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.
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.)
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
