3.3 Feeder Sizing & General Demand Factors

Feeder Sizing & General Demand Factors

A feeder consists of all circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device (NEC Article 100). Sizing feeder conductors and associated overcurrent protective devices (OCPD) requires applying the continuous duty rules of Article 215 and the demand factors of Article 220.

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Continuous and Noncontinuous Feeder Sizing (NEC 215.2)

According to NEC Article 100, a continuous load is a load where the maximum current is expected to continue for 3 hours or more. Office lighting, store display lighting, and commercial parking lot luminaires are common continuous loads. Residential domestic lighting and general appliance loads are typically noncontinuous.

Conductor Ampacity Sizing Formula

Under NEC 215.2(A)(1), feeder conductors must have an allowable ampacity not less than the sum of the noncontinuous load plus 125% of the continuous load:

$\text{Minimum Feeder Ampacity} = (1.25 \times \text{Continuous Load}) + (1.00 \times \text{Noncontinuous Load})$

If the feeder overcurrent device assembly and enclosure are specifically listed for operation at 100% of their rating, the conductor ampacity is permitted to equal simply $100\% \times (\text{Continuous Load} + \text{Noncontinuous Load})$. Standard commercial circuit breakers are rated for 80% continuous operation, so the 125% multiplier applies to virtually all standard installations.

Overcurrent Protection Sizing (NEC 215.3)

Feeders must be protected against overcurrent in accordance with Article 240. The overcurrent protective device rating must not be smaller than $(1.25 \times \text{Continuous Load}) + \text{Noncontinuous Load}$. Furthermore, under NEC 240.4(B), if the calculated ampacity does not correspond to a standard fuse or breaker size listed in NEC 240.6, the next higher standard overcurrent device rating is permitted, provided the rating does not exceed 800 amperes and the conductors are not part of a multi-outlet branch circuit.

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Feeder Neutral Sizing & Reductions (NEC 220.61)

The feeder neutral conductor carries return current from line-to-neutral loads. In many distribution systems, the neutral conductor does not need to be as large as the ungrounded phase conductors because phase currents cancel out in the neutral.

Maximum Unbalanced Load

Under NEC 220.61(A), the feeder neutral load is calculated based on the maximum unbalanced load between the neutral conductor and any one ungrounded conductor. Phase-to-phase 240V loads (such as electric water heaters, central air conditioners, and baseboard heaters without 120V controls) do not connect to the neutral and contribute zero amperes to the neutral calculation.

Permitted Neutral Reductions (NEC 220.61(B))

Once the maximum unbalanced load is determined, two specific demand factor reductions are permitted on the neutral conductor:

1. Cooking Equipment & Clothes Dryers: A 70% demand factor (0.70) is permitted on the calculated feeder demand for household electric ranges, wall-mounted ovens, counter-mounted cooking units, and electric clothes dryers.
2. Loads Exceeding 200 Amperes: For 3-wire single-phase systems or 4-wire, 3-phase wye systems, that portion of the maximum unbalanced load in excess of 200 amperes is permitted a demand factor of 70%:

$\text{Calculated Neutral Load} = 200\text{ A} + [0.70 \times (\text{Unbalanced Load} - 200\text{ A})]$

Prohibited Neutral Reductions — Harmonic Non-Linear Loads (NEC 220.61(C))

Exam Trap — Harmonic Distortion: Under 220.61(C), no reduction is permitted for the portion of the neutral load that consists of nonlinear loads supplied from a 4-wire, wye-connected, 3-phase system. No reduction is permitted either for a 3-wire circuit made of two ungrounded conductors and the neutral of a 4-wire wye system. Non-linear loads—including electronic data processing equipment, computers, variable frequency drives (VFDs), and solid-state LED or fluorescent lighting drivers—introduce triplen harmonics (3rd, 9th, 15th harmonics) into the system.

In a 3-phase, 4-wire wye system, triplen harmonic currents do not cancel each other out in the neutral; they add arithmetically in the neutral conductor. Consequently, neutral current can actually exceed phase current ($I_{\text{neutral}} > I_{\text{phase}}$), leading to dangerous conductor overheating. Sizing the neutral smaller than phase conductors is a code violation where non-linear loads predominate, and the neutral must be counted as a current-carrying conductor for ampacity derating per NEC 310.15(E)(3).

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General Lighting Demand Factors (NEC Table 220.45)

General lighting load is the floor area (220.5(C)) times the unit load: $3\text{ VA/ft}^2$ for dwelling units (220.41), or the Table 220.42(A) value for other occupancies. The 2023 edition renumbered Article 220:

• the demand-factor table formerly numbered Table 220.42 is now Table 220.45; and
• non-dwelling receptacle demand factors moved from 220.44 to 220.47 (first 10 kVA at 100%, the remainder at 50%).

Because not all luminaires are on at once, Table 220.45 lets you apply demand factors to the calculated general lighting load.

Summary of Table 220.45 Demand Factors

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Electric Clothes Dryers Demand Factors (NEC Table 220.54)

The feeder load for household electric clothes dryers must be calculated at 5,000 watts (volt-amperes) or the nameplate rating, whichever is greater (NEC 220.54).

• 1 to 4 Dryers: A $100\%$ demand factor applies.
• 5 Dryers: $85\%$
• 6 Dryers: $75\%$
• 7 Dryers: $65\%$
• 8 Dryers: $60\%$
• 9 Dryers: $55\%$
• 10 Dryers: $50\%$
• 11 to 23 Dryers: Demand factor decreases by $1\%$ per dryer (e.g., 11 dryers = $47\%$, 23 dryers = $35\%$).
• 24 to 42 Dryers: $35\%$ minus $0.5\%$ for each dryer over 23 (42 dryers = $25.5\%$)
• 43 and over: $25\%$

Example: Four identical 5,000 VA dryers have a calculated demand of $4 \times 5{,}000\text{ VA} \times 1.00 = 20{,}000\text{ VA}$. Five identical 5,000 VA dryers have a calculated demand of $5 \times 5{,}000\text{ VA} \times 0.85 = 21{,}250\text{ VA}$.

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Household Electric Ranges & Cooking Appliances (NEC Table 220.55)

Table 220.55 provides demand loads for household electric ranges, wall-mounted ovens, and counter-mounted cooking units. Column C is used for single ranges rated over $8.75\text{ kW}$ through $12\text{ kW}$:

• 1 Range: $8\text{ kW}$ (8,000 VA)
• 2 Ranges: $11\text{ kW}$ (11,000 VA)
• 3 Ranges: $14\text{ kW}$ (14,000 VA)
• 4 Ranges: $17\text{ kW}$ (17,000 VA)
• 5 Ranges: $20\text{ kW}$ (20,000 VA)

Ranges Exceeding 12 kW (Table 220.55 Note 1)

For ranges rated over $12\text{ kW}$ through $27\text{ kW}$, the maximum demand in Column C must be increased by 5% for each additional kilowatt of rating or major fraction thereof by which the rating exceeds 12 kW.

Worked Example: Calculate the feeder demand for one 14 kW household range.

1. Find excess kW over 12 kW: $14\text{ kW} - 12\text{ kW} = 2\text{ kW}$.
2. Calculate percentage increase: $2\text{ kW} \times 5\% = 10\%$.
3. Base Column C demand for 1 range: $8\text{ kW}$.
4. Increase base demand: $8\text{ kW} \times (1 + 0.10) = 8\text{ kW} \times 1.10 = 8.8\text{ kW}$ (8,800 VA).

If the range rating includes a major fraction (over $0.5\text{ kW}$), round up to the next whole kilowatt. A $13.6\text{ kW}$ range is treated as exceeding 12 kW by 2 kW (10% increase).

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Worked Feeder Sizing Example

Problem: A commercial building feeder supplies a 208Y/120V, 3-phase, 4-wire subpanel with the following loads:

• Continuous commercial lighting load: $36{,}000\text{ VA}$
• Noncontinuous receptacle load, already reduced by Table 220.47: $48{,}000\text{ VA}$
• Noncontinuous fixed-appliance load (no motors): $15{,}000\text{ VA}$

Step 1: Calculate Minimum Feeder Conductor Ampacity

Apply the $125\%$ continuous load multiplier per NEC 215.2(A)(1):

$\text{Continuous Portion} = 36{,}000\text{ VA} \times 1.25 = 45{,}000\text{ VA}$ $\text{Noncontinuous Portion} = 48{,}000\text{ VA} + 15{,}000\text{ VA} = 63{,}000\text{ VA}$ $\text{Total Minimum Feeder Load} = 45{,}000\text{ VA} + 63{,}000\text{ VA} = 108{,}000\text{ VA}$

Step 2: Calculate Feeder Phase Current

For a three-phase system, current is calculated using the formula $I = \frac{\text{Total VA}}{\sqrt{3} \times V_{\text{L-L}}}$:

$I = \frac{108{,}000\text{ VA}}{\sqrt{3} \times 208\text{ V}} = \frac{108{,}000\text{ VA}}{360.26} = 299.78\text{ A}$

Step 3: Conductor & Overcurrent Selection

• Minimum Conductor Ampacity: $299.78\text{ A}$.
• Using NEC Table 310.16 (75°C terminal rating, THHN/THWN-2 Copper):
  • 300 kcmil Copper has an ampacity of $285\text{ A}$ (insufficient).
  • 350 kcmil Copper has an allowable ampacity of 310 A (adequate).
• Overcurrent Protection: Under 215.3 the device must be rated at least 299.78 A, so 300 A is the smallest standard rating that qualifies (240.6(A)). Because 310 A is not a standard rating, 240.4(B) would also let these conductors be protected at the next higher standard rating, 350 A.