4.2 Branch Circuit & Feeder Calculations (0302)

NEC Article 220 — Load Calculations

Article 220 establishes the methods for computing branch-circuit and feeder loads. The inspector verifies that the installed conductors, overcurrent devices, and equipment are sized to carry the calculated load. Three concepts drive nearly every calculation: unit loads (Table 220.42(A) for non-dwelling occupancies, 220.41 for dwelling units), demand factors (Table 220.45 for lighting, 220.44 for receptacles, 220.51–220.56 for specific loads), and the 125% continuous load rule (215.3, 210.20(A)).

2023 renumbering: what the 2020 NEC called Table 220.12 (unit loads) is now Table 220.42(A), and what it called Table 220.42 (lighting demand factors) is now Table 220.45. The requirements did not change; the numbers did.

General Lighting Load (220.41, 220.42, 220.45)

Table 220.42(A) gives unit loads in volt-amperes per square foot for non-dwelling occupancies; 220.41 covers dwelling units at 3 VA/ft² for general lighting and general-use receptacles combined. Representative unit loads:

The general lighting load = unit load × floor area, using outside dimensions. Table 220.45 then permits a demand reduction, but only for four occupancy families:

Stores, offices, restaurants, schools, and assembly occupancies fall in "All others" and get no reduction at all. Note also that the hospital and hotel/motel factors do not apply to areas where the entire lighting is likely to be used at one time — operating rooms, ballrooms, dining rooms — which are calculated separately at 100%.

Receptacle Loads (220.14, 220.44)

220.14(I) sets non-dwelling general-purpose receptacle loads at 180 VA per strap (yoke); a single yoke carrying one, two, or three receptacles counts as one 180 VA unit, and a multi-gang yoke of four or more receptacles counts as 90 VA per receptacle. Dwelling-unit receptacles are already inside the 220.41 general lighting figure, so they are not counted separately. 220.44 then applies a demand to the non-dwelling receptacle load: 100% of the first 10,000 VA and 50% of the remainder. Two related rules the exam likes: 220.46 requires show-window lighting to be calculated at 200 VA per linear foot of show window, and track lighting outside dwelling units and hotel/motel guest rooms at 150 VA per 2 ft of track.

Specific Loads (220.51, 220.54, 220.55)

• 220.51 Fixed electric space heating: calculated at 100% of the connected load with no demand factor.
• 220.54 Electric clothes dryers (dwelling): 5,000 VA each, or the nameplate if larger, with demand factors per Table 220.54.
• 220.55 Electric ranges and ovens (dwelling): demand factors in Table 220.55 based on the number of appliances served.

Motor Loads (220.50, 430.24)

Motor loads are calculated per Article 430. 430.24 requires a feeder supplying two or more motors plus other loads to have an ampacity of not less than 125% of the largest motor full-load current plus the sum of the other motor full-load currents plus the non-motor load. 220.50 points motor load calculations at 430.22, 430.24, and 440.6. The full-load current used is the table value from Table 430.248 (single-phase) or Table 430.250 (three-phase), not the motor nameplate — see Section 6.3.

The 125% Continuous Load Rule (215.3, 210.20(A))

Both 215.3 (feeder OCPD) and 210.20(A) (branch-circuit OCPD) require the overcurrent device to be rated at not less than 125% of the continuous load plus 100% of the noncontinuous load. The same 125% factor applies to conductor sizing (215.2, 210.19(A)). A load is treated as continuous when the maximum current is expected to continue for 3 hours or more.

Neutral Load (220.61)

220.61 permits the grounded (neutral) conductor of a 3-wire or multiwire circuit to be sized for the maximum unbalanced load. For feeders with largely balanced 3-phase loads, the neutral may be reduced in limited cases, but it must never be smaller than the EGC required for fault clearing and must carry the maximum unbalanced current.

Worked Example — Office Building Feeder (4,000 sq ft)

A single-tenant office building of 4,000 sq ft is served by a 208Y/120 V, 3-phase feeder. Calculate the minimum feeder load and size the conductor and OCPD.

Step 1 — General lighting (Table 220.42(A)): office buildings are 3.5 VA/ft², not 3. Lighting load = 4,000 × 3.5 = 14,000 VA. Office buildings fall under "All others" in Table 220.45, so the demand factor is 100%: lighting = 14,000 VA. Office lighting runs more than 3 hours, so it is a continuous load.

Step 2 — Receptacles (220.14(I), 220.44): 80 general-purpose receptacles x 180 VA = 14,400 VA. Apply 220.44 demand: 100% of the first 10,000 VA = 10,000 VA; 50% of the remaining 4,400 VA = 2,200 VA. Receptacle load = 12,200 VA.

Step 3 — Other continuous load: constant-on equipment load of 6,000 VA, treated as continuous.

Step 4 — Other noncontinuous load: Miscellaneous equipment = 4,000 VA (noncontinuous).

Step 5 — Feeder load (215.2, 215.3):

• Continuous = 14,000 (lighting) + 6,000 (equipment) = 20,000 VA → × 1.25 = 25,000 VA
• Non-continuous = 12,200 (receptacles) + 4,000 (misc) = 16,200 VA
• Total feeder load for sizing = 25,000 + 16,200 = 41,200 VA

Step 6 — Conductor ampacity: at 208Y/120 V, three phase, I = VA ÷ (√3 × V) = 41,200 ÷ (1.732 × 208) = 41,200 ÷ 360.3 = 114.4 A. Using the 75°C column of Table 310.16, 2 AWG copper has an ampacity of 115 A, which covers 114.4 A. (1 AWG at 130 A is the common design choice for headroom.)

Step 7 — Overcurrent device: 215.3 requires the device to be rated at least 114.4 A. The standard sizes in 240.6(A) bracketing that value are 110 A and 125 A, so the device is 125 A. Because 115 A is not itself a standard rating, 240.4(B) permits the next higher standard device — 125 A — to protect the 2 AWG conductor, since the device does not exceed 800 A and 2 AWG is not a small conductor under 240.4(D).

Key point: the 125% continuous factor is built into the 41,200 VA total before the ampacity is computed. Also note how much the Step 1 unit load matters: using 3 VA/ft² instead of the correct 3.5 VA/ft² for an office would have understated the feeder by 2,500 VA after the 125% adder and produced a 100 A answer instead of 125 A.