Voltage Drop -- Part 3

As discussed in Part 1 of this series (Jan. 2000 issue, page 44), the NEC does not generally consider voltage drop a safety issue. As a result, it contains Fine Print Notes (FPNs) recommending you size circuit conductors large enough to provide reasonable efficiency of equipment. However, two NEC rules require you to increase the size of circuit conductors to accommodate voltage drop: Grounding Conductors in Sec. 250-122(b) and Fire Pumps in Sec. 695-7.

When sizing conductors, make sure you size the conductor to meet NEC requirements, such as continuous load, motors, conductor bundling, etc. Then, determine if the conductor is sufficiently large enough to meet the NEC recommendations or requirements for voltage drop.

In Part 2 (Feb. 2000 issue, page 50), I demonstrated two methods used to calculate the voltage drop of conductors using Ohm’s Law and the Voltage Drop (VD) formula. You can rearrange the VD formulas to solve for CM (circular mils of the circuit conductor necessary to prevent voltage drop).

CM (single-phase) = (2 x K x I x D)/VD

CM (3-phase) = (1.732 x K x I x D)/VD

Let’s go through a couple of examples to show you how to calculate appropriate conductor size.

Single-phase example:

A 5 hp motor is located 100 ft from a 120/240V panelboard. What size conductor should you use if the motor nameplate indicates the voltage range is between 208V and 230V? Limit voltage drop to the NEC recommendation of 3% of the voltage source. Assume terminals are rated 75°C.

(a) No. 10 THHN
(b) No. 8 THHN
(c) No. 6 THHN
(d) No. 4 THHN

Answer: (a) No. 10 THHN

Sec. 430-22(a) requires you to size motor conductors not less than 125% of the motor full-load current (28A) as listed in Table 430-148. A No. 10 is rated 35A at 75°C [Table 310-16 and Sec. 110-14(c)], and it is suitable to meet the NEC requirements (28A x 1.25 = 35A). A No. 10 conductor also limits the voltage drop to meet the manufacturer’s voltage limitation rating [Sec. 110-3(b)].

Next, calculate conductor size to limit voltage drop to 3% of voltage source:

CM = (2 x K x I x D)/VD, where
CM = wire size (Chapter 9, Table 8)

K = 12.9 ohm, copper
I = 28A
D = 100 ft
VD = 240V x 3% = 7.2V

CM = (2 x 12.9 ohms x 28A x 100 ft)/7.2V
CM = 10,033, which calls for a No. 10 conductor per Chapter 9, Table 8.

Three-phase example:

A 25 hp, 208V, 3-phase fire pump motor is 175 ft from the service. The fire pump motor controller is 150 ft from the service (the motor is 25 ft from the controller). What size conductor must you install to the fire pump motor? Assume the terminals are rated at 75°C.

(a) No. 4 THHN
(b) No. 3 THHN
(c) No. 2 THHN
(d) No. 1 THHN

Answer: (b) No. 3 THHN

When sizing conductors for a fire pump motor, size the conductor to meet these requirements:

Calculation 1: Sec. 695-6(c)(2)

You must size branch-circuit conductors no less than 125% of the fire pump motor full-load current as listed in Table 430-148 or 430-150, based on a 75°C terminal rating [Sec. 110-14(c)(1)] as listed in Table 310-16.

74.8A x 1.25 = 93.4A,
No. 3 THHN at 75°C is rated 100A

Calculation 2: Sec. 695-7

Operating voltage at the motor controller terminals shall not drop more than 15% below the controller-rated voltage when the motor starts (lock-rotor current).

CM = (1.732 x K x I x D)/VD
CM = wire size (Chapter 9, Table 8)
K = 12.9 ohms, copper
I = 404A (locked-rotor, Table 430-151B)
D = 150 ft
VD = 31.2V (208V x 15%)
CM4(1.732 x 12.9 ohms x 404A x 150 ft)/31.2V
CM = 43,396, which calls for a No. 3 conductor per Chapter 9, Table 8.

Calculation 3: Sec. 695-7

The operating voltage at the terminals of the motor shall not drop more than 5% below the motor’s voltage rating while the motor operates at 115% of its full-load current rating.

CM = (1.732 x K x I x D)/VD
CM = wire size (Chapter 9, Table 8)
K = 12.9 ohms, copper
I = 86A (74.8A x 115%), Table 430-150
D = 175 ft
VD = 10.4V (208V x 5%)
CM = (1.732 x 12.9 ohms x 86A x 175 ft)/10.4V
CM = 32,332, which calls for a No. 4 conductor per Chapter 9, Table 8.

Caution: For voltage drop, the No. 4 wire is okay from the controller to the motor, but Sec. 695-6(c)(2) requires the branch-circuit conductors to be no smaller than No. 3.