5.3 Feeder Load Calculations

Feeder Load Calculations

Quick Answer: NEC Article 220 Part III governs feeder load calculations — the process of summing connected loads and applying demand factors to determine the minimum feeder conductor and OCPD size. The general method adds general lighting + small appliance + laundry + appliance loads, then applies Table 220.45 demand factors for dwelling general lighting. Continuous loads must be added at 125% of their continuous portion per 215.2(A)(1) when sizing the conductor. The neutral feeder load is calculated per 220.61, where 220.61(B)(2) permits a 70% demand factor only on the portion of the unbalanced load above 200 A. For multifamily dwellings, Table 220.84 provides optional demand factors when the qualifying conditions are met.

NEC 220 Part III — Feeder Load Components

A feeder load calculation for a dwelling unit sums the following components:

Demand Factors — Table 220.45 (Dwelling General Lighting)

For dwelling unit general lighting loads, Table 220.45 applies the following demand factors:

Small-appliance and laundry loads are included in the general lighting load for demand factor purposes — they are added to the 3 VA/sqft lighting load BEFORE applying Table 220.45, per 220.52 and long-standing NEC interpretation.

Continuous Loads and 125%

NEC 215.2(A)(1) requires the feeder conductor to have an ampacity of at least 125% of the continuous load plus 100% of the noncontinuous load. A continuous load is defined in Article 100 as a load where the maximum current is expected to continue for 3 hours or more. Common continuous loads on a feeder include:

• Electric vehicle supply equipment (EVSE) — 625.41 rated continuous
• Motors — 430.22 requires 125% of the full-load current
• Commercial lighting and refrigeration
• Data processing equipment

The load calculation in Article 220 includes the continuous load at its nameplate; the 125% factor is applied when sizing the conductor and OCPD, not when summing the load.

Neutral Feeder Load — NEC 220.61

The feeder neutral carries only the unbalanced load — the difference between the line-to-neutral loads on each phase. NEC 220.61 permits reductions:

• 220.61(A) — for 3-wire, 120/240V single-phase dwelling feeders, the neutral may be sized for the unbalanced load (small appliance + laundry + lighting on one leg minus the other).
• 220.61(B)(1) — where the unbalanced load includes household electric ranges, wall-mounted ovens, counter-mounted cooking units, or electric dryers taken from Table 220.55 / Table 220.54, that range and dryer portion is counted at 70%.
• 220.61(B)(2) — a further 70% demand factor applies only to the portion of the unbalanced load that exceeds 200 amperes on a 3-wire DC or single-phase AC system, a 4-wire 3-phase system, a 3-wire 2-phase system, or a 5-wire 2-phase system. The first 200 A is always taken at 100%.
• 220.61(C) — no reduction is permitted for the portion consisting of nonlinear loads supplied from 4-wire, 3-phase wye systems (discharge lighting, VFDs, computers produce harmonics that add on the neutral).

Read 220.61(B)(2) carefully — it is one of the most commonly missed rules on the Calculations portion. The 70% is not applied to the whole neutral load and is never applied to a conductor's ampacity. The correct form is:

Neutral load ≤ 200 A          → 200 A at 100%
Neutral load portion > 200 A  → that excess × 70%
Neutral (A) = 200 + (unbalanced load − 200) × 0.70

A 260 A unbalanced load is therefore 200 + (60 × 0.70) = 242 A, not 260 × 0.70 = 182 A.

Feeder vs Service Load Calculation — Key Differences

Worked Example — 12-Unit Apartment Feeder

Problem: Compute the feeder load for a 12-unit apartment building. Each unit is 1,200 sqft and has a 10 kW electric range and a 5 kW electric clothes dryer. Use the Table 220.84 optional calculation for multifamily dwellings.

Step 1 — Verify Table 220.84 eligibility. Table 220.84 applies to multifamily dwellings when ALL four conditions are met: (a) 3 or more dwelling units — 12 units, satisfied; (b) every unit has electric cooking equipment — 10 kW range per unit, satisfied; (c) every unit has electric space heating and/or air conditioning — assume each unit has electric HVAC (problem implies all-electric units); (d) each unit is supplied by no more than one feeder — one feeder per unit, satisfied. The optional method is allowed.

Step 2 — Sum connected loads.

General lighting per unit: 3 VA × 1,200 sqft = 3,600 VA. Small appliance (2 circuits) + laundry (1 circuit) per unit: 1,500 × 3 = 4,500 VA. Range per unit (Table 220.55, Column C, 12 kW nameplate → use 12 kW basis; for 10 kW nameplate use 8 kW per Table 220.55 Note 1): 8,000 VA. Dryer per unit (Table 220.54, min 5,000 VA): 5,000 VA.

Per unit total = 3,600 + 4,500 + 8,000 + 5,000 = 21,100 VA.

12 units × 21,100 VA = 253,200 VA connected.

Step 3 — Apply Table 220.84 demand factor. For 12 dwelling units, Table 220.84 (11–13 unit bracket) gives a demand factor of 42%.

Calculated load = 253,200 × 0.42 = 106,344 VA.

Step 4 — Convert to current and size the feeder. At 240V single-phase (or 120/240V 3-wire for the building feeder from a 3-phase service with phase balancing, assume 240V):

I = 106,344 / 240 = 443.1 A.

Apply 215.2(A)(1) — assuming no continuous portion in this dwelling feeder calc, the conductor ampacity must be ≥ 443.1 A. From Table 310.16 (75°C copper): 500 kcmil THWN = 380 A (too small); 600 kcmil THWN = 420 A (still below 443.1 A, too small); 700 kcmil THWN = 460 A (exceeds 443.1 A ✓). A single-conductor 700 kcmil THWN copper per phase satisfies the calculated load. Alternatively, parallel 4/0 AWG THWN copper (2 × 230 A = 460 A) is a common field choice that also meets 443.1 A — use this when conductor handling is a concern. The feeder OCPD per 215.3 and 240.6(A): the next standard size at or above 443.1 A is 450 A. With 700 kcmil (460 A) the 450A OCPD is permitted directly (ampacity ≥ OCPD rating); no 240.4(B) next-size-up is needed.

Step 5 — Neutral sizing per 220.61. The neutral carries the unbalanced 120V load. For a multifamily dwelling feeder with mixed 120V and 240V loads, the unbalanced load is the 120V portion (lighting + small appliance + laundry + portion of range/dryer on 120V). Apply the demand factor to the load, never to the conductor's ampacity. Taking the whole 443.1 A as unbalanced (the conservative reading, since the range and dryer 120 V portions already sit in the total):

First 200 A            × 100% = 200.0 A
Excess (443.1 − 200)   × 70%  = 170.2 A
Neutral load                  = 370.2 A

From Table 310.16 (75 °C Cu), 500 kcmil THWN = 380 A covers the 370.2 A neutral. Use a 500 kcmil neutral. Note the common wrong answer: multiplying the entire load by 70% (443.1 × 0.70 = 310 A) understates the neutral by 60 A and would land you on an undersized 400 kcmil conductor.

Answer: 12-unit apartment feeder = 106,344 VA calculated load, ~443.1 A, with 700 kcmil THWN copper ungrounded conductors (or parallel 4/0 AWG THWN Cu, 2 × 230 A = 460 A), a 500 kcmil neutral, and a 450A OCPD. (Note: a 3-phase 208Y/120V service would split the units across phases; the per-phase current would differ — the exam typically uses single-phase or asks for the total VA, not the per-phase current.)

Common Exam Traps

• Forgetting to apply Table 220.45 demand to the SUM of general lighting + small appliance + laundry, not to lighting alone.
• Applying Table 220.84 when the eligibility conditions are not met — all four conditions (3+ units; electric cooking in every unit; electric space heating and/or A/C in every unit; no more than one feeder per unit) must be satisfied. Using the optional method for a building without electric cooking, or for a 2-unit building, is a Code error.
• Counting the neutral as current-carrying for 310.15(C)(1) when it qualifies for Exception No. 1 — a 3-wire single-phase dwelling feeder with balanced 120V loads does not count the neutral.
• Using 100% of the range nameplate instead of Table 220.55 demand (a 12 kW range is taken at 8 kW for one unit, not 12 kW).
• Forgetting the 5,000 VA minimum dryer load — Table 220.54 sets 5,000 VA as the minimum even if the nameplate is lower.

Feeder Calculation Checklist

1. General lighting = 3 VA × sqft (220.41).
2. Add small appliance (1,500 × 2) + laundry (1,500 × 1) per 220.52.
3. Apply Table 220.45 demand factors to the lighting + small appliance + laundry sum.
4. Add appliance loads (range per Table 220.55, dryer per Table 220.54, water heater, fixed appliances per 220.53).
5. Add continuous loads at nameplate; apply 125% to continuous portion when sizing the conductor.
6. Sum the VA; divide by voltage to get current.
7. Apply 310.15(B) and 310.15(C)(1) corrections; select conductor from Table 310.16.
8. Size neutral per 220.61 (70% reduction for 200A+ dwelling feeders when conditions allow).

Takeaways

• NEC 220 Part III is the feeder calculation backbone; demand factors in Table 220.45 apply to the lighting + small appliance + laundry sum.
• Continuous loads use 125% for conductor sizing (215.2(A)(1)), but nameplate for the load sum.
• Table 220.84 optional multifamily demand factor can drastically reduce a 12-unit calculated load (42% demand for 12 units) when all four eligibility conditions are met.
• Per 220.61(B)(2) the neutral load is 200 A at 100% plus 70% of the portion above 200 A — not 70% of the whole load, and never 70% of a conductor's ampacity. No reduction applies to nonlinear load on a 4-wire wye system per 220.61(C).