5.3 Feeder Load Calculations

Feeder Load Calculations

Quick Answer: NEC Article 120, standard method 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 120.45 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 120.61, where 120.61(B)(2) permits a 70% demand factor only on the portion of the unbalanced load above 200 A. For multifamily dwellings, Table 120.84 provides optional demand factors when the qualifying conditions are met.

NEC Article 120 — Feeder Load Components

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

Demand Factors — Table 120.45 (Dwelling General Lighting)

For dwelling unit general lighting loads, Table 120.45 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 2 VA/sqft lighting load BEFORE applying Table 120.45, per 120.52 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 120 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 120.61

The feeder neutral carries only the unbalanced load — the difference between the line-to-neutral loads on each phase. NEC 120.61 permits reductions:

• 120.61(A) — for 3-wire, 120/240V single-phase dwelling feeders, the neutral may be sized for the maximum load between the neutral and any one ungrounded conductor; use the load schedule rather than assuming perfect balance.
• 120.61(B)(1) — where the unbalanced load includes household electric ranges, wall-mounted ovens, counter-mounted cooking units, or electric dryers taken from Table 120.55 / Table 120.54, that range and dryer portion is counted at 70%.
• 120.61(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%.
• 120.61(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 120.61(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          → actual load 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 — Single-Dwelling Feeder, Standard Method

Problem: A feeder supplies the entire load of a 2,000 ft² dwelling at 120/240 V single phase. The schedule gives two small-appliance circuits, one laundry circuit, a 10 kW range, a 5.5 kW dryer, four qualifying fixed appliances totaling 9 kVA, and a 12 kW electric furnace. There is no air-conditioning, EVSE or other load in this exercise.

1. General load: 2,000 × 2 + 2 × 1,500 + 1,500 = 8,500 VA.
2. Table 120.45: 3,000 + (8,500 − 3,000) × 0.35 = 4,925 VA.
3. Range: Table 120.55 gives 8,000 VA for one qualifying range not over 12 kW.
4. Dryer: The greater of 5,000 VA and its 5,500 VA nameplate is 5,500 VA.
5. Qualifying appliances: 9,000 × 0.75 = 6,750 VA under 120.53. Do not include the range, dryer, space heating or EVSE in that appliance group.
6. Heating: Add the stated 12,000 VA. Apply noncoincident-load treatment only when operation is actually noncoincident.
7. Load total: 4,925 + 8,000 + 5,500 + 6,750 + 12,000 = 37,175 VA; divide by 240 V to obtain 154.9 A.

This is a load calculation. Next check feeder conductor and overcurrent requirements, including the treatment of continuous loads, terminations, correction/adjustment factors and whether the dwelling allowance in 310.12 applies. Under 120.5(E) the 125% continuous-load sizing rule is not an extra blanket multiplier inside the Article 120 load sum.

Why Optional Multifamily Calculations Need Their Own Worksheet

A calculation under 120.84 must use that method's specified connected loads and conditions. Do not first reduce each apartment's range to the single-range demand from Table 120.55, then multiply the reduced total by the multifamily factor: that mixes two different methods and applies diversity twice. Include the prescribed heating/cooling and other loads; eligibility alone does not make an omitted load disappear.

Read the current 120.84 text and table for the actual installation. Keep common-area loads separately as required. A building's voltage and phase arrangement must also be stated: dividing total three-phase VA by 240 V as though it were a single-phase feeder is invalid.

Common Exam Traps

• Forgetting to apply Table 120.45 demand to the SUM of general lighting + small appliance + laundry, not to lighting alone.
• Applying Table 120.84 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.
• Mixing methods — standard range demand and optional multifamily demand are not cumulative discounts.
• Forgetting the 5,000 VA minimum dryer load — Table 120.54 sets 5,000 VA as the minimum even if the nameplate is lower.

Feeder Calculation Checklist

1. General lighting = 2 VA × sqft (120.41).
2. Add small appliance (1,500 × 2) + laundry (1,500 × 1) per 120.52.
3. Apply Table 120.45 demand factors to the lighting + small appliance + laundry sum.
4. Add appliance loads (range per Table 120.55, dryer per Table 120.54, water heater, fixed appliances per 120.53).
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 120.61 (70% reduction for 200A+ dwelling feeders when conditions allow).

Takeaways

• NEC Article 120 is the feeder calculation backbone; demand factors in Table 120.45 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 120.84 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 120.61(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 120.61(C).