Load lists start in watts—panels and overcurrent devices think in amps. Learn how to convert each electrical load from rated power to current so branch circuits, inverters, and backup systems are sized consistently.
Benefits
- Builds a repeatable load table: device name, watts, voltage, calculated amps, and notes on continuous vs. non-continuous duty.
- Supports mixed 120 V branch loads and 12/24/48 V DC appliances on the same worksheet with separate voltage columns.
- Highlights when nameplate VA differs from real watts so you do not undersize motor and compressor circuits.
How it works
- List each load with nameplate or measured watts and the voltage at its connection point.
- Apply I = P ÷ V per row; sum amps only within the same voltage and phase group.
- Compare totals to breaker, inverter, or generator continuous-current ratings with code margins.
FAQ
Why convert watts to amps per load instead of totaling watts first?
Summing watts is fine on a single voltage. Mixed-voltage systems (e.g., 120 V AC plus 12 V DC) require converting each load to amps at its bus before comparing to breaker or cable limits on that bus.
What counts as continuous electrical load?
Loads running three hours or more in a normal duty cycle often need 125% conductor and OCPD sizing under NEC practice. Mark continuous rows in your table before converting watts to amps.
How do motors affect the conversion?
Use running watts for steady amps; check locked-rotor or LRA for breaker coordination. Watts ÷ volts gives running current, not startup peak.
Technical specifications
- Per load: I (A) = P (W) ÷ V (V) at connection voltage.
- Panel subtotals: sum I per leg/bus; do not sum unlike voltages without conversion.
- Continuous load margin: ×1.25 on calculated amps where applicable.
- Tool cross-check: watts-to-amps calculator for single-load spot checks.
From load survey to amp column
Walk the site with a clamp meter or nameplate camera. Record watts and voltage per device, convert to amps immediately, and flag anything with poor power factor or high inrush so it is not treated as a simple resistive row.
Generator and inverter limits
Inverter continuous amps and generator prime ratings bind on current, not watts alone—at your operating voltage. Converting each load prevents approving a 3 kW inverter for a 3 kW load that draws overcurrent at motor start on a 120 V leg.