Wire Sizing Calculator
Find the right electrical wire size based on load current, distance, and voltage drop requirements. Supports both NEC (USA) and CEC (Canada) with appropriate derating factors for temperature and conduit fill conditions.
Results
• AWG sizesProper wire sizing ensures safe operation and code compliance. The calculator considers ampacity requirements, voltage drop limitations, and applies appropriate derating factors for temperature and conduit fill conditions.
Wire Size Formats
AWG (US): American Wire Gauge - Standard in USA/Canada (14, 12, 10, 8, 6, 4, etc.)
mm² (Metric): Cross-sectional area in square millimeters - Standard in Europe/International (2.5mm², 4mm², 6mm², etc.)
AWG + mm²: Shows both formats for reference and comparison
Ampacity Calculation
I_adj = I_load × 1.25
125% factor for continuous loads
A_derated = A_base × TF × CF
TF = Temp Factor, CF = Conduit Factor
A_derated ≥ I_adj
Voltage Drop Check
VD = 2 × K × I × L / CM
VD = 1.732 × K × I × L / CM
VD% = (VD / V) × 100
VD% ≤ 3% (branch circuits)
Derating Factors
Temperature Correction
• 70-77°F (21-25°C): 1.08
• 79-86°F (26-30°C): 1.00 (reference)
• 88-95°F (31-35°C): 0.91
• 97-104°F (36-40°C): 0.82
• 106-113°F (41-45°C): 0.71
• 115-122°F (46-50°C): 0.58
Conduit Fill Adjustment
• 1-3 conductors: 1.00
• 4-6 conductors: 0.80
• 7-9 conductors: 0.70
• 10-20 conductors: 0.50
• 21-30 conductors: 0.45
• 31+ conductors: 0.40 or less
Example Calculation
30A continuous load, 100 feet, 167°F (75°C) wire, 95°F (35°C) ambient, 6 conductors in conduit:
• Adjusted Load: 30A × 1.25 = 37.5A
• Derating: 0.91 × 0.80 = 0.728
• Required Base Ampacity: 37.5A / 0.728 = 51.5A
• Minimum Wire: 8 AWG (NEC: 50A, CEC: 50A base × 0.728 = 36.4A less than 37.5A) → 6 AWG needed
How to Calculate Wire Size: Step-by-Step
Selecting the right wire size requires balancing ampacity, voltage drop, and code requirements. Follow these steps to pick the correct gauge every time.
Step 1: Determine the Load Current
Find the circuit's full load current from the equipment nameplate or by dividing watts by voltage. If the load is continuous (runs 3+ hours), multiply by 125% per NEC 210.20.
Step 2: Check NEC 310.16 Ampacity Table
Look up the minimum wire size that has an ampacity equal to or greater than your adjusted load current. Use the 75 degrees C column for most installations with standard terminations.
Step 3: Apply Derating Factors
If more than 3 current-carrying conductors share a conduit, reduce the ampacity per NEC 310.15(C). Also apply temperature correction if the ambient temperature exceeds 30 degrees C.
Step 4: Calculate Voltage Drop
Use VD = 2 x K x I x L / CM to ensure the voltage drop stays within NEC recommendations. Long runs often require upsizing the wire beyond what ampacity alone would dictate.
Step 5: Select the Larger of Both Results
Compare the wire size from ampacity (Steps 2-3) and from voltage drop (Step 4). Always use the larger wire. This ensures both safe current capacity and acceptable voltage at the load.
Formula
Adjusted Current = Load Amps x 1.25 (continuous) | VD = 2 x K x I x L / CM
Where: K = 12.9 (copper) or 21.2 (aluminum), I = Current (amps), L = One-way distance (feet), CM = Circular mil area of wire
Worked Example
Scenario: A 30A continuous load at 240V runs 150 feet using copper wire.
- Step 1: Adjusted current = 30 x 1.25 = 37.5A
- Step 2: NEC 310.16 at 75C: #8 AWG is rated 50A (meets 37.5A)
- Step 3: Only 3 conductors in conduit, no derating needed
- Step 4: VD for #8 (16,510 CM) = 2 x 12.9 x 30 x 150 / 16,510 = 7.0V (2.9%)
- Step 5: #8 AWG satisfies both ampacity and voltage drop
Result: Use #8 AWG copper, which handles 37.5A adjusted load with only 2.9% voltage drop.
Wire Sizing Questions & Answers
Should I size wire based on breaker size or actual load?
Always size wire for the actual load current, not the breaker size. A 20-amp breaker might protect a 16-amp motor, so you size the wire for 16 amps (plus 125% if it's a continuous load). The breaker protects the wire, but the wire must handle the actual equipment current safely.
When do I need to derate wire for temperature?
Anytime ambient temperature exceeds 86°F (30°C). Hot attics, near furnaces, or Arizona summer heat all require derating. At 104°F (40°C) you derate to 88% of normal ampacity, at 122°F (50°C) it drops to 71%. That 12 AWG rated for 20 amps might only handle 14 amps in a hot attic. Both NEC and CEC use similar temperature derating factors.
How many wires in conduit before I need to derate?
You start derating at 4 current-carrying conductors in the same conduit. With 4-6 conductors, multiply ampacity by 80%. With 7-9 conductors, it's 70%. Remember that neutrals carrying unbalanced current count as current-carrying conductors, but equipment grounding conductors don't count. This applies to both NEC and CEC installations.
What's the difference between THHN and THWN wire?
THHN is rated for dry locations only, while THWN works in wet locations too. Most modern wire is dual-rated THHN/THWN-2, so it works anywhere. The ampacity ratings are the same - it's just about where you can install it. Wet locations need the "W" rating for both NEC and CEC compliance.
Can I use the 167°F (75°C) or 194°F (90°C) ampacity columns?
Depends on your terminations. Most standard breakers and devices are rated for 167°F (75°C), so you use the 167°F (75°C) column even with 194°F (90°C) wire. You can use 194°F (90°C) ratings for derating calculations, but the final ampacity can't exceed what your terminations can handle. Check equipment labeling for both NEC and CEC installations.
Why does voltage drop matter for wire sizing?
Wire sized only for ampacity might still cause problems with voltage drop on long runs. A motor 200 feet away might need 10 AWG for ampacity but 6 AWG to keep voltage drop under 3%. Motors especially hate low voltage - they draw more current and run hot when voltage drops too much.
Is aluminum wire really that much cheaper for large sizes?
For large feeders, absolutely. A 400-amp service might save you $500-1000 using aluminum instead of copper. But you need larger wire sizes (aluminum 3/0 equals copper 1/0 in ampacity), larger conduit, and proper terminations. Factor in the total installation cost, not just wire cost.
What size wire do I need for a 50-amp electric range?
6 AWG copper is the minimum for a 50-amp range, but check your specific range nameplate. Some ranges allow demand factor calculations that might let you use 8 AWG, while others require the full 50 amps. For a straight 50-amp circuit, 6 AWG copper or 4 AWG aluminum with 50-amp breaker.
Can I run 240V equipment on 208V power?
Usually yes, but it affects performance. Most 240V equipment works fine on 208V but produces about 75% of rated output (since power varies with voltage squared). Check the nameplate - many modern devices are rated 208-240V. For wire sizing, use the actual operating voltage and current.
Do I need to upsize the neutral for non-linear loads?
Sometimes. Non-linear loads like computers, LED drivers, and electronic ballasts create harmonics that don't cancel in the neutral. For office buildings with lots of computers, you might need to size the neutral at 100% instead of using demand factors. Check NEC 220.61 for the rules.
What's the biggest wire size mistake electricians make?
Not accounting for continuous loads properly. Any load running 3+ hours needs wire sized at 125% of the load current. A 16-amp continuous load needs wire sized for 20 amps minimum. Lighting circuits are almost always continuous loads, but many electricians forget this NEC/CEC requirement.
Can I parallel smaller wires instead of using one large wire?
Yes, but only for 1/0 AWG and larger per NEC 310.10(H) and similar CEC requirements. The wires must be identical length, type, and termination, and each conduit needs a full set (including grounds). It's usually easier and cheaper to just use one larger wire unless you're dealing with very large loads or difficult pulls.
What's the difference between NEC and CEC wire sizing?
The Canadian Electrical Code (CEC) generally has higher ampacity ratings for larger conductors compared to NEC, particularly for wire sizes 8 AWG and larger. CEC also uses metric measurements alongside AWG designations. Both codes use similar derating factors for temperature and conduit fill, but CEC ampacities can allow for smaller wire sizes on some applications. Always verify local adoption and amendments.
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