Boat Shore Power Load Calculator – Amps, Watts & Circuit Capacity

Total simultaneous AC loads in amps, watts, or volt-amps; account for power factor; and compare each shore-power leg with its circuit rating and an editable planning target. All inputs stay in this browser tab.

Load-planning estimate — not electrical approvalIncorrect shore-power equipment or loading can cause shock, electric-shock drowning, overheating, fire, or equipment damage. Verify the pedestal supply, cordset, inlet, main protection, vessel distribution, grounding, polarity, leakage protection, and applicable rules with the marina, equipment instructions, and a qualified marine electrician.

Shore service and simultaneous loads

Verified shore-power service

Confirm actual labels; do not choose by connector appearance alone.

For 125/250 V split phase, enter 125 V.

Use the lowest verified controlling rating.

Two-leg mode assumes line-to-line voltage is twice the entered leg voltage.

Editable margin only—not a universal code limit.

Loads expected to run together

Use nameplate or manufacturer operating data. Enter starting or inrush current separately in an engineering check.

Privacy: calculations are performed locally. Input values are not sent, saved, or included in analytics events.

Circuit capacity result

Advertisement

How to use the calculator

  1. Identify the shore supply and every controlling rating from the pedestal to the boat's AC main protection.
  2. Add the operating value for every appliance or branch load that may be on at the same time.
  3. Choose watts, amps, or VA to match the source data and enter a documented power factor where available.
  4. On 125/250 V service, assign each 125 V load to its actual leg and each 250 V load across both.
  5. Review the most-loaded leg, not only total power, and separately assess inrush and continuous-load requirements.

What the result means

Full service capacity is a nameplate apparent-power comparison, not a promise that all of it may be used continuously. The planning target applies the editable percentage to each leg.

  • Real power (W) is the power doing useful work or producing heat.
  • Apparent power (VA) combines RMS voltage and current and is the relevant capacity total.
  • Per-leg current exposes imbalance on a split-phase supply.
Do not add breaker handles: two 50 A legs are not a 100 A leg. Each leg remains limited to its verified 50 A rating.

Formulas and assumptions

Apparent power (VA) = voltage (V) × current (A)
Real power (W) = apparent power (VA) × power factor
Current for a watt-rated load = watts ÷ (voltage × power factor)

For a two-leg split-phase service, a leg-to-neutral load counts only on its selected leg. A line-to-line load uses twice the entered leg voltage and adds the calculated line current to both legs. Total service capacity is leg voltage × circuit amps × number of legs.

The model assumes nominal RMS voltage, sinusoidal single-phase or split-phase supply, one entered operating value per simultaneous load, and a power factor from 0.01 to 1.00. It does not model voltage sag, harmonic current, neutral current from nonlinear loads, duty cycling, diversity, starting surge, generator/inverter limits, transfer switching, cable loss, breaker trip curves, frequency compatibility, or multiple shore inlets.

Frequently asked questions

How many watts are available from 30 amp boat shore power?

At a nominal 125 V, 30 A corresponds to 3,750 VA. At unity power factor that is numerically 3,750 W; with lower power factor, real watts are lower for the same current. Actual permitted and available capacity depends on the complete installation.

How much capacity does 50 amp 125/250 volt shore power provide?

It provides two 125 V legs rated at 50 A each: 6,250 VA per leg and 12,500 VA total when evenly used. One heavily loaded leg can still be the constraint.

Why does shore-power leg balance matter?

A 125 V load uses one leg. If too many loads are assigned to the same leg, that leg can exceed its rating even while the combined VA total appears acceptable. A 250 V line-to-line load adds equal current to both legs.

What power factor should I use?

Use manufacturer data at the relevant operating condition. Resistive heating is often near 1.00. Motors, compressors, chargers, and power electronics can be lower and can change with load. Do not guess a favorable value to make a plan pass.

Is the 80% planning target a required maximum?

No. It is an editable conservative screening margin. The applicable continuous-load treatment and permitted design come from current vessel standards, electrical code, marina rules, equipment listings, and the installation—not from this calculator.

Does the result include air-conditioner or pump starting surge?

No. The calculator totals steady operating values. Motor and compressor starting current, transformer energization, charger inrush, and breaker trip behavior require separate equipment-specific checks.

Can this calculator select a shore cord, adapter, breaker, ELCI, or RCD?

No. Never use an adapter to infer more capacity or defeat a protective device. Selection and installation require compatible listed equipment, conductor and protection sizing, polarity and grounding checks, leakage protection, and compliance with current requirements.

References and safety limits

Safety limitThis educational load screen cannot establish compliance or safe operation. Use current ABYC/ISO standards as applicable, local electrical requirements, marina rules, equipment instructions, and a qualified marine electrician. If a connection is hot, damaged, wet, discolored, loose, arcing, or repeatedly trips, disconnect safely and obtain professional help.

Explore more calculators

Marine & BoatingInfrastructureAll Calculators

Explore more tools