Voltage Drop - Volts Electrical Design Software

IEEE Exact Voltage Drop Formulae

NEC® Specifications Concerning Voltage Drop

NEC® Sections 210-19(a) (FPN No.4) and 215-2(b) (FPN No.2) recommend sizing both feeders and branch circuits to prevent a voltage drop exceeding 3 percent at the farthest outlet, where the maximum total voltage drop of the feeders and branch circuits does not exceed 5 percent. NEC® section 110-3(b) requires equipment to be installed in accordance with the equipment instructions. Therefore, electrical equipment must be installed so that it operates within its voltage rating as specified by the manufacturer. Additionally, NEC® Section 310-15(a) (FPN No.1) states that the ampacities provided in it's Tables do not take voltage drop into consideration.

Ground Conductors

NEC® 250-122(b) states that where circuit conductors are increased in size to compensate for voltage drop, the equipment grounding conductors, where installed, shall be adjusted proportionately according to circular mil area.

The following sections are the ANSI / IEEE Std 141 formulae used by Volts for voltage drop computations.

AC Circuits

For AC circuits, where AC resistance and inductive reactance are considered, the following is the IEEE Std 141 exact voltage drop formula.

where:

Vd = Voltage drop (Line to Neutral)

V = Voltage (source)

I = Current in amperes (A)

R = AC Resistance from NEC® Chapter 9 Table 9 (Ohms to Neutral)

X = AC Reactance from NEC® Chapter 9 Table 9 (Ohms to Neutral)

distance (L) is considered from the Resistance & Reactance Tables

where Ohms per unit / 1000 * L in same unit = R or X

θ = Angle of Phase Offset = Arc Cosine (device or circuit Power Factor)

Line to Line is computed by Line to Neutral VD / Sqrt(3).

VOLTS™ automatically computes the correct conductor, neutral and ground sizes based on the conductor / cable type, voltage, raceway type, ambient and terminal temperature, distance, hertz (Hz) and the user defined voltage drop limit. The resulting voltage drop information is also automatically updated from the accumulation of resistance / impedance from the entry of upstream panels and transformers.

Volts also features a very comprehensive Voltage Drop Analysis with Graphing report.

DC Circuits

where:

Vd = Voltage drop

R = DC Resistance from NEC® Chapter 9 Table 8

L = Distance

I = Current in amperes (A)

K = Material Resistivity constant - 12.9 for Cu & 21.2 for Al

CM = Circular mils of conductor

Ambient Temperature

Additionally, ambient temperature is considered with the following ratio of temperatures formula. This formula is used to adjust the Chapter 9 Table 9 values from 75ºC to the installation ambient temperature in Celsius (C).

R2 = New Conductor Resistance

R1 = Original Conductor Resistance

α = Material Resistivity

T2 = Ambient Temperature in Celsius (TA)

Magnetic (Eddy) Currents

Eddy currents are induced currents in surrounding magnetic or non-magnetic metal. These currents create heating in the metal and therefore act as an energy loss that translates into an increase in resistance of the circuit. NEC® Chapter 9, Table 9 segregates conduits into three groups, PVC, Aluminum and Steel to account for the added resistance with each of the three conduit group types. PVC, being non-metallic, does not produce any eddy currents and therefore has the least resistance value of the three. Steel and Aluminum conduit, being metallic, do produce eddy currents and their respective resistance values are reflective of this.

Volts utilizes these factors in determining the correct conductor size and voltage drop computations along with the accumulation of the impedance / resistance of all upstream conductors.

VOLTS™ utilizes both ambient and terminal temperatures separately to size the conductor and compute the resulting voltage drop.