How We Answer
How a Load Calculation Works
Harlow Street Motors. 15 frames, about 2 minutes.
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The showroom needs chargers for its EVs.
Harlow Street Motors is adding 4 chargers to keep the electric cars on its lot and showroom floor charged. Before the electrician can file for a permit, somebody has to show that the 400 A service can carry them.
First, we check the capacity with the utility's data.
Connecting the utility account brings in 12 months of 15-minute demand and the tariff. There is no visit, no photograph and no sensor at this step.
The Preliminary Capacity Report gives the first answer.
The highest demand of the year is 220 A on a 400 A service, and the chargers fit. The report arrives in 2 days. It carries no seal, so it is not submittable to a permit office. The Free Panel Read gives a first look at no cost.
Then we digitize the panel.
A technician photographs the open panel, the schedule and every nameplate, with the panel live. The record is needed for the seal and for anything below the meter.
Every circuit is written down properly.
Each breaker becomes a typed row with its circuit number, what it serves and its rating.
The panel now has a record.
A QR code goes on the door. Scanning it opens this panel's schedule, its photographs and the calculation drawn from them.
We record what is on every circuit.
Each circuit is traced to the equipment it feeds, and that equipment's nameplate is recorded beside it.
This panel needs no sensors.
Sensors go on only when the utility data cannot settle the question. None of the three reasons applies here, so the calculation stands on the utility's own record.
The existing load is the measured peak.
NEC 220.87 takes the existing load from the highest demand actually recorded over a year: 220 A here. It does not guess at everything that could run at once.
The code adds 25% to it.
The 2023 NEC, which California has adopted, multiplies the recorded peak by 125%, which makes 275 A. The 2026 edition keeps it and numbers the section 120.87. The calculation cites whichever edition the authority has adopted.
The chargers are added on top.
4 chargers of 7.7 kW, calculated under the code's own rule for chargers, come to 85 A.
It fits, with 40 A to spare.
360 A against a 400 A rating. The chargers can go in on the service the showroom already has.
A licensed Professional Engineer seals it.
The engineer reviews the record, the data and the arithmetic, and seals the Determination of Load Capacity under their own licence. It is 6 sheets: the determination, the calculation line by line, the single-line, the panel schedule, the basis for every input, and the certification. It arrives in 3 days.
The electrician files it with the permit.
The determination goes in with the permit application. Plan check comments on it are answered at no charge until the submittal is accepted.
The chargers go in on the service the showroom has.
No new switchgear and no utility upgrade. The panel's record stays on its door for the next project.
How Capacity Recovery Works
Alder Street Grocery. 28 frames, about 3 minutes.
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A contractor has quoted an upgrade.
Alder Street Grocery wants EV chargers for its customers. A contractor has quoted an upgrade of the 400 A service to 800 A, with new switchgear and utility work. Nobody has yet measured what the panel actually carries, circuit by circuit.
A first estimate says the panel is tight.
The panel already has a record: every breaker scheduled, every load written down, and a QR code on the door. The Determination is a high-level estimate of the capacity the service has, and it says the 400 A service is tight. The next step is to measure it.
We fit sensors to every circuit that matters.
Split-core sensors close around each circuit's conductor, and flexible coils wrap the large conductors of the mains. The panel stays live and nothing is disconnected. Each sensor is labelled against the panel record.
They record through a full operating cycle.
Current and voltage are recorded on every sensor for 30 days, through the facility's busiest days. Then the sensors are collected.
A reading every 15 minutes, on every circuit.
That is the interval your utility bills demand on. The chart shows one week of current on the main service, with its 400 A rating.
The peak crosses the line the code allows.
The code counts the recorded peak at 125%, so on a 400 A service the peak has to stay under 320 A before anything is added. Every weekday morning it goes over. The highest reading is 336 A, on Tuesday, 7:15 am.
The peak comes from a few circuits at once.
Each band is one circuit on that Tuesday. At 7:15 the rooftop units are starting, the water heater is recovering, the kitchen is on and the parking lights are still lit.
Here is your service.
The same peak, drawn as one bar of amperes. The line is the 400 A rating.
This is what the code counts.
336 A at the peak, and the code adds 25% to it. Counted, that is 420 A, 20 A over the rating before anything is added.
So the new load has nowhere to go.
The EV chargers need 48 A. On this reading there is no room for them, which is why the quote exists.
We look at it circuit by circuit.
Each segment is one circuit's share of the 336 A peak, drawn at its real width.
Life safety, and anything you tell us to leave alone.
These circuits are set aside before anything else is decided, and they are never touched. Here that is life safety, refrigeration and the kitchen.
This much of the peak does not need to be there.
- The parking lights are on day and night. 16 A.
- The water heater recovers during the morning peak. 12 A.
- The supply fan runs at full speed around the clock. 14 A.
- The 3 rooftop units start at the same moment. 34 A.
76 A is recoverable.
The Load Study finds the capacity that can be unlocked, circuit by circuit, and leaves the rest of the operation as it is.
Each finding is matched to an intervention.
An intervention is named by what it does to the peak, such as moving a load to another hour or starting loads one at a time. The design says which loads change, what sheds at the peak and how many kW come off. It is fitted to 12 months of the facility's utility demand and 4 to 8 weeks of its key circuits recorded in the peak season.
Simulated, the Tuesday peak comes down by 76 A.
The chart shows the main service on the peak day, as measured. With the design applied, the 7:15 peak falls from 336 A to 260 A. The same chart is drawn again once the panel has been measured after the work.
An electrician fits two timers.
One turns the parking lights on at 7 pm and off at 6:30 am. The other has the water heater recover between 4:30 and 6 am, before the store opens. The work is done under the electrician's own licence.
The controller staggers the rooftop units.
The 3 units used to start together at 6:45 am. The controller Wattif runs on site now starts them one at a time from 4:45, so no two pull their starting current at once. Nothing is replaced.
An electrician fits a drive to the supply fan.
The fan used to run at full speed around the clock. With the drive it slows whenever the space needs less air.
Every change is photographed and checked.
The field app records what was set and photographs what was fitted, against the circuit. We check each one against the specification before the sensors go back in.
We measure again, on the same instruments.
The sensors go back on the same conductors for another window. The result is the facility's own measured peak.
76 amps back.
Measured, the peak falls from 336 A to 260 A. Counted at 125%, that is 325 A, and 75 A of the rating is free.
Now it fits.
The EV chargers' 48 A lands inside the 400 A rating, with 27 A to spare.
Two of the changes also cut energy.
Staggering the rooftop units and the water heater timer move energy to another hour. The parking-light schedule and the fan drive remove it. The shaded area is energy no longer used.
63,400 kWh a year less.
The same changes take 25 kW off the peak. Both figures are read off the same instruments as the amperes.
Sealed by a licensed Professional Engineer.
The panel measured again states the recovered result, and a licensed Professional Engineer seals it under their own licence. It is the document the permit is filed on.
The panel's record shows its new capacity.
It lists each change, what it acts on and what settings it carries. The controller stays on site, and Wattif keeps it running.
No upgrade.
The EV chargers go in on the 400 A service you already have.
How Peak Determination Works
Linden Court Hotel. 14 frames, about 2 minutes.
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One 15-minute interval sets the demand charge.
Linden Court Hotel is a specimen hotel, and it pays two charges for electricity. The energy charge follows the kilowatt-hours used across the month. The demand charge follows the single busiest 15 minutes, and in August that one interval cost the hotel $11,475.
We read 12 months of the utility's interval data.
Connecting the utility account brings in every 15-minute reading of the year, with the tariff. There is no visit at this step.
Each month's charge was set by one interval.
The report finds that interval in every month. The highest of the year is 510 kW, in August, on a Friday at 4:45 pm. Summer months set the largest charges.
The same climb, every afternoon.
The week around August's peak shows one shape each day: demand rises through the afternoon as guests arrive, then eases through the night. Friday, the busiest check-in, is the highest.
It lands at 4:45 pm, as guests check in.
Rooms cooling down, the laundry still drying and the kitchen starting dinner all land in the same quarter hour. That quarter hour is what August is billed on.
The panel record says what could be running.
The capacity report already digitized the hotel's main panel. Every circuit's equipment and nameplate are on record, so the interval is matched to real loads.
Each circuit's share of that interval.
Where circuits have been measured, each share is read directly. Where they have not, it is attributed from the panel record and the equipment's own schedule, and the report says which.
510 kW, split by what drew it.
The same interval as one bar, one segment per circuit, at its real width.
Four things put the peak where it is.
- Every occupied room cools down at once as guests check in. 40 kW.
- The laundry dryers run through the afternoon. 45 kW.
- The kitchen hood fans run at full speed from 4 pm. 12 kW.
- The pool heater runs through the peak. 24 kW.
Together they account for 121 kW of the 510 kW peak, which cost $2,723 in August alone.
The charge is priced on the hotel's own tariff.
This hotel's tariff charges $22.50 for each kW of the month's peak. August's 510 kW cost $11,475, and across the year its demand charges came to $109,598. The report reads each tariff's own terms, including any ratchet, before it prices anything.
Chargers that start at check-in raise the peak.
8 chargers for guests add 62 kW. Plugged in at check-in, they land on the interval that already sets the charge: the peak rises to 572 kW and August's charge to $12,870.
The same charging overnight leaves it where it was.
Held until after 10 pm, the same energy stays under today's peak of 510 kW, and the charge stays at $11,475. The report states both.
What you receive.
- The interval that set each month's charge, with its date and time.
- What set each one, named circuit by circuit.
- What it costs on your tariff, today and with the load you are planning.
It is engineer-reviewed and not sealed, because it explains a bill rather than calculating capacity.
Where the peak is the problem, it can be taken down.
Taking the four causes out of the peak would leave 389 kW. Capacity Recovery goes on to design the changes on the hotel's own demand, and the same panel is measured again once they are in.