Small Business Panel and Load Assessment

Purpose

Answer "can my building take this" with a measured number instead of an opinion. On an existing commercial building, adding up nameplates gives a total that no occupied building has ever actually drawn, and counting empty breaker spaces measures the enclosure rather than the conductors feeding it. Both produce a confident wrong answer, in opposite directions.

This SOP owns the measured-demand method for the whole Electrical shelf. Other procedures that need an existing load figure take it from here rather than deriving their own, so the number in a tenant circuit addition, a panel change quote and an equipment install all come from one recording with one window and one measurement point attached to it.

Scope

Covers assessing whether an existing commercial service and panel can carry a proposed addition, in small businesses on services up to 400 A: retail suites, offices, salons, print shops, small shops and professional practices.

Does not cover the standard Article 220 calculation for a dwelling, which the Load Calculation Before a Service Upgrade SOP owns; does not cover performing the service or panel replacement the assessment may recommend; and does not cover the condition assessment of a discontinued or damaged panel, which the Obsolete Panel Assessment and Replacement Recommendation SOP owns and which this SOP hands off to.

Roles and handoffs

Procedure

1. Pin down exactly what is being added, in nameplate terms. Acceptance: every proposed load listed with nameplate volt-amperes or amperes, voltage, phase, and whether it is continuous, meaning expected to run three hours or more. Wrong looks like "a couple of machines," which cannot be calculated and which changes after the quote. Stop rule: a proposed load with no nameplate data available stops the assessment until the customer supplies the manufacturer's data; a guess entered here reappears as an undersized feeder. Hazard: none at this step, it is a list built at a desk from documents the customer supplies.

2. Record what the service and panel actually are, from the labels. Acceptance: service rating, panel bus rating, main overcurrent device rating, the panel's marked maximum number of overcurrent devices, conductor size and material, and the terminal temperature rating, all read and photographed. Wrong looks like assuming the service is 200 A because the main breaker says 200, when the conductors are sized for less. Stop rule: unreadable or missing labels means the panel goes to the obsolete-panel assessment rather than into a capacity calculation. Hazard: reading inside an energized enclosure is qualified-person work under 29 CFR 1910.332 and 1910.399, in arc-rated clothing per your employer's program, and the cover is removed by someone standing to the hinge side.

3. Choose the calculation method and write down why. NEC 220.87, in the edition your jurisdiction has adopted, permits the existing load to be taken as 125 percent of the maximum demand from a year of data, or, where that data is unavailable, from a recording of at least 30 consecutive days, provided the conditions that section states are met. Acceptance: the chosen method named with the reason, and the section's conditions confirmed one by one in writing. Wrong looks like using 220.87 on a building whose occupancy or equipment is about to change, which makes the recording a measurement of a building that will not exist. Stop rule: conditions not met sends the job to a standard Article 220 calculation instead. Hazard: none at this step, it is a decision made at a desk.

4. Install the recorder at a stated measurement point, over a stated window. Acceptance: current transformers on the service or feeder conductors at a named point, a demand interval of 15 minutes or shorter, and at least 30 consecutive days that include the season this business loads hardest. Wrong looks like a 30-day window run in a salon's slowest month, which produces a low number the equipment will exceed in December. Stop rule: a recorder that loses power or drops data restarts the full 30 days rather than reporting a partial window. Hazard: putting current transformers around live service conductors is energized work; do it de-energized where the utility can drop the service, and otherwise only under an energized work permit with shock and arc flash boundaries established per NFPA 70E-2021, Article 130, as the Energized Work Permit Process SOP sets out.

5. Retrieve the data and read the maximum demand per phase, not just the total. Acceptance: maximum demand in amperes for each phase with its timestamp, plus the phase spread stated as a percentage of the highest phase. Wrong looks like reporting only the average, which hides the peak that trips things. Stop rule: a phase spread over our shop flag of 10 percent goes on the report as a balancing item to be corrected before load is added, because adding load to the heaviest phase is how a panel that calculated fine still nuisance-trips. Hazard: opening the enclosure again to remove the transformers carries the same energized-work condition as step 4 and gets the same treatment, not a shortcut because the job is nearly over.

6. Do the arithmetic in one unit and show every step. Take 125 percent of the recorded maximum demand as the existing load, add the new load with continuous loads taken at 125 percent per NEC Article 210 or 215 as applicable, and compare to the service rating and to the conductor ampacity at its terminal temperature rating per NEC 110.14(C) and Table 310.16. Acceptance: existing load, added load, total and headroom all stated in amperes at the same voltage and phase, with the governing limit named. Wrong looks like comparing a total to the main breaker rating when the conductors, not the breaker, are the smaller number. Stop rule: a total at or above the governing limit ends the "add it" answer and starts the service-upgrade conversation. Hazard: none at this step, it is arithmetic done at a desk.

7. Check the constraints that fail before capacity does. Count usable spaces against the device the addition needs, confirm the panel's marked maximum number of overcurrent devices is not already met, confirm a listed breaker is available for that panel, and confirm the working clearance and the bonding are sound. Acceptance: each of those four confirmed or failed by name. Wrong looks like a report that says yes on capacity while the panel is full of tandem breakers in an enclosure not marked for them. Stop rule: any one of the four failing changes the recommendation from "add a circuit" to "add a panel" or "replace the panel," and the report says which of the four drove it. Hazard: none at this step where the cover stays on; where the cover comes off to count, step 2's conditions apply again.

8. Give the customer two separate answers, in writing. Acceptance: a report stating service capacity as one answer and panel capacity as another, each with the number that governs it, plus the recording window and measurement point. Wrong looks like a single yes or no, which is what leads a customer to buy equipment on the strength of a service that has room and a panel that does not. Stop rule: no verbal answer is given on site before the arithmetic is checked, because the number the customer remembers is the one said out loud first. Hazard: none at this step, it is a report written at a desk.

The record this produces

One assessment per building: the proposed load list with nameplate data; the service and panel data from step 2 including the marked maximum number of overcurrent devices; the method chosen with its conditions confirmed; the recorder's measurement point, interval and exact window dates; maximum demand per phase with timestamps and the phase spread; the step 6 arithmetic line by line with the governing limit named; the four step 7 constraints each marked; and the two-part answer.

The customer keeps the report. The office keeps the recording, because every later job in that building starts from this number and re-recording a building the shop already measured is billable work nobody should have to sell twice. Re-record when the occupancy, the equipment or the operating hours change, since those are the conditions 220.87 was applied under.

Worked pass: 200 A, 208Y/120 V service, print shop adding a press

Step 1: one addition, a press with a nameplate of 40 A at 208 V three-phase, run continuously through a production shift.

Step 2: service rating 200 A, panel bus 200 A, main 200 A, service conductors 3/0 copper with 75 C terminations, and the panel label marks a maximum of 42 overcurrent devices.

Step 3: no utility year-data available, so the 30-day recording path under NEC 220.87 is chosen, and the section's conditions are confirmed in writing, including that the existing loads are staying as they are.

Step 4: transformers installed on the service conductors at the main with the utility's disconnect pulled, so the work is done de-energized. Demand interval 15 minutes, window 30 consecutive days across the shop's busiest month.

Step 5: maximum demand comes in at 96 A on phase B, 88 A on phase A and 91 A on phase C. The spread is 96 minus 88, which is 8 A, and 8 divided by 96 is 8.3 percent, inside the shop's 10 percent flag, so no balancing item is raised.

Step 6: existing load is 96 times 1.25, which is 120 A. The press is continuous, so it is taken at 40 times 1.25, which is 50 A. Total is 120 plus 50, or 170 A, against a 200 A service. Headroom is 30 A and the service sits at 85 percent of rating. The conductor check runs the same way: 3/0 copper at the 75 C column of NEC Table 310.16 carries 200 A, and 170 A is inside it, so the service rating and the conductors give the same governing limit of 200 A. The service passes.

Step 7 fails. The press needs a three-pole device, and there are 4 open positions. But counting devices in the panel gives 42 already installed, because 12 positions carry tandem breakers, and the label marks a maximum of 42. The panel is at its marked device limit, so no device may be added regardless of open positions. Step 7's stop rule fires and the recommendation changes.

Step 8: the report says the service has 30 A of headroom and can carry the press, and the panel cannot accept the device that press requires. Those are two answers, and the second one is the one the customer needs to act on. The options quoted are a subpanel fed from the existing panel, which does not add devices to the panel beyond one feeder breaker, or a panel replacement on the same service. The service upgrade the customer expected to be told they needed is not on the list, and the recording is what makes that defensible.

References

• NEC 220.87 for the measured-demand method and its conditions, NEC Article 210 and 215 for continuous load factors, NEC 110.14(C) and Table 310.16 for conductor ampacity at the terminal temperature rating, all in the edition your jurisdiction has adopted
• NFPA 70E-2021, Article 130, in the edition your employer's electrical safety program has adopted, for the energized work permit and the shock and arc flash boundaries at steps 4 and 5
• 29 CFR 1910.332 and 1910.399 for the qualified person opening an energized enclosure at step 2
• The panel's own marking for bus rating and maximum number of overcurrent devices, which governs step 7
• See related: Load Calculation Before a Service Upgrade SOP, Obsolete Panel Assessment and Replacement Recommendation SOP, Energized Work Permit Process SOP, Tenant Space Circuit Additions SOP