Question about voltage drop

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crtemp

Senior Member
Location
Wa state
I was playing around with a voltage drop app (southwire) for iphone and plugged in the following

14 gauge copper
120 volts
14 amps at end of run

I wanted it to show me the longest run that could handle this and still be under the 3% allowable drop that code allows
It claims that I can only run 47.98 feet. How is this correct? What if I had a 15 amp circuit in a house that is 125 feet from the panel? Should I be running 12-2? This makes no sense.
Am I doing something wrong on the calculation?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
130629-2337 EDT

Do some basic calculations and you will see why.

What is 3% of 120? At 14 A what is the loop resistance? How many feet of #14 copper wire at at 14 A and a reasonable ambient temperature equals the calculated resistance? Divide that by 2 and you have your distance.

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It claims that I can only run 47.98 feet. How is this correct?
The app is rounding down anything under 3.5%. The actual answer is closer to 40 feet.
What if I had a 15 amp circuit in a house that is 125 feet from the panel? Should I be running 12-2?
In this example: for your home run, even #10 wire is 3.9%. Then change to 14-2 after the first outlet.
Am I doing something wrong on the calculation?
Probably not. But for real kicks and giggles try this calculator.
http://www.electriciancalculators.com/calculators/wireocpd_ver_1.html

Tom Henry has an in depth article on just this subject in the May - June 2013 issue of The Informer, Volume XVI, No. 3.

BTW, 3% is in a Fine Point Note. It's not enforceable as law. You can't flunk inspection for going over.
 
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iwire

Moderator
Staff member
Location
Massachusetts
I wanted it to show me the longest run that could handle this and still be under the 3% allowable drop that code allows
It claims that I can only run 47.98 feet. How is this correct?

It is not correct, but not for the reasons you think.

The NEC does not limit voltage drop on branch circuits. The NEC only suggests trying to limit voltage drop, it does not require it.

If you go out in the real world with a 14 amp load and plug it in to the end of many 14 AWG circuits you will have more than 3% voltage drop.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
I was playing around with a voltage drop app (southwire) for iphone and plugged in the following

14 gauge copper
120 volts
14 amps at end of run

I wanted it to show me the longest run that could handle this and still be under the 3% allowable drop that code allows
It claims that I can only run 47.98 feet. How is this correct? What if I had a 15 amp circuit in a house that is 125 feet from the panel? Should I be running 12-2? This makes no sense.
Am I doing something wrong on the calculation?

Shouldn't that be 12 amps? I realize the difference won't be a lot, but keeping 210.23(A)(1) in mind...
 

iwire

Moderator
Staff member
Location
Massachusetts
Let's face how often does a household branch circuit see 15 amps on a 14 gauge wire.

15 amps, not often for sure but 10-12 amps no problem with a space heater, hair dryer or window AC unit.

But my question would be how many household appliances are really that effected by some under-voltage? :)
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
15 amps, not often for sure but 10-12 amps no problem with a space heater, hair dryer or window AC unit.

But my question would be how many household appliances are really that effected by some under-voltage? :)
Ceiling fans with synchronous motors, maybe?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
But my question would be how many household appliances are really that effected by some under-voltage? :)
Appliances, not really, but for incandescent lights (I think that is what they used to be called) the CTI and output of the bulbs will be reduced.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
130702-1242 EDT

One problem appliance is a small computer UPS. These typically transfer at about 105 V and also do not charge very well at low line voltage. My nominal voltage is around 123 to 125 and for the last hour was within 123 to 124. This does not present a problem for me.

A year or so ago I measured the voltage in a different area about 5 miles away. Their source was stiffer than mine based on a 1500 W heater test load, but their nominal was just around 110 V. They could easily have their UPS clicking fairly often.

A number of years ago at the shop my DeWalt radial arm saw was about 100 ft from the main panel and fed with about 125 ft of #12 copper. Source voltage is usually 120 to 125 V and much stiffer than at my home. Start up of the saw was slow, current about 80 A, and voltage drop more than 30 V. This is not the way to run this saw, but it was a convenient location. Note: a large diameter saw blade is a high inertia load.

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