SFB-4 — Feeder Sizing and Installation

Confirm that the conductors are a feeder

Article 100 defines a feeder as all circuit conductors between service equipment, the source of a separately derived system, or another power-supply source and the final branch-circuit overcurrent device. Conductors from the final overcurrent device to outlets are branch-circuit conductors. Conductors before the service disconnect are service conductors. Name the segment correctly before applying Article 215.

Begin with the feeder load calculated under Article 220. Record voltage, phase, continuous and noncontinuous portions, motors, neutral load, and any permitted demand or noncoincident treatment. The panelboard bus rating is not the feeder load, and an oversized panel does not require an equally oversized feeder unless the calculated load and protection call for it.

Apply continuous-load rules

Under 2017 NEC 215.2(A)(1), feeder conductor ampacity generally must be at least the noncontinuous load plus 125 percent of the continuous load. The section contains an allowance where both the feeder assembly and overcurrent device are listed for operation at 100 percent of rating. Do not assume ordinary equipment has that listing. Section 215.3 applies corresponding continuous-load treatment to feeder overcurrent protection, subject to its exception.

Example: assume a 240 V single-phase feeder supplies 80 A noncontinuous plus 32 A continuous. Assume copper THWN conductors, 75°C-rated equipment terminals, no ambient correction, no adjustment for conductor count, no dwelling allowance, no motor adder, and no 100-percent-rated equipment.

Minimum ampacity is 80 A + (32 A × 1.25) = 120 A. In the 75°C copper column of 2017 Table 310.15(B)(16), 2 AWG copper is 115 A and is too small; 1 AWG copper is 130 A and is the minimum listed table size under these assumptions. The feeder overcurrent device must cover 120 A; 125 A is a standard rating in 240.6(A), and 1 AWG copper has sufficient 75°C ampacity for a 125 A device. The result changes if adjustment, correction, terminal, load, or listing assumptions change.

Select the correct ampacity

Table lookup follows a sequence:

1. Identify copper or aluminum and the conductor insulation.
2. Determine the terminal-temperature limitation under 110.14(C), not merely the insulation's printed rating.
3. Use the correct 2017 ampacity table and column.
4. Apply ambient-temperature correction under Table 310.15(B)(2)(a) and current-carrying-conductor adjustment under Table 310.15(B)(3)(a), where required.
5. Confirm the adjusted ampacity still carries the calculated load and does not exceed terminal limitations.
6. Coordinate overcurrent protection under 240.4 and the rules for the load or equipment.

For equipment rated over 100 A, or for conductors larger than 1 AWG, 110.14(C)(1)(b) generally uses the 75°C termination provisions. For equipment rated 100 A or less or marked for 14 AWG through 1 AWG, the ordinary starting point is 60°C unless equipment is listed and identified for 75°C conductors. A 90°C insulation value can often serve as the starting ampacity for adjustment or correction, but the final allowable ampacity cannot violate the terminal rule.

The next-higher standard overcurrent rule in 240.4(B) is not automatic. It applies only where conductor ampacity falls between standard device ratings, the next rating does not exceed 800 A, and the conductors are not part of a branch circuit supplying more than one receptacle for cord-and-plug loads. Equipment-specific rules can prohibit its use.

Size the neutral deliberately

Calculate the feeder neutral load from the maximum unbalance determined under 220.61. Section 215.2(A)(2) also sets a minimum grounded-conductor size by reference to 250.122, subject to the subsection's limits. Do not reduce the neutral simply because the phase conductors are equal. Nonlinear loads on a 4-wire, 3-phase wye system can produce additive harmonic current, and 310.15(B)(5) determines whether the neutral counts as a current-carrying conductor for adjustment. Grounded-conductor identification follows 200.6; it is not an equipment grounding conductor.

Treat taps as conditioned exceptions

A feeder tap is a conductor supplied from a feeder overcurrent device with a rating greater than would normally protect that tap conductor. Section 240.21(B) permits feeder taps only under detailed conditions involving length, ampacity, physical protection, routing, termination, and the rating of the supplied device or conductors. “Ten-foot tap” or “25-foot tap” is a label for a rule set, not permission to omit protection. Verify every condition from the 2017 text.

Finish the installation checks

Feeder ungrounded conductors need system identification under 215.12(C) where more than one nominal voltage system exists. Grounded and equipment grounding conductors follow 200.6 and 250.119. Raceway or cable use, fill, support, wet-location rating, and physical protection remain applicable.

The voltage-drop material associated with 215.2 is an Informational Note and supports efficient design; it is not a blanket enforceable percentage by itself. Calculate voltage drop when asked, but do not mislabel guidance as mandatory text.

A feeder to a separate building or structure triggers Article 225 supply and disconnect rules and 250.32 grounding and bonding rules. A grounded conductor is normally isolated from equipment grounding conductors downstream of the service or separately derived bonding point. Feeder sizing is complete only after load, ampacity, terminals, protection, neutral, installation method, and destination rules agree.

Feeder calculation audit

In the worked example, connected current is 80 A + 32 A = 112 A, while the sizing basis is 120 A because the continuous portion contributes an additional 25 percent. At 240 V single phase, those values correspond to 26,880 VA connected and a 28,800 VA sizing basis. This comparison explains the increase and provides a unit-based check without pretending the load itself changed.