Electrician · Voltage Drop
Voltage Drop Calculator Practice Problems
Free, unlimited practice for the Journeyman & Master Electrician exam. Every click generates a new problem with different wire size, current, distance, and voltage — not a fixed question bank.
How to calculate it
Voltage drop, worked by hand
The calculator above checks your work instantly, but the exam expects you to run this by hand. Here's a full example using the same formula and constants the calculator uses.
Problem: A single-phase, 240V circuit carries 40A over a one-way distance of 150 ft, using 10 AWG copper conductor. Find the voltage drop and %VD.
1. Look up constants: K (copper) = , circular-mil area of 10 AWG = (NEC Ch. 9 Table 8)
2. Apply the single-phase formula:
3. Substitute:
4. Convert to a percentage of system voltage:
— this exceeds the commonly recommended 3% branch-circuit maximum, so a real installation would need a larger conductor (e.g. 8 AWG) or a shorter run.
FAQ
Voltage drop — frequently asked questions
What voltage drop percentage is considered acceptable?
NEC Chapter 2 informational notes (210.19(A) and 215.2(A)) suggest keeping voltage drop to 3% on the branch circuit, 3% on the feeder, and 5% combined total — but these are informational notes, not enforceable requirements for most installations. Some jurisdictions or specs do make them mandatory, so always check the exam's own reference material and any local amendment.
Why is the K constant different for copper and aluminum?
K is an approximation of a conductor's DC resistance in circular-mil-ohms per foot, derived from the resistance values in NEC Chapter 9 Table 8. Aluminum has roughly 64% more resistance than copper for the same cross-sectional area, so its K (21.2) is much higher than copper's (12.9) — using the wrong K for the conductor material in the problem is the single most common mistake on this calculation.
What's the difference between the single-phase and three-phase formulas?
Single-phase uses a multiplier of 2, because current travels out on one conductor and back on another over the same one-way distance D. Three-phase uses 1.732 (√3) instead, reflecting the line-to-line voltage relationship in a balanced three-phase circuit. Both formulas use the same one-way distance — don't double it yourself before plugging it in.
Does upsizing the conductor always fix a voltage drop problem?
Yes — a larger circular-mil area (CM) directly reduces VD, since CM is in the denominator. But the conductor still has to be sized for ampacity and its overcurrent device per NEC Article 240 first; you can't undersize for ampacity just because a smaller wire happens to pass the voltage-drop check.
Is this the exact method the NEC requires?
The NEC does not publish one official voltage-drop formula. K = 12.9 (copper) and K = 21.2 (aluminum) are long-standing industry-standard approximations derived from the conductor resistance data in NEC Chapter 9 Table 8, used the same way by most code-reference tools and textbooks. See our Sources & References page for exactly how these were verified.