Voltage drop questions are common on Journeyman electrician exams because they test one skill: choosing a conductor size that keeps voltage loss within a target percent.
The NEC provides recommended limits often used on exams:
- 3% max voltage drop for a branch circuit (at the farthest outlet)
- 5% max total for feeder + branch combined
Exam tip: Even though these are typically shown as Informational Notes, exams still test them heavily.
Study smarter with VoltageLab
Built for electricians, apprentices, and electrical engineers who want faster practice and better exam prep.
⭐️ Join thousands of electricians upgrading their skills
How to spot a voltage drop question fast (exam pattern)
If you see 3 or more of these, it’s a voltage drop problem:
- “voltage drop” or “percent drop”
- “maximum allowable” 3% or 5%
- “distance from source/panel” (feet or meters)
- “k-factor” (12.9 copper / 21.2 aluminum)
- “minimum conductor size” to stay under a limit
If the question is asking only “Can this wire handle the load?” that’s ampacity, not voltage drop.
The voltage drop method most exams use (K-constant / Circular mil)
Most licensing exams use the K-constant method with Circular Mil Area (CMA).
Step 1) Pick the right formula
Single-phase (or DC):
CMA = (2 × K × I × D) / VD
Three-phase:
CMA = (√3 × K × I × D) / VD
Where:
- CMA = circular mil area needed
- K = material constant (12.9 copper, 21.2 aluminum)
- I = load current (amps)
- D = one-way distance (feet)
- VD = allowable voltage drop (volts)
Step 2) Convert percent drop into volts (VD)
VD = System Voltage × (% drop / 100)
Example: 240V at 3% → VD = 240 × 0.03 = 7.2V
Step 3) Find the conductor size using NEC Chapter 9 Table 8
Once you calculate CMA, use NEC Chapter 9 Table 8 and pick the next size UP (never down).
Walkthrough example (typical exam style)
Question (typical):
A 40A, 240V, 1-phase load is 230 ft from the supply. Copper conductor, K = 12.9. Choose a conductor size that keeps voltage drop ≤ 3%.
1) Known values
- I = 40A
- V = 240V
- D = 230 ft (one-way)
- K = 12.9 (copper)
- % drop = 3%
2) Allowable VD
VD = 240 × 0.03 = 7.2V
3) Use single-phase CMA formula
CMA = (2 × 12.9 × 40 × 230) / 7.2
CMA ≈ 32,967 circular mils
4) Table 8 → choose next size up
Find the next conductor size with CMA ≥ 32,967 using Chapter 9 Table 8.
(You’d select the first standard size above that CMA.)
Study smarter with VoltageLab
Built for electricians, apprentices, and electrical engineers who want faster practice and better exam prep.
⭐️ Join thousands of electricians upgrading their skills
Common exam traps (avoid these)
- Using total distance instead of one-way K-method already accounts for the return path (2× for 1φ, √3 for 3φ). Use one-way distance.
- Wrong formula (1φ vs 3φ) If it’s 3-phase, use √3, not 2.
- Wrong K factor
- Copper: 12.9
- Aluminum: 21.2
- Forgetting to convert percent to volts You must calculate VD in volts before using the CMA formula.
- Not rounding up If your CMA falls between sizes, always choose the next size larger.
Quick checklist (what the exam wants)
Before you calculate, confirm:
- ✅ System: 1-phase or 3-phase
- ✅ Material: copper or aluminum
- ✅ Distance is one-way
- ✅ Target drop: 3% or 5% (or given)
- ✅ Use Table 8 for CMA reference
