3.3 Service Calculations, Clearances & Working Space (depth)

Service and Feeder Load Calculations — Article 220

The 2023 renumbering matters. Article 220 was reorganized for the 2023 NEC. The unit-load table that used to be Table 220.12 is now Table 220.42(A), General Lighting Loads by Occupancy, and the demand-factor table that used to be Table 220.42 is now Table 220.45, Lighting Load Demand Factors. Dwelling-unit general lighting moved to 220.41, and show-window and track lighting to 220.46. If your jurisdiction still tests the 2020 NEC, the rules are the same but the numbers are the old ones.

The standard method computes the service or feeder load by summing the following:

• General lighting — area (ft²) × the unit load for the occupancy from Table 220.42(A). Representative values: stores 3 VA/ft², office buildings and banks 3.5 VA/ft², hospitals 2 VA/ft², warehouses (storage) 0.25 VA/ft². Watch the 2023 change: schools and sports arenas dropped from 3 to 1.5 VA/ft². Floor area is taken from the outside dimensions.
• Lighting demand factors — Table 220.45. This is where most candidates lose points. The table gives a reduction to only four occupancy types: dwelling units (first 3000 VA at 100%, 3001–120,000 VA at 35%, over 120,000 VA at 25%), hospitals, hotels and motels without cooking provisions, and warehouses (storage). Every other non-dwelling occupancy — stores, offices, restaurants, schools, assembly — falls under "All others," which is 100%: no reduction at all. Applying the dwelling 3000/35% ladder to a retail store is the single most common arithmetic error on a commercial load calculation. The table also does not apply to areas of hospitals, hotels, and motels where the entire lighting is likely to be in use at one time, such as operating rooms, ballrooms, and dining rooms.
• Receptacle loads — 220.14(I) assigns 180 VA per receptacle strap (yoke) in non-dwelling occupancies. 220.44 then permits a demand: first 10 kVA at 100%, remainder at 50%.
• Continuous loads — loads expected to operate at maximum current for 3 hours or more (store and office lighting, signage, refrigeration, data equipment). Per 215.2/215.3 and 230.42(A)(1), conductors and overcurrent devices are sized at 125% of the continuous load plus 100% of the non-continuous load.
• Existing installations — 220.87. This is not a flat percentage of connected load. 220.87 lets you calculate a feeder or service load for an existing installation from the actual recorded maximum demand, provided (1) maximum demand data is available for a 1-year period (or, by the exception, a 15-minute-interval demand recorded continuously over a minimum 30-day period), (2) the maximum demand at 125% plus the new load does not exceed the feeder ampacity or the service rating, and (3) the feeder or service has the required overcurrent protection. Remember it as 125% of measured demand, not 35% of connected load.

The 125% Continuous-Load Rule (215.3, 230.42(A)(1))

For both feeders and service-entrance conductors, the ampacity shall not be less than the non-continuous load plus 125% of the continuous load. The OCPD is similarly rated at 125% of the continuous load (unless the assembly is listed for 100% continuous duty, per 215.3 Exception and 210.20(A) Exception).

Example: A feeder carries 100 A continuous and 80 A non-continuous. Minimum conductor ampacity = (100 × 1.25) + 80 = 205 A. Select a 225 A overcurrent device (next standard size, 240.6(A)) and a conductor with at least 205 A ampacity — 4/0 Cu at 75 °C is 230 A, which works.

Minimum Service-Disconnect Rating — 230.79

Beyond the calculation, 230.79 floors the service disconnect at:

• 30 A for ≤two 2-wire branch circuits
• 100 A, 3-wire for one-family dwellings
• 60 A for all other installations

A calculated 45 A load in a small office cannot use a 30 A disconnect; it must be at least 60 A per 230.79(D).

Working Space Clearances — 110.26(A)(1)

The inspector verifies minimum clearances around all electrical equipment likely to be examined, adjusted, serviced, or maintained while energized. 110.26(A)(1) Table gives the working-space depth by voltage-to-ground and "Condition":

The NEC numbers these Condition 1, 2, and 3 — not a, b, c. Condition 1 applies where the opposite side has no live or grounded parts (an open wood-frame wall, for example). Condition 2 applies where grounded parts face the equipment, and concrete, brick, tile, and similar masonry walls are considered grounded for this purpose — which is why a panel on a block wall in a 480 V room needs 3½ ft, not 3 ft. Condition 3 requires the deepest clearance because live parts are exposed on both sides and a worker could be between them.

Working-Space Width and Headroom — 110.26(A)(2), 110.26(A)(3)

• Width (110.26(A)(2)): minimum 30 in. (762 mm) wide OR the width of the equipment, whichever is greater. The space shall permit at least 90° opening of equipment doors or hinged panels.
• Headroom (110.26(A)(3)): the work space must be clear and extend from grade, floor, or platform to a height of 6½ ft (2.0 m, about 78 in.) or the height of the equipment, whichever is greater.

Dedicated Equipment Space — 110.26(E)

For indoor installations, 110.26(E)(1) reserves a dedicated space equal to the footprint of the equipment, extending from the floor to a height of 6 ft (1.83 m) above the equipment, or to the structural ceiling, whichever is lower. The space is reserved for electrical equipment only — no foreign systems (HVAC, plumbing, ducts) may occupy it. Sprinkler protection is permitted, but the clearance to the sprinkler deflector still applies.

Service-Conductor Clearances Recap

• 230.24(A) 8 ft above roofs (with exceptions); 230.24(B) 10 / 12 / 15 / 18 / 24.5 ft to grade depending on what is underneath and, in the 12 vs. 15 ft rows, on whether the voltage exceeds 300 V to ground
• 230.26 point of attachment not less than 10 ft above finished grade
• 230.28 service-mast strength and fittings identified for service-mast use
• 230.54 service head above the point of attachment, with drip loops arranged so water cannot enter the raceway

Worked Service-Load Calculation

A 4000 ft² retail store is supplied at 120/240 V, single phase. The store lighting and the exterior sign both run more than 3 hours, so both are continuous.

1. General lighting — Table 220.42(A), stores = 3 VA/ft²: 4000 × 3 = 12,000 VA. No lighting demand factor applies. Table 220.45 gives a reduction only to dwelling units, hospitals, hotels/motels, and warehouses; a store falls in "All others" at 100%. This is the step where candidates wrongly borrow the dwelling 3000-VA/35% ladder and undersize the service by nearly half.
2. Receptacles — 220.14(I), 180 VA per strap: 20 straps × 180 = 3600 VA. 220.44 demand: the first 10 kVA at 100% covers all of it → 3600 VA, non-continuous.
3. Exterior sign — 2000 VA, continuous. (Note that 600.5(A) separately requires at least one 20 A branch circuit dedicated to the sign, and 220.14(F) requires that circuit to be computed at a minimum of 1200 VA; here the actual 2000 VA load is larger and governs.)
4. HVAC — 12,000 VA, non-continuous.

Now apply the 125% continuous rule of 230.42(A)(1):

1. Service current = 33,100 / 240 = 137.9 A
2. Service overcurrent device — next standard size at or above 137.9 A per 240.6(A) = 150 A
3. Service-entrance conductors — ampacity at least 137.9 A. 1/0 AWG Cu at 75 °C = 150 A, which satisfies both the calculated load and the 150 A device.

The inspector then verifies that the 230.79(D) 60 A floor is met, that the conductor and device satisfy 230.42(A)(1), and that 110.26 working space is provided around the equipment. Note the actual connected demand is only 29,600 VA (123 A) — the 125% adder on the continuous portion is what drives the service from 125 A to 150 A.