# Voltage Drop Residential Style



## jar546 (Jun 25, 2013)

I would like to bring this simple, everyday problem to the residential electrical inspector side of our group.  We are in the middle of a voltage drop (Vd) discussion on the commercial side so I thought this would be appropriate and a learning tool for the residential only electrical inspectors.  I really hope that we learn from this and that we have good discussion and some takers for something that we should all now how to do.  I first learned this in 1987 but the code has changed since then and Vd has been demoted to an FPN in the NEC.  Here it goes.

You have received a permit for a homeowner who wants to install an exhaust fan in his accessory pole barn that he uses to store his personal classic car collection.  There is currently no electricity at the pole barn and he just wants an exhaust fan as there is plenty of ambient light when he is there during the day.  (don't look into this too much).  He is proposing to run PVC conduit underground in a trench for the single 120v circuit from the house to the pole barn, a total of 120' from the main service panel.  Taking the FPN into account with a maximum 3% Vd for this branch circuit, what is the minimum size THWN copper wire he will need?

Here is a photo of the nameplate for the motor he will be installing:







A few things about answering this question:

1) We would like to see your answer and how you got that answer.

2) Assume a permit is required, your local conditions not a factor, this is a simple, straightforward question.

3) Telling them they have to go to an engineer is not acceptable.

4) Let us please have some fun with this and learn.

If you need any other info I will give it to you.

Have at it!!


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## jwelectric (Jun 25, 2013)

I would install #14 THW conductors and nothing more.

Table 430.248 1 HP equals 16 amps

430.22 125% of 16 equals 20

Table 310.15(B)(16) THW #14 good for 20 amps

Voltage drop is a design issue not a code issue but this wouldn’t much matter as in the header of Table 430.248 it clearly states that for voltages between 110 and 120 we use 120 for calculations.

Using this conductor there would be a voltage at the load of 110.3 well within the parameters of the voltages mentioned in the header of the Table.


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## Dennis (Jun 25, 2013)

If you want to stay within a 3% VD then you would need a #8 if the unit is wired 120V.  If you use the 240V rating then you could install a #14


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## jar546 (Jun 25, 2013)

jwelectric said:
			
		

> I would install #14 THW conductors and nothing more.Table 430.248 1 HP equals 16 amps
> 
> 430.22 125% of 16 equals 20
> 
> ...


Spoken like an electrician and there is nothing wrong with that.

Next opinion?


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## Dennis (Jun 25, 2013)

I could show you the calculation but I used the table in art. 430  because the nameplate is not allowed to be used for sizing conductors.  I then used this online calculator  Voltage Drop Calculator JavaScript


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## Dennis (Jun 25, 2013)

If this is a hypothetical that is fine but if this is real then look at 210.52(G).  This will require a receptacle.  That being the case a single circuit is not sufficient unless it is a multiwire branch circuit.

Look at 210.23(A)(2)


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## Gregg Harris (Jun 25, 2013)

Dennis said:
			
		

> If this is a hypothetical that is fine but if this is real then look at 210.52(G).  This will require a receptacle.  That being the case a single circuit is not sufficient unless it is a multiwire branch circuit.Look at 210.23(A)(2)


(A) 15- and 20-Ampere Branch Circuits. A 15- or 20-ampere branch circuit shall be permitted to supply lighting units or other utilization equipment, or a combination of both, and shall comply with 210.23(A)(1) and (A)(2).

Exception:  The small-appliance branch circuits, laundry branch circuits, and bathroom branch circuits required in a dwelling unit(s) by 210.11©(1), ©(2), and ©(3) shall supply only the receptacle outlets specified in that section.

(1) Cord-and-Plug-Connected Equipment Not Fastened in Place. The rating of any one cord-and-plug-connected utilization equipment not fastened in place shall not exceed 80 percent of the branch-circuit ampere rating.

(2) Utilization Equipment Fastened in Place. The total rating of utilization equipment fastened in place, other than luminaires, shall not exceed 50 percent of the branch-circuit ampere rating where lighting units, cord-and-plug-connected utilization equipment not fastened in place, or both, are also supplied.

(G) Basements, Garages, and Accessory Buildings. For a one-family dwelling, the following provisions shall apply:

(1) At least one receptacle outlet, in addition to those for specific equipment, shall be installed in each basement, in each attached garage, and in each detached garage or accessory building with electric power. See related


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## jar546 (Jun 25, 2013)

The issue with the receptacle is an absolute requirement but not something we are taking into consideration as this is just an exercise in voltage drop.  Thanks for sharing that info so we don't forget about that requirement.  (often overlooked)


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## jar546 (Jun 26, 2013)

I have to say that I am a bit disappointed that we have so many electrical inspectors on this forum but the same, seasoned, heavy hitters with electrical experience are the only ones participating.


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## gfretwell (Jun 26, 2013)

OK so we are pulling 3 wires plus ground.

I disagree with JW about not being able to use the nameplate. It is far more likely that the FLA is correct than the HP rating (look at the number of class action suits about lies about HP ratings).

In this care it is academic tho since #14 will meet the code either way.

The voltage drop issue will have you in . #14 will drop 9.04v or 7.5%. You have to get up to #10 to get under 3% (2.97%)

There is another plan since we are running 3wire + ground. Wire that motor for 240, drawing 6a at well under 3% V/D. You could then have 2 120v circuits and be below the 50% in 120.52(A)(2) on each of them.


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## gfretwell (Jun 26, 2013)

er  ...  210.52(A)(2)


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## ICE (Jun 26, 2013)

jar546 said:
			
		

> I have to say that I am a bit disappointed that we have so many electrical inspectors on this forum but the same, seasoned, heavy hitters with electrical experience are the only ones participating.


These threads that you have been posting where there is a problem to work out are great.  It gives people a basis to work out thier real world situations.  I for one think that it is best left to the heavy hitters to work out the problem.  So often I have been at a forum and found wrong info and nobody knows the difference.  Hell I see Mike Holmes bump his head...Norm and Bob too.  I can count on the guys here to give the right info.

Not to mention that I come home tired.

These threads will develop into a resource.  Imagine the day when an inspector will be looking at his first nuclear reactor and he comes here to search the archives.  There will come a day, and trust me on this Jeff,  there will be a day when you can charge $40 per year.


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## jar546 (Jun 26, 2013)

gfretwell said:
			
		

> OK so we are pulling 3 wires plus ground. I disagree with JW about not being able to use the nameplate. It is far more likely that the FLA is correct than the HP rating (look at the number of class action suits about lies about HP ratings).
> 
> In this care it is academic tho since #14 will meet the code either way.
> 
> ...


yes gfretwell, I too am a fan of higher voltage to save money on electric bills and wire size for branch circuits.


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## jar546 (Jun 26, 2013)

ICE said:
			
		

> These threads that you have been posting where there is a problem to work out are great.  It gives people a basis to work out thier real world situations.  I for one think that it is best left to the heavy hitters to work out the problem.  So often I have been at a forum and found wrong info and nobody knows the difference.  Hell I see Mike Holmes bump his head...Norm and Bob too.  I can count on the guys here to give the right info. Not to mention that I come home tired.
> 
> These threads will develop into a resource.  Imagine the day when an inspector will be looking at his first nuclear reactor and he comes here to search the archives.  There will come a day, and trust me on this Jeff,  there will be a day when you can charge $40 per year.


I hope that one day these threads develop into a resource, but only after we have discussion and different viewpoints so people can see there is more than 1 way to do something and they can choose the method that they like.  $40 per year?  I can only wish every member had to pay that.  Then I can retire and run the board full time.  Thanks for the positive thoughts....


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