voltage drop 20a 120v

[ibew501ed]

given the formula Vd= 2KIL/Cmil with K=12.9 @75degC or 10.4 @20 degC
#12 = 6530 Cmil The 1 way length of a 20 amp 120volt circuit = 46' or 56'
Can anyone explain? It doesn't seem practical

 

[iwire]

Why would you use 20 amps to figure the voltage drop on a 20 amp circuit?

Would you intend to load a 20 amp circuit to 20 amps?

 

[Dennis Alwon]

I would bet that most circuits if loaded to their capacity would have VD beyond the 3% recommended by the FPN. Remember this 3% is not code but an FPN.

 

[ibew501ed]

OK 16a load max length 57' or 70' is this now practical

 

[iwire]

I would bet that most circuits if loaded to their capacity would have VD beyond the 3% recommended by the FPN.

That is exactly my thought as well.

 

[Dennis Alwon]

OK 16a load max length 57' or 70' is this now practical

Not sure what practical has to do with it. It is what it is. If you are using equipment that is sensitive to lower voltages then you would have to increase your wire size.

 

[ibew501ed]

I would bet that most circuits if loaded to their capacity would have VD beyond the 3% recommended by the FPN. Remember this 3% is not code but an FPN.

no need to go on the offense
if a circuit is 20 amps not 15 theres probably a reason
then if would be advisable to CONSIDER vd

I simply mention it because I was under the impression Vd on normal circuits was not an issue before + 100'

 

[Dennis Alwon]

Remember the equations usually are based on 3%-- at least the online one I use. Online VD

The equation itself is a bit of an oddity. Let's look at a single phase copper conductor on a 240 volt. Let use 100 feet for the distance one way and 40 amps for the ampacity. We get a #6 wire. Okay now reverse it.

Since the equation is multiplication the communicative laws take effect. Instead of 100 feet use 40 feet and instead of 40 amps use 100 amps. We still get #6 which we know is not correct.-- Explain that.

 

[Dennis Alwon]

no need to go on the offense

Sorry if I came across negatively. I did not mean that at all-- it's the NYer in me but I really did not mean it as an attack at all.

 

[augie47]

The keyword there may be "normal" in that the 3% rule is "normally" ignored, especially in runs less than 100 ft.
You calculations are correct, and I have often seen engineered drawings where 50-100 ft circuits with an actual full load of 20 amps (such as a hand dryer) specify a #10 on a 20 amp circuit.

 

[ibew501ed]

Remember the equations usually are based on 3%-- at least the online one I use. Online VD

The equation itself is a bit of an oddity. Let's look at a single phase copper conductor on a 240 volt. Let use 100 feet for the distance one way and 40 amps for the ampacity. We get a #6 wire. Okay now reverse it.

Since the equation is multiplication the communicative laws take effect. Instead of 100 feet use 40 feet and instead of 40 amps use 100 amps. We still get #6 which we know is not correct.-- Explain that.

HOLD ON I need a calculator and 10 minutes:roll:

 

[ibew501ed]

you are correct but the we know 40a wire need only be14333 cmil or #8
but even if the answer was #12 wire we know the code require larger

 

[ibew501ed]

The keyword there may be "normal" in that the 3% rule is "normally" ignored, especially in runs less than 100 ft.
You calculations are correct, and I have often seen engineered drawings where 50-100 ft circuits with an actual full load of 20 amps (such as a hand dryer) specify a #10 on a 20 amp circuit.

THANK YOU
thats what I needed to ensure I'm stating it correctly

 

[iwire]

you are correct but the we know 40a wire need only be14333 cmil or #8
but even if the answer was #12 wire we know the code require larger

Currently the NEC does not require we do anything at all about voltage drop except for Fire Pumps and couple other specific instances.

For general circuits there is no NEC voltage drop requipment which brings us to what Gus said.

The keyword there may be "normal" in that the 3% rule is "normally" ignored, especially in runs less than 100 ft.

I am not trying to be negative but for the most part I feel 'if it's not broke, don't fix it'. Most equipment can deal with more than a total of 10% VD anyways (5% from the utility and 5% more on the feeder and branch circuit)

 

[ibew501ed]

in addition IWIRE probably the real opperation temp of the wire allow for less Vd typically (20deg C)

 

[skeshesh]

Others have answered properly as usual. I'll only add that a decent engineering practice I personally use, specially if its a fire alarm system or some other case where I feel like voltage drop could become an issue is try to limit the load on each circuit to give less than 8% or so. That way if field conditions make the run longer or loads get added one way or another the drop doesnt exceed 10% for sensitive equip. As far as a general 20A circuit with some receptacles or other typical loads I think you're fine.