Voltage drop
VD = 2 × K × I × L / CM
Single-phase and three-phase, solving for volts, percent, size or length.
Read the worked examplesJourneyman electrician exam / calculations
The exam calculations, worked out step by step.
Voltage drop, conduit fill, box fill, load calcs, Ohm's law, ampacity and derating. Every line of the math written out, with the NEC table you would open for each number.
CH1 source / CH2 loadLive
Source120.0 V
At load112.1 V
Drop7.90 V
Drop %6.58%
12 AWG copper, 20 A, 100 ft one way, 120 V single-phase.
What it covers
Six calculation types. One method for each.
Each topic is broken into the same pattern: the formula, where every number comes from, then worked problems with the traps marked.
Conduit fill
Σ conductor area ≤ 40% of raceway
Chapter 9 Tables 1, 4 and 5, the ground you must count, and the 0.8 rule.
Read the worked examplesBox fill
count × volume allowance ≤ box volume
What counts as 1, what counts as 2, and what does not count at all.
Read the worked examplesLoad calculations
3,000 VA at 100% + rest at 35%
The dwelling standard method in order, ending at the service conductors.
Read the worked examplesOhm's law and power
E = I × R / P = E × I
Heaters, parallel loads, power factor and three-phase power.
Read the worked examplesAmpacity and derating
base × temp factor × fill factor
Temperature correction, bundling, and the terminal check.
Read the worked examplesThe format
A problem, then every step. Nothing skipped.
This is how each exercise is laid out: the givens, one step per line, and the answer with the code reference. In the course, the steps animate in one at a time and you score points for each one you get right before it is shown.
Check it in the calculatorVoltage drop: 240 V branch circuit+10 pts
A 240 V single-phase load draws 30 A. It is 150 ft from the panel on 10 AWG copper. What is the voltage drop in percent, and does it meet the 3% recommendation?
K (copper)12.9
CM, 10 AWG10,380
I30 A
L (one way)150 ft
Pick the single-phase formula
VD = 2 × K × I × L / CMPlug in the numbers
VD = 2 × 12.9 × 30 × 150 / 10,380Multiply the top, then divide
VD = 116,100 / 10,380 = 11.18 VConvert to percent and compare
11.18 / 240 × 100 = 4.66% > 3%
4.66%, over the 3% recommendation. 8 AWG copper (16,510 cmil) brings it to 7.03 V, or 2.93%. CM values: NEC Chapter 9, Table 8.
Free tool
Free voltage drop calculator
Single-phase or three-phase, copper or aluminum, 14 AWG to 500 kcmil. It shows the drop in volts and percent, compares it to the 3% recommendation, and writes out every step so you can check your own work line by line.
Open the calculatorSingle-phaseVD = 2 × K × I × L / CM
Three-phaseVD = 1.732 × K × I × L / CM
K copper12.9 Ω·cmil/ft
K aluminum21.2 Ω·cmil/ft
CM sourceNEC Ch. 9, Table 8
3% figureInformational note
Recent articles
Worked examples, one topic at a time
How to Calculate Voltage Drop for the Electrician Exam
Voltage drop for the journeyman exam: single-phase and three-phase formulas, where K and CM come from, and four worked problems with every step.
Conduit Fill Calculation, Step by Step
How to calculate conduit fill with NEC Chapter 9 Tables 1, 4 and 5: the 53/31/40 rule, the 0.8 rounding rule, and three worked EMT examples.
Box Fill Calculation Examples (NEC 314.16)
Box fill counting rules from NEC 314.16 in plain terms, with worked examples for receptacle boxes, switch boxes and mixed conductor sizes.
Residential Load Calculation: The Standard Method
A full dwelling load calculation using the NEC standard method: lighting, small appliance, demand factors, range, dryer, appliances, heat or AC, service size.
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