at 12A on 14 gauge copper is about 4% drop. There's no NEC requirement that I'm aware of for voltage drop. Some states mandate 3%. Source for calculator: https://www.southwire.com/calculator-vdrop

Most modern things are very tolerant of voltage drop. I use 16 gauge extension cords, outdoors, all the time at 100'. Most things that are 120V run at 100V, and that's something like 16% drop.

You said "not direct burial" and that's fine, but pretty sure you have to run moisture resistant wire even if in conduit underground, because it's going to get wet. Romex in conduit is a bit of a hack, and because of the moisture issue may be a code violation, plus it just sucks to run in conduit. I think most box stores sell 14 copper THHN/THWN wire by the foot, and that would be the way I'd go, in pvc conduit. Needs to be on a GFI, either outlet or breaker. Probably easiest to make the last outlet you're tapping off of a GFI outlet.
 
Spring Ford said:
There is NOT a voltage drop over distance. It's ALL about amps, and the cable cross sectional area and material, and the distance.

V = IR — — Voltage = Current * Resistance

The voltage drop is the product of the resistance (for a given cable, proportional to the length) and the current. The AMPS.

If there is no current the voltage drop is zero.
Click to expand...
If ‘…there is NOT a voltage over distance…’ then what does ‘…the voltage drop is the product of the resistance…’ mean?
 
Albany Tom said:
The distance isn't a big deal. The voltage drop calculation starts at the main panel, 80' at 12A on 14 gauge copper is about 4% drop. There's no NEC requirement that I'm aware of for voltage drop. Some states mandate 3%. Source for calculator: https://www.southwire.com/calculator-vdrop

Most modern things are very tolerant of voltage drop. I use 16 gauge extension cords, outdoors, all the time at 100'. Most things that are 120V run at 100V, and that's something like 16% drop.

You said "not direct burial" and that's fine, but pretty sure you have to run moisture resistant wire even if in conduit underground, because it's going to get wet. Romex in conduit is a bit of a hack, and because of the moisture issue may be a code violation, plus it just sucks to run in conduit. I think most box stores sell 14 copper THHN/THWN wire by the foot, and that would be the way I'd go, in pvc conduit. Needs to be on a GFI, either outlet or breaker. Probably easiest to make the last outlet you're tapping off of a GFI outlet.
Click to expand...
Good link, thanks.
 
luvflyin said:
If ‘…there is NOT a voltage over distance…’ then what does ‘…the voltage drop is the product of the resistance…’ mean?
Click to expand...
If there’s no load on the circuit, there wont be a voltage drop.

If you put a load on the circuit, you’ll see the voltage drop specified in the documentation. In the example @Albany Tom noted:
80' at 12A on 14 gauge copper is about 4% drop.
Click to expand...
 
MauleSkinner said:
If there’s no load on the circuit, there wont be a voltage drop.

If you put a load on the circuit, you’ll see the voltage drop specified in the documentation. In the example @Albany Tom noted:
Click to expand...
Ok. Now I get it. I just put a volt meter to it at the breaker box. At the main feed I get 120.3 on one of the hots and 119.5 on the other. Get 236.5 across both. Doesn't quite add up exactly. From the breaker I'll be using, nothing on it right now, I get 120.3. In a week or two it'll be hooked up to the outlet box outside. I've got a 9.5 amp sump pump there. I'll fire that up and some other things like a compressor and some hand tools. See what I get then.
 
I wish I’d known more about electricity when I was running my shop on a 20-amp fuse at the house with 80 feet of wire to the shop. I could run a window air conditioner, lights, table saw, and dust collector all at once, but I had to turn them on in the proper order. ;)

I did upgrade the electrical shortly after I bought the house.
 
MauleSkinner said:
I wish I’d known more about electricity when I was running my shop on a 20-amp fuse at the house with 80 feet of wire to the shop. I could run a window air conditioner, lights, table saw, and dust collector all at once, but I had to turn them on in the proper order. ;)

I did upgrade the electrical shortly after I bought the house.
Click to expand...
It's almost like figuring out the proper sequence to fire up the computers on Apollo 13, lol.
 
You must log in or register to reply here.
Back
Top