Electrical Load Calculator – Calculate Circuit Load
Calculate the total electrical load on a circuit. Enter voltage and connected loads to determine current, breaker size, and wire requirements.
Step 1: Enter your system voltage (120V, 230V, or 400V depending on your region and application).
Step 2: Add all devices connected to the circuit with their wattage ratings.
Step 3: Click Calculate to see total load, current draw, recommended breaker size, and minimum wire ampacity.
What Is Electrical Load
Electrical load is the total power consumed by all devices on a circuit. Think of it like passengers in an elevator - each device adds weight, and the circuit has a maximum capacity. Exceeding that capacity trips the breaker or, worse, overheats wires.
Why Load Calculations Matter
Proper load calculations prevent three common problems:
- Nuisance tripping: Breakers that trip randomly because the load exceeds 80% of rating for continuous use
- Overheated wires: Undersized wires get hot and can start fires inside walls
- Code violations: Electrical codes require specific safety margins that this calculator applies automatically
The 80% Rule Explained
Electrical codes require circuits to be sized at 125% of continuous load (anything running 3+ hours). A 15A circuit should only carry 12A of continuous load. This gives a safety margin so wires stay cool and breakers don't trip unnecessarily.
| Breaker Rating | Max Continuous Load (80%) | Max Power at 120V | Max Power at 230V |
|---|---|---|---|
| 15 A | 12 A | 1,440 W | 2,760 W |
| 20 A | 16 A | 1,920 W | 3,680 W |
| 25 A | 20 A | 2,400 W | 4,600 W |
| 30 A | 24 A | 2,880 W | 5,520 W |
| 40 A | 32 A | 3,840 W | 7,360 W |
| 50 A | 40 A | 4,800 W | 9,200 W |
Continuous loads (3+ hours) must not exceed 80% of breaker rating per NEC and IEC standards.
Common household device wattages. Add up all devices on a circuit to check if you're within safe limits.
Breaker Sizing Formula
Breaker Rating ≥ Load Current × 1.25
Multiply your total current by 1.25 to get the minimum breaker size. Round up to the nearest standard rating (15A, 20A, 25A, 30A, etc.).
Wire Ampacity Requirements
Wire Ampacity ≥ Load Current × 1.25
Wire must handle the same 125% of continuous load as the breaker. Common copper wire ratings at 60°C:
- 14 AWG: 15 A (for 15A circuits)
- 12 AWG: 20 A (for 20A circuits)
- 10 AWG: 30 A (for 30A circuits)
- 8 AWG: 40-50 A (for 40-50A circuits)
Voltage Drop Considerations
For runs over 100 feet, increase wire size to reduce voltage drop. More than 3% voltage drop wastes energy and can damage motors. Use a voltage drop calculator for long runs.
How do I calculate electrical load for a circuit?
Add up the wattage of all devices on the circuit, then divide by voltage to get current. For continuous loads, multiply by 1.25. A 15A circuit at 120V handles 1,800W total, or 1,440W for continuous use.
What size breaker do I need for my load?
Divide total watts by voltage to get amps, then multiply by 1.25 for continuous loads. Round up to standard breaker sizes: 15A, 20A, 25A, 30A, 40A, or 50A. Always match wire size to breaker rating.
Can I put multiple outlets on one breaker?
Yes, but the total load from all outlets cannot exceed the breaker rating. A 20A circuit can have 10-12 outlets, but you need to consider what devices will plug in. Kitchen and bathroom outlets require dedicated 20A circuits.
Why does my breaker keep tripping?
Either the load exceeds the breaker rating, or the breaker is old and weak. Check your total wattage - a space heater (1,500W) plus hair dryer (1,500W) on a 15A circuit will trip it. If load is fine, the breaker may need replacement.
What wire size for a 20 amp circuit?
Use 12 AWG copper wire for 20A circuits. This is standard for kitchen, bathroom, and outdoor receptacles. For runs over 100 feet, consider 10 AWG to reduce voltage drop.
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