# Voltage Drop Calculations

#### andrew.tkelly

##### Member
Quick question about voltage drop calculations.

Let's say there are 15 amps of continuous current running through a wire.

When calculating voltage drop, should I use 15 amps as my current value, or 15 amps x 1.25 (18.75 amps) in my calculations? Wasn't sure what to do about that continuous duty factor.

#### synchro

##### Senior Member
You should use 15 amps without the 1.25 multiplier. Voltage drop is essentially instantaneous and is dierctly proportional to the current (Vdrop = I x R).
The 1.25 factor is for continuous loads, which are loads that are expected to persist for 3 hours or more at a time. This is to account for the extra temperature rise that can occur from I**2 x R power dissipation in conductors when the load currents continue for extended periods.

#### MAC702

##### Senior Member
Voltage drop is [practically] independent of the temp rise from continuous duty. So, just use the actual current.

#### synchro

##### Senior Member
Voltage drop is [practically] independent of the temp rise from continuous duty. So, just use the actual current.
That's correct. The temp coefficient of resistance for copper is about 0.4% / deg C. So even with a 50 deg C temp rise it would be only be a 20% increase in resistance.

#### oldsparky52

##### Senior Member
That's correct. The temp coefficient of resistance for copper is about 0.4% / deg C. So even with a 50 deg C temp rise it would be only be a 20% increase in resistance.
So if you had a 3% VD w/out consideration for the temp rise you mention, you would have a 3.6% VD if you had that 50C temp rise? (checking if I understand what you posted correctly).

#### kwired

##### Electron manager
The K factors commonly used used in VD calculations assume a certain temperature - I think usually 30 degrees C. If you have an extreme high or low ambient temp and want to be more accurate with the calculation you should adjust that factor accordingly. 30C is commonly used I think mostly because the base for ampacity in T310.15(16) is also 30C.

#### david luchini

##### Moderator
Staff member
The K factors commonly used used in VD calculations assume a certain temperature - I think usually 30 degrees C. If you have an extreme high or low ambient temp and want to be more accurate with the calculation you should adjust that factor accordingly. 30C is commonly used I think mostly because the base for ampacity in T310.15(16) is also 30C.
The K factor commonly used in VD calculations are based on a conductor operating temperature of 75 degrees C. Ambient temperature is irrelevant to the K factor.

#### synchro

##### Senior Member
So if you had a 3% VD w/out consideration for the temp rise you mention, you would have a 3.6% VD if you had that 50C temp rise? (checking if I understand what you posted correctly).
That is a correct application of the temperature coefficient.
Chapter 9, Table 8 of the NEC provides resistance at standard temps like 75 deg C which David mentioned, so you don't have to do any correction yourself.

If you have heavier wire gauges then the inductive reactance at 60Hz starts getting significant compared to the resistance, and it adds to the total impedance. In most branch circuit wiring it can be ignored.

#### Julius Right

##### Senior Member
In my humble opinion, the VD has to be calculated considering the prospected current ,the actual impedance including skin and proximity effect and of course the power factor. A correct formula is from IEEE 141/1993 ch.3.11.1 General mathematical formulas:

#### Attachments

• 9.9 KB Views: 0

#### Julius Right

##### Senior Member
I am sorry. For 15 A the voltage drop is correct enough if we multiply current by resistance only, indeed. Sorry!:ashamed:

#### ggunn

##### PE (Electrical), NABCEP certified
In my humble opinion, the VD has to be calculated considering the prospected current ,the actual impedance including skin and proximity effect and of course the power factor. A correct formula is from IEEE 141/1993 ch.3.11.1 General mathematical formulas:
Yeah, if you are concerned about that third decimal place. The difference between an exam question and the real world:
Exam: What's the minimum wire size needed to service such and such loads under X conditions?
Real world: I have a buttload of #10 THWN-2 in the truck; can I use some of that?

:thumbsup:

#### kwired

##### Electron manager
The K factor commonly used in VD calculations are based on a conductor operating temperature of 75 degrees C. Ambient temperature is irrelevant to the K factor.
I can understand that, still corresponding to what I said, isn't ampacity tables somewhat the same concept, if carrying x amps in an ambient of 30C won't the conductor ampacity in the table be based on 60, 75 or 90 C conductor operating temp? Maybe some fudging in there for some safety factor but kind of the base of the ampacity tables?

#### david luchini

##### Moderator
Staff member
I can understand that, still corresponding to what I said, isn't ampacity tables somewhat the same concept, if carrying x amps in an ambient of 30C won't the conductor ampacity in the table be based on 60, 75 or 90 C conductor operating temp? Maybe some fudging in there for some safety factor but kind of the base of the ampacity tables?
I think I understand what you are getting at, but no, the two concepts are unrelated.

For instance, the K factor is directly related to conductor operating temperature, not ambient temperature, so a #1/0 conductor operating at a conductor 75deg in a 30degC ambient and a #1/0 conductor operating at a conductor temperature in 40degC ambient will have the same resistance and therefore, the same K factor. Change in ambient temperature does not directly affect the K factor.

A change in ambient would change the voltage drop, either through proper design or misdesign, A feeder of the same conductor size and length, operating at the same conductor temperature will have less current at the 40deg ambient, therefore less voltage drop. A feeder of the same conductor size and length, with the same current will operate at a higher conductor temperature, therefore increasing the conductor resistance (and K factor) and increasing the voltage drop.

#### LPS

##### Member
I just started teaching a class and got up to go through a voltage drop powerpoint that came with the curriculum. Voltage drop is too easy so I didn't need to prepare, right?... Next thing I know they're using different tables and K at 20C, and I'm just stumbling and bumbling my way through the lecture... I was raised on 2KIL/Cmil=VD along with 1.732 and CU@12.9 and AL@21.2. and Q factor over 1/0. Is my old formula only used for exam prep..? It's in Ugly's, so I figured that was the final word. I thought I knew what I was doing.. Guess not. LOL...