Puzzled Voltage Drop Calculation

Don't know - my math says 1.37V drop. (intuitively, a 5.7V drop on a 150' run seems high)

Resistivity of #12 Cu wire is 0.00187 ohm/ft.

150 ft of wire gives 0.2805 ohm.

2 wires in parallel gives effective resistance of 0.14025 ohm.

V=IR=9.8*0.14025=1.37445V.

Maybe someone else can find the flaw in my logic.

You considered the 2 wires in parallel, while in reality they are connected in series. Therefore your total voltage drop should be multiplied by 4, since the ratio difference between 2 in parallel and 2 in series is 4.

Tomad

According to the 2008 NEC chapter 9 table 8 conductor properties the resistance of a #12 bare copper conductor is 1.93ohm/kft.I am just trying to understand because i am preparing for the state exam and the calculations are kicking my butt.I have 15 years of electrical experience but the man i worked for gave no education on the technical aspect of the business so any help clearing up my misunderstandings is greatly appreciated.The question also states an estimated k factor of 12.9 for copper wire if that has any bearing on the outcome.

thank you very much for clearing that up so for more than one conductor is listed then you obtain the total wire resistance for that particular circuit?

Yes. You need to remember that voltage in a series circuit is additive. The total voltage available (115v) is split proportionally among the circuit elements according to the resistance of each.

The actual voltage available to the load will be reduced by the voltage drop across every other component in the circuit. In this circuit there are 3 components: The line wire, the load, and the neutral wire. Therefore, the voltage available at the terminals of the load is 115 - 5.6742 = 109.3258v (still well within the normal tolerance of ±10%).

thank you very much for that explanation it clarifies alot of info for me.Would you mind if i contacted you if i have anymore questions?

You are right about the voltage drop being additive along the circuit. But you have to consider the quoted voltage drop only along the 2 conductors (hot wire and neutral). The load itself influences the total voltage drop along the 2 conductors (by its current). The available voltage at the load (109.3258 V) is actually the voltage drop along the load (the third component), so the 5.6742 V are only along the 2 conductors.

Tomad

Number 12 copper wire has a resistance of 1.6215 ohms/1000 feet.

2x150=300 feet of wire

(300/1000)X 1.6215 = 0.48645 ohms

V=IXR = 9.8 X .48645 = 4.76721 V

Should have included this URL in previous submission

http://www.stealth316.com/2-wire-resistance.htm