Existing Load by Metered Demand Calculator
Size a feeder or service to an existing building from its measured maximum demand, per the NEC 220.87 optional method. Enter the recorded demand and any new load, and the tool applies the 125% factor, checks the total against your feeder ampacity or service rating, and reports how much new load the existing service can still accept.
How to use this calculator
- Pick whether you are checking a service (rating) or a feeder (ampacity), and enter its ampere value.
- Enter the recorded maximum demand. Use amperes if you have a recording-ammeter reading, or switch to kVA and enter the voltage and phase to convert an apparent-power reading from a power meter.
- Enter the new calculated load you want to add. Leave it at 0 to see the spare capacity of the existing service by itself.
- Read the verdict: whether the projected load fits, the demand taken at 125%, the headroom, and the maximum new load the service can accept.
- Confirm your demand data meets the 220.87 conditions (1-year data or a 30-day recording taken while occupied). The notes list what the code requires.
NEC reference
This calculator implements NEC 2020 220.87 (Determining Existing Loads). The existing load may be taken from the actual maximum demand when the maximum-demand data is available for a 1-year period, or, by the exception, when the maximum demand (the 15-minute average) is continuously recorded over a minimum 30-day period with a recording ammeter or power meter on the highest-loaded phase, taken while occupied and capturing the larger of the heating or cooling load. Per 220.87(2) the maximum demand at 125% plus the new load must not exceed the feeder ampacity or the service rating; per 220.87(3) the feeder must have overcurrent protection per 240.4 and the service overload protection per 230.90. Recommended standard ratings are the NEC 240.6(A) sizes.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Worked example: adding 20 A to a 200 A service
An existing 200 A service has a recorded maximum demand of 130 A over the past year. You want to add a 20 A calculated load. Does 220.87 permit it on the existing service? These are the calculator's default inputs, so you can check the tool against the arithmetic directly.
| Step | Amperes |
|---|---|
| Recorded maximum demand | 130.0 |
| Existing demand at 125% (130 x 1.25) | 162.5 |
| New load (added at 100%) | 20.0 |
| Projected load (162.5 + 20) | 182.5 |
| Service rating | 200.0 |
| Headroom (200 - 182.5) | 17.5 |
182.5 A is within the 200 A service, so the addition is permitted with 17.5 A of headroom. Before this load was added, the service could accept up to 200 minus 162.5 equals 37.5 A of new load. Note that the 125% factor applies only to the measured 130 A demand, not to the 20 A addition.
The canonical 400 A example
The textbook 220.87 case: a 400 A service records a 300 A maximum demand. The existing load is 300 x 1.25 equals 375 A, so the service can accept only 400 minus 375 equals 25 A of new load. It is a common surprise on a retrofit that a 400 A service loaded to 300 A has almost no room left once the 125% factor is applied. Enter 300 A demand, a 400 A service, and 0 new load to reproduce it.
Frequently asked questions
Amperes or kVA for the demand reading?
A recording ammeter reports amperes directly, and 220.87 is written around an ammeter on the highest-loaded phase, so amperes is the most direct entry. If your reading came from a power meter in kVA, switch the unit to kVA and enter the line-to-line voltage and phase; the tool converts using amps equals kVA x 1000 / V for single-phase and kVA x 1000 / (root 3 x V) for three-phase. Use apparent power (kVA) rather than real power (kW): apparent power sets the current, and using it never undersizes.
Does the 125% factor apply to the new load too?
No. NEC 220.87(2) applies 125% to the existing maximum demand only; the new load is added at 100%. If the new load is a continuous load, it should already be sized at 125% under 210.20(A) or 215.3 as part of its own calculation before you enter it here, so the factor is not applied a second time by this tool.
Can I use metered demand if the building has solar or a load controller?
Be cautious. On-site generation such as solar, or a peak-shaving or load-management system, can hold the metered demand below the true connected load, so a 220.87 recording taken with those systems active may understate what the service would carry if they were offline. Where the measured demand does not reflect the true worst-case load, fall back to a connected-load method such as 220.83 or the Article 220 standard calculation.
Is 220.87 only for services?
No. 220.87 applies to feeders as well as services. Set the tool to feeder and enter the feeder ampacity; the 125% demand rule is the same. The only difference is the overcurrent-protection condition: 220.87(3) points a feeder to 240.4 and a service to 230.90. This tool checks the ampere capacity and reports the applicable citation, but it does not size the overcurrent device.
Related calculators
- Existing Dwelling Load Calculator: the NEC 220.83 connected-load method for an existing dwelling addition, when you do not have metered demand.
- Service Size Calculator: turn a calculated load into a service disconnect, conductors, and GEC.
- Electrical Load Calculator: the NEC 220.82 optional method for sizing a whole new dwelling service.
- Breaker Size Calculator: size the overcurrent device and conductor for the new load you are adding.
Studying for the licensing exam? Our own NEC Code Quickstart works all twelve exam calculations start to finish, each with the code reference. Also in paperback.
Running an electrical contracting business? Jobber handles scheduling, invoicing, and job management. Start a 14-day free trial.
For deeper NEC training on load calculation and service sizing, Mike Holt's NEC courses are the industry standard.
Working from the printed code? NEC code books and tab sets are available from Mike Holt.
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