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Voltage Drop Calculation, Step by Step
Every journeyman exam has voltage drop questions, and they all run on one formula. Learn the formula, learn where the numbers come from in your Code book, and these become the fastest points on the test. Here's the whole voltage drop calculation, start to finish.
Step 1 — The Formula
Open your Code book to Chapter 9, Table 8 — that's where this lives. For a single-phase circuit, the voltage drop calculation is:
VD = (2 × K × I × L) ÷ CM
- VD — voltage drop, in volts. This is what you're solving for.
- K — the resistance constant for the conductor material. Memorize both: 12.9 for copper, 21.2 for aluminum.
- I — the load current in amps. Use the load the question gives you.
- L — the one-way length of the run, in feet.
- CM — the conductor's area in circular mils, from Chapter 9, Table 8.
The 2 in the formula is the round trip: current travels out on one conductor and back on the other, so the wire's resistance counts twice. It's already in the formula — which is why L is always the one-way distance. If a question hands you the total round-trip length, don't double it again. That's the single most common way these questions get missed.
Step 2 — Circular Mils from Chapter 9, Table 8
You don't memorize circular mils — you look them up. Open your Code book to Chapter 9, Table 8 (Conductor Properties) and read the circular mil area for the wire size in the question. The sizes that show up constantly:
| Wire size | Circular mils (CM) |
|---|---|
| 14 AWG | 4,110 |
| 12 AWG | 6,530 |
| 10 AWG | 10,380 |
| 8 AWG | 16,510 |
| 6 AWG | 26,240 |
| 4 AWG | 41,740 |
| 2 AWG | 66,360 |
| 1/0 AWG | 105,600 |
| 2/0 AWG | 133,100 |
| 4/0 AWG | 211,600 |
Notice the pattern: bigger wire, bigger CM number, smaller drop. When a run gets long, the only lever you have is a bigger CM — that's the whole game in Step 5.
Step 3 — The 3% and 5% Targets
A voltage drop answer isn't finished until you've turned it into a percent:
%VD = (VD ÷ system voltage) × 100
The Code recommends keeping voltage drop under 3% on a branch circuit and under 5% total for feeder plus branch circuit combined — see the informational note at 210.19(A). It's a recommendation, not a hard rule, but the exam treats it as the target. When a question asks whether a run is acceptable, or which wire size to choose, 3% is almost always the number it's testing against.
Do this math once now so it's automatic on test day:
| System voltage | 3% = | 5% = |
|---|---|---|
| 120V | 3.6V | 6.0V |
| 208V | 6.24V | 10.4V |
| 240V | 7.2V | 12.0V |
| 277V | 8.31V | 13.85V |
| 480V | 14.4V | 24.0V |
Step 4 — Worked Example: Find the Drop
The question: A 10-amp load sits 90 feet from the panel, fed with 12 AWG copper on a 120V circuit. What's the voltage drop, and is it within the 3% recommendation?
Pull the numbers: K = 12.9 (copper), I = 10A, L = 90 ft, and 12 AWG = 6,530 CM from Table 8.
- VD = (2 × 12.9 × 10 × 90) ÷ 6,530
- VD = 23,220 ÷ 6,530 = 3.56 volts
- %VD = 3.56 ÷ 120 = 2.96%
Verdict: it passes — just under the 3% target. Notice how close it is, though. Stretch that same circuit to 100 feet and a heavier load and it falls apart fast, which is exactly the next example.
The follow-up: Same panel, now a 20-amp load 100 feet out on 12 AWG copper, 120V. VD = (2 × 12.9 × 20 × 100) ÷ 6,530 = 7.90 volts = 6.6%. That fails the 3% branch target and the 5% total. Upsize to 10 AWG (10,380 CM): 51,600 ÷ 10,380 = 4.97V = 4.1% — better, still over 3%. Upsize again to 8 AWG (16,510 CM): 51,600 ÷ 16,510 = 3.13V = 2.6%. Now it passes. Long runs eat wire sizes — expect the exam to make you climb two sizes.
Step 5 — Worked Example: Size the Wire
Half the exam's voltage drop questions flip the formula around: they give you the maximum allowed drop and ask for the minimum wire size. Same formula, solved for CM:
CM = (2 × K × I × L) ÷ VD
The question: A 20-amp, 120V load is 150 feet from the panel. Keep the drop at or under 3%. What's the smallest copper conductor that works?
- Allowed drop: 3% of 120V = 3.6 volts
- CM = (2 × 12.9 × 20 × 150) ÷ 3.6 = 77,400 ÷ 3.6 = 21,500 CM minimum
- Open your Code book to Chapter 9, Table 8 and pick the first size at or above 21,500 CM: 8 AWG is 16,510 — too small. 6 AWG is 26,240 — that's your answer.
- Check it: VD = 77,400 ÷ 26,240 = 2.95V = 2.46%. Under 3%. Done.
Two rules for this version of the question: always round up to the next wire size (a CM requirement never rounds down), and always run the check calculation if you have time — it takes twenty seconds and catches the trap answers.
🔎 Lookup tip: tab Chapter 9 in your Code book — Table 8 is one of the highest-value tabs you'll place. Every voltage drop question on the exam is the same three moves: read K for the material, pull CM from Table 8, run the formula. Drill it until the moves are boring. And when you check your work, run it through the free voltage drop calculator → to confirm your arithmetic before you trust it.
Exam Traps
- Doubling the length twice. The 2 is already in the formula. L is one-way, always.
- Wrong K. Aluminum questions are copper questions with K = 21.2. Read the question for the conductor material before you touch the calculator.
- Stopping at volts. If the question asks about percent — or about whether the run meets the recommendation — a volts-only answer is half an answer.
- Forgetting the ground wire grows too. When you upsize ungrounded conductors for voltage drop, 250.122(B) requires the equipment grounding conductor to be increased in size proportionately. Example: upsizing a circuit from 12 AWG to 6 AWG multiplies the circular mils by about four (6,530 → 26,240 CM) — so a 12 AWG equipment ground on that circuit grows to 6 AWG as well. This is a favorite follow-up question.
- Treating 3% as a Code violation. It's an informational-note recommendation. If an answer choice calls a 4% branch-circuit drop a Code violation on that basis alone, it's the distractor.
That's the whole calculation: K in, Table 8 out, percent to finish. It prints on one card of the Sparky formula sheet — memorize the formula and the two K values, look up everything else.
Keep Drilling
Voltage drop is one of 33 NEC topics, right alongside box fill and motor calculations. Sparky's 1,087 codebook-first questions drill all of them — every question a lookup rep with your book open, 15 minutes a day.