# Electrical Load Calculator

Calculate total electrical load in amps for any circuit or building. Applies NEC 80% rule for continuous loads. Shows breaker size and wire gauge recommendations.

## What this calculates

Sizing breakers and panels correctly keeps your wiring safe and your inspection passing. This calculator takes the total wattage on a circuit, applies NEC continuous-load rules (125% factor), and tells you exactly what breaker size and wire gauge you need. Works for both single-phase residential and three-phase commercial circuits.

## Inputs

- **Total Connected Watts** (W) — min 0 — Sum of all device wattages on the circuit
- **Circuit Voltage** — options: 120V (Standard), 208V (3-Phase), 240V (Dryer/Range), 277V (Commercial), 480V (Industrial)
- **Continuous Load Portion** (%) — min 0, max 100 — Percentage of load that runs 3+ hours continuously (NEC 210.20)
- **Power Factor** — min 0.5, max 1 — 1.0 for resistive loads (heaters), 0.8-0.9 for motors
- **Number of Phases** — options: Single Phase, Three Phase

## Outputs

- **Circuit Amps** (A) — Actual current draw at rated load
- **NEC Adjusted Amps** (A) — With 125% continuous load factor per NEC 210.20
- **Minimum Breaker Size** (A) — Next standard breaker size up
- **Recommended Wire Gauge** — formatted as text — Copper conductor size (NEC Table 310.16)
- **Apparent Power** (VA) — Volt-amps (watts / power factor)
- **Panel Load Contribution** (W) — NEC-adjusted wattage for panel sizing

## Details

## How Electrical Load Calculation Works

The core formula is simple: **Amps = Watts / Voltage** for single-phase, or **Amps = Watts / (Voltage x 1.732)** for three-phase.

But the NEC (National Electrical Code) adds an important safety layer. Section 210.20 requires that any load running continuously for 3 hours or more must be multiplied by **125%** when sizing the breaker. This is the "80% rule" contractors talk about -- a 20A breaker should carry no more than 16A of continuous load.

## Standard Breaker Sizes

Residential panels typically use these breaker sizes:

- **15A** -- Lighting circuits
- **20A** -- General receptacles, kitchen, bathroom
- **30A** -- Dryers, water heaters
- **40A** -- Ranges, cooktops
- **50A** -- Ovens, large appliances
- **100-200A** -- Main panel breakers

## Wire Gauge Quick Reference (Copper, 75C)

| Breaker | Wire Gauge |
|---------|-----------|
| 15A | 14 AWG |
| 20A | 12 AWG |
| 30A | 10 AWG |
| 40A | 8 AWG |
| 50A | 6 AWG |

Always verify wire sizing with NEC Table 310.16 and check local code amendments.

## Frequently Asked Questions

**Q: What is the NEC 80% rule for breakers?**

A: NEC Section 210.20 requires that continuous loads (running 3+ hours) use no more than 80% of the breaker rating. In practice, this means you multiply continuous loads by 125% when selecting breaker size. A 16A continuous load needs a 20A breaker.

**Q: How do I add up total watts for a circuit?**

A: List every device on the circuit and add their wattages. For lighting, use actual bulb wattage. For outlets, estimate based on what will be plugged in. Appliance nameplates show rated watts. Motors may list horsepower (1 HP = 746W).

**Q: When do I need 240V vs 120V?**

A: Most residential outlets are 120V. Large appliances like dryers, ranges, ovens, water heaters, and central AC use 240V. The higher voltage carries the same power at half the amps, allowing smaller wire.

**Q: What is power factor and when does it matter?**

A: Power factor measures how efficiently a device uses electricity. Resistive loads (heaters, incandescent bulbs) have a power factor of 1.0. Motors and fluorescent ballasts typically run 0.8-0.9. For residential calculations, using 1.0 is safe for most circuits.

**Q: Can I put two circuits on one breaker?**

A: No. Each circuit requires its own breaker. Multi-wire branch circuits (sharing a neutral) use a double-pole breaker or handle-tied single-pole breakers per NEC 210.4. Never tap multiple circuits to a single breaker.

---

Source: https://vastcalc.com/calculators/construction/electrical-load
Category: Construction
Last updated: 2026-04-08