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Electrical Panel Capacity Calculator

Spare service capacity after adding a water heater, with the 125 percent continuous factor and the next standard service rating.

Inputs

The service

The service disconnect rating, from the main breaker or the meter base. Sizes are the standard ratings of NEC 240.6(A).

120/240 V single phase or 208/120 V three phase are the usual residential services.

Single phase for most homes.

Bring this from a load calculation or from a utility demand reading. Do not use the main breaker rating - that is the service size, not the load.

What you are adding

The water heater's rated current, usually the nameplate amperage or watts divided by the voltage.

A storage water heater up to 120 gallons is treated as continuous by NEC 422.13, so 125 percent applies to the added load.

Result

Service demand with the new load
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Spare capacity
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Service utilisation
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Added load after the continuous factor
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Added load at nameplate
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Existing demand
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Next standard service that would cover it
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Total demand
—
—
Where this stops applying
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Calculation details
Continuous-load multiplier
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Phase multiplier
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What to check

  • Existing calculated or recorded demand, not the main breaker rating
  • Nameplate amperage of the water heater being added
  • Service rating, voltage and phase from the panel label

Standards referenced

  • NFPA 70 National Electrical Code (NEC) — 2023 edition · Branch circuits, conductors, protection, raceways, and services. Local AHJ adoptions vary; the enforcing edition is set by the jurisdiction.

Related calculators

Screening estimate only. The result panel lists the limits that apply to this calculation. Verify nameplate data, rates, groundwater temperatures and installer quotes before deciding, and have gas, venting and electrical installations checked against the local code by a licensed contractor.

Saved calculations

Keep the assumptions with the result.

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