Voltage Drop Calculator

Calculate voltage drop across conductors per NEC guidelines

One-way distance from source to load

Ohm's Law Calculator

Enter any two values to calculate the remaining electrical properties

Power Calculations

Calculate apparent, real, and reactive power for AC circuits

Ratio of real to apparent power (0-1)

Wire Sizing Calculator

Determine proper conductor size with NEC derating factors

NEC 310.15(B)(1) correction
Current-carrying conductors
NEC 210.20(A) continuous load factor
NEC Table 310.16 - Ampacity Reference
AWG Cu 60°C Cu 75°C Cu 90°C Al 75°C Al 90°C
14152025--
122025302025
103035402535
84050553545
65565754060
47085955575
29511513075100
111013014585115
1/0125150170100135
2/0145175195115150
3/0165200225130175
4/0195230260150205

Ampacity based on NEC 2023 Table 310.16, not more than 3 current-carrying conductors in raceway, 30°C ambient.

Conduit Fill Calculator

Calculate conduit fill percentage per NEC Chapter 9

Conductors

Box Fill Calculator

Calculate junction box fill per NEC 314.16

For clamp & ground volume allowance
Each device = 2x largest conductor volume
1x largest conductor volume
1x largest conductor volume

Motor FLA Calculator

Look up full-load amperes per NEC Table 430.250 and calculate protection

NEC 430.32 overload multiplier
NEC Table 430.52

Transformer Sizing Calculator

Determine the required transformer kVA rating for your load

Safety margin for future load growth

Unit Converter

Convert between common electrical and measurement units

Definitions & Reference

Comprehensive glossary of electrical terms used throughout the calculators

General Electrical Terms
Voltage (V)
The difference in electric potential between two points in a circuit, measured in volts. It is the "pressure" that pushes electric charge through a conductor. Also called electromotive force (EMF).
Current (I)
The rate of flow of electric charge through a conductor, measured in amperes (amps). One ampere equals one coulomb of charge passing a point per second.
Resistance (R)
The opposition to the flow of electric current in a conductor, measured in ohms (Ω). Higher resistance means less current flow for a given voltage.
Power (P)
The rate at which electrical energy is transferred or consumed, measured in watts (W). Power equals voltage multiplied by current (P = V × I).
Conductor
A material that allows electric current to flow through it with low resistance. Copper and aluminum are the most common electrical conductors.
Insulation
A non-conductive material that surrounds a conductor to prevent unintended current flow, short circuits, and electric shock. Common types include THHN, THWN, and XHHW.
Circuit
A closed loop or pathway through which electric current flows from a source, through a load, and back to the source.
Ground / Grounding
A safety connection between electrical equipment and the earth. It provides a low-resistance path for fault current, helping to prevent electric shock and enabling overcurrent devices to operate.
AWG (American Wire Gauge)
A standardized system for measuring wire diameter. Smaller AWG numbers indicate larger wire. For example, 4 AWG is larger than 12 AWG. Sizes larger than 1 AWG are designated as 1/0, 2/0, 3/0, and 4/0.
NEC (National Electrical Code)
A set of standards published by the NFPA (National Fire Protection Association) for the safe installation of electrical wiring and equipment in the United States. Updated on a three-year cycle.
Ampacity
The maximum current, in amperes, that a conductor can carry continuously under conditions of use without exceeding its temperature rating. Defined in NEC Table 310.16.
Voltage Drop
Voltage Drop
The decrease in electrical potential (voltage) along a conductor as current flows through it due to the conductor's inherent resistance. Calculated as Vdrop = I × R × L × K / 1000, where K is 2 for single-phase and 1.732 for three-phase.
Voltage Drop Percentage
The voltage drop expressed as a percentage of the source voltage. The NEC recommends no more than 3% for branch circuits and 5% total (branch + feeder combined) per NEC 210.19(A) Informational Note No. 4.
End Voltage
The remaining voltage at the load after subtracting the voltage drop from the source voltage. Low end voltage can cause equipment malfunction, motor overheating, or flickering lights.
Wire Resistance
The resistance of a conductor per unit length (ohms per 1000 feet), which depends on the wire material (copper or aluminum), wire gauge (AWG), and temperature. Listed in NEC Chapter 9, Table 8.
Single Phase
An AC power system using two conductors (one hot, one neutral) with a single alternating voltage waveform. Common in residential applications at 120V and 240V.
Three Phase
An AC power system using three conductors, each carrying an alternating current offset by 120 degrees. More efficient for large loads and motors. Common at 208V, 480V, and 600V in commercial/industrial settings.
Ohm's Law
Ohm's Law
The fundamental relationship between voltage, current, and resistance: V = I × R. It states that current is directly proportional to voltage and inversely proportional to resistance.
Ohm (Ω)
The SI unit of electrical resistance. One ohm is the resistance that produces a current of one ampere when a voltage of one volt is applied across it.
Watt (W)
The SI unit of power equal to one joule per second. In electrical terms, one watt equals one volt multiplied by one ampere (W = V × A).
Ampere (A)
The SI unit of electric current. One ampere is defined as one coulomb of charge flowing past a point per second.
Power Wheel
A circular diagram showing all 12 formulas relating voltage (V), current (I), resistance (R), and power (P). Any two known values can be used to calculate the other two.
Power Calculations
Apparent Power (S)
The total power in an AC circuit, which is the product of voltage and current without regard to phase angle. Measured in volt-amperes (VA). S = V × I (single phase) or S = √3 × V × I (three phase).
Real Power (P)
The actual power consumed by the load and converted into useful work (heat, light, motion). Measured in watts (W). Also called active power or true power. P = S × PF.
Reactive Power (Q)
The power that oscillates between the source and reactive components (inductors and capacitors) without performing useful work. Measured in volt-amperes reactive (VAR). Q = S × sin(θ).
Power Factor (PF)
The ratio of real power to apparent power (PF = P / S), expressed as a decimal between 0 and 1. A power factor of 1.0 means all power is being used effectively. Low power factor wastes energy and can result in utility penalties.
Phase Angle (θ)
The angular difference (in degrees) between the voltage and current waveforms in an AC circuit. Caused by reactive loads. θ = arccos(PF).
kVA (Kilovolt-Ampere)
A unit of apparent power equal to 1,000 volt-amperes. Used to rate transformers, generators, and UPS systems because they must handle total current regardless of power factor.
Power Triangle
A right triangle illustrating the relationship between apparent power (hypotenuse), real power (adjacent side), and reactive power (opposite side). S² = P² + Q².
Wire Sizing & Ampacity
Temperature Rating
The maximum temperature that a conductor's insulation can withstand continuously without degradation. Common ratings are 60°C (TW, UF), 75°C (THW, THWN), and 90°C (THHN, XHHW). Higher ratings allow more current.
Temperature Correction Factor
A multiplier applied to the ampacity of a conductor when the ambient temperature differs from the standard 30°C (86°F). Found in NEC Table 310.15(B)(1). Higher ambient temperatures reduce ampacity.
Conductor Fill Derating
A reduction factor applied when more than 3 current-carrying conductors are bundled in a raceway or cable. Per NEC 310.15(C)(1): 4-6 conductors = 80%, 7-9 = 70%, 10-20 = 50%, 21-30 = 45%, 31-40 = 40%.
Continuous Load
A load where the maximum current is expected to continue for 3 hours or more. Per NEC 210.20(A), conductors and overcurrent devices must be rated at 125% of the continuous load.
Adjusted Current
The effective current that the conductor must handle after applying all derating factors (temperature correction, conductor fill, continuous load factor). The wire must have an ampacity equal to or greater than this value.
NEC Table 310.16
The primary ampacity table in the NEC, listing allowable ampacities for insulated conductors rated up to 2000 volts in raceways, cables, or direct burial, based on not more than 3 current-carrying conductors at 30°C ambient.
Conduit Fill
Conduit
A tube or trough used to protect and route electrical wiring. Types include EMT (Electrical Metallic Tubing), Rigid Metal Conduit (RMC), IMC (Intermediate Metal Conduit), and PVC (polyvinyl chloride).
EMT (Electrical Metallic Tubing)
A thin-walled, unthreaded metal conduit commonly used in commercial and industrial buildings. Lightweight and easy to bend. Connected with set-screw or compression fittings.
Rigid / IMC
Heavy-walled, threaded metal conduit providing maximum physical protection. Required for exposed outdoor installations, hazardous locations, and where subject to physical damage.
PVC Conduit
A non-metallic, plastic conduit used in underground, wet, and corrosive environments. Schedule 40 is used for most applications; Schedule 80 is required where subject to physical damage.
Conduit Fill Percentage
The ratio of total conductor cross-sectional area to the conduit's internal cross-sectional area. NEC Chapter 9, Table 1 limits: 1 wire = 53%, 2 wires = 31%, 3+ wires = 40%.
NEC Chapter 9
The section of the NEC containing tables for conduit and tubing dimensions, conductor properties, and fill calculations. Table 4 gives conduit areas; Table 5 gives conductor areas including insulation.
Box Fill
Junction Box
An enclosure used to protect electrical connections (splices) and provide access for maintenance. Must be sized per NEC 314.16 based on the number and size of conductors, devices, and fittings.
Box Fill Volume
The total volume required for all conductors, devices, clamps, and grounds in a box. Each conductor size has a specific volume allowance per NEC Table 314.16(B): 14 AWG = 2.0 cu in, 12 AWG = 2.25 cu in, 10 AWG = 2.5 cu in, 8 AWG = 3.0 cu in, 6 AWG = 5.0 cu in.
Device (Yoke) Volume
Each switch, receptacle, or device mounted on a yoke counts as 2 times the volume of the largest conductor connected to it, per NEC 314.16(B)(4).
Clamp Volume
If internal cable clamps are present, one volume allowance (based on the largest conductor) is added regardless of the number of clamps, per NEC 314.16(B)(2).
Equipment Ground Volume
All equipment grounding conductors together count as one volume allowance based on the largest equipment grounding conductor entering the box, per NEC 314.16(B)(5).
Motor FLA & Protection
Full Load Amperes (FLA)
The current a motor draws when operating at its rated horsepower and voltage under full mechanical load. Listed in NEC Table 430.250 for 3-phase motors. Used for sizing conductors, overloads, and branch circuit protection.
Horsepower (HP)
A unit of power equal to 746 watts. Used to rate the mechanical output of electric motors. One HP can lift 550 pounds one foot in one second.
Service Factor (SF)
A multiplier indicating how much above the nameplate rating a motor can operate continuously without damage. A 1.15 SF motor can run at 115% of rated load. Affects overload protection sizing per NEC 430.32.
Overload Protection
A device that protects a motor from sustained overcurrent (overload) that could cause overheating. Per NEC 430.32, overloads are set at 115% of FLA (1.15 SF motors) or 125% (40°C rise motors).
Branch Circuit Protection
Overcurrent protection (breaker or fuse) sized to handle motor starting current without nuisance tripping. Per NEC Table 430.52: inverse time breaker = 250% FLA, dual element fuse = 175% FLA, instantaneous breaker = 300% FLA.
NEC Table 430.250
Lists full-load current for three-phase AC motors at various voltages (208V, 230V, 460V, 575V). These values, not the motor nameplate, must be used for sizing conductors and protection per NEC 430.6(A).
Motor Branch Circuit Conductor
Per NEC 430.22, branch circuit conductors supplying a single motor must have an ampacity not less than 125% of the motor's full-load current rating.
Transformer Sizing
Transformer
An electrical device that transfers energy between circuits through electromagnetic induction. Used to step voltage up or down. Rated in kVA (kilovolt-amperes) because it must handle both real and reactive power.
kVA Rating
The apparent power capacity of a transformer, measured in kilovolt-amperes. Standard sizes include 15, 25, 37.5, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, and 2000 kVA.
Primary / Secondary
The primary winding connects to the power source (higher voltage). The secondary winding connects to the load (lower voltage for step-down transformers). The turns ratio determines the voltage transformation.
Growth Factor
An additional capacity margin (typically 20-25%) added to the calculated load to accommodate future load growth. Industry best practice to avoid undersizing a transformer that is expensive to replace.
Secondary Current
The current available at the transformer secondary: I = kVA × 1000 / (V × √3) for three-phase, or I = kVA × 1000 / V for single-phase. Used to size secondary conductors and protection.
Units & Conversions
kcmil (Thousand Circular Mils)
A unit of area used for large conductors. One circular mil is the area of a circle with a diameter of one mil (0.001 inch). Wire sizes larger than 4/0 AWG are measured in kcmil (e.g., 250 kcmil, 500 kcmil).
mm² (Square Millimeters)
The metric unit of conductor cross-sectional area used internationally (IEC standards). 1 mm² ≈ 1.974 kcmil. For example, 2.5 mm² is roughly equivalent to 14 AWG.
BTU (British Thermal Unit)
A unit of heat energy. One BTU is the energy needed to raise one pound of water by one degree Fahrenheit. 1 watt = 3.412 BTU/hr. Used for HVAC load calculations.
VAR (Volt-Ampere Reactive)
The unit of reactive power in an AC circuit. Reactive power does no real work but is required to maintain magnetic fields in motors, transformers, and other inductive loads.
VA (Volt-Ampere)
The unit of apparent power in an AC circuit. Equal to the product of RMS voltage and RMS current. Used to rate transformers, UPS systems, and generators.

Calculation History