12.3 Feeders: Sizing, Overcurrent Protection & Article 215 Requirements
Key Takeaways
- A feeder is defined in Article 100 as all circuit conductors between the service equipment, the source of a separately derived system, or another power supply source and the final branch-circuit overcurrent device.
- NEC 215.2(A)(1) requires feeder conductors to have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load, before applying any adjustment or correction factors.
- NEC 215.3 requires the feeder overcurrent device to have a rating not less than the noncontinuous load plus 125 percent of the continuous load.
- NEC 215.10 requires ground-fault protection of equipment on feeder disconnects rated 1,000 amperes or more on solidly grounded wye systems of more than 150 volts to ground but not exceeding 1,000 volts phase-to-phase, unless that protection is already provided upstream at the service.
- NEC 215.12 requires feeder conductors to be identified by phase or line and system where a premises has feeders supplied from more than one nominal voltage system, using color coding, tagging, or another approved means documented at each feeder distribution equipment location.
12.3 Feeders: Sizing, Overcurrent Protection & Article 215 Requirements
Exam Focus: The Article 100 feeder definition, 215.2 conductor sizing, 215.3 overcurrent device sizing, feeder EGC and grounded conductor sizing, 215.10 GFPE, and 215.12 identification.
Feeders are the middle tier of the distribution system, and the PSI subject list names them separately from services and branch circuits. Article 215 is short, which makes it a reliable source of quick points if you know where its five or six substantive rules live.
What Makes a Conductor a Feeder
Article 100 defines a feeder as all circuit conductors between:
- the service equipment,
- the source of a separately derived system, or
- another power supply source (a generator, a photovoltaic system, an energy storage system),
and the final branch-circuit overcurrent device.
+-------------------------------------------------------------------------+
| IDENTIFYING THE THREE CONDUCTOR TIERS |
| |
| Utility --> [service point] --> SERVICE CONDUCTORS |
| | |
| [ SERVICE DISCONNECT ] |
| | |
| FEEDER CONDUCTORS <-- Article 215 |
| | |
| [ Subpanel / MDP ] |
| | |
| FEEDER CONDUCTORS (still a feeder) |
| | |
| [ FINAL BRANCH-CIRCUIT OCPD ] |
| | |
| BRANCH CIRCUIT <-- Article 210 |
| | |
| [ Outlet ] |
+-------------------------------------------------------------------------+
[!IMPORTANT] A feeder can pass through several panelboards. The tier does not change at a subpanel; it changes at the final overcurrent device protecting the circuit that reaches the outlet. A conductor from a main distribution panel to a 200 A subpanel is a feeder; a conductor from that subpanel to a second 100 A subpanel is still a feeder.
Sizing the Feeder Conductors — NEC 215.2(A)(1)
Feeder conductors must have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load, determined before the application of any adjustment or correction factors.
They must also have an ampacity not less than the maximum load to be served after applying any adjustment or correction factors.
That is a two-part test, and both parts must pass:
- Before derating: conductor ampacity from the appropriate column of Table 310.16 must meet the 125% calculation.
- After derating: the ampacity adjusted for ambient temperature and conductor bundling must meet the actual calculated load.
Worked Example
A feeder supplies a panel with 90 A of continuous load and 40 A of noncontinuous load. The terminals at both ends are rated 75 °C.
- $(90 \times 1.25) + 40 = 112.5 + 40 = 152.5\ \text{A}$
- Because the circuit exceeds 100 A, 110.14(C)(1)(b) permits the 75 °C column.
- From Table 310.16, 1/0 AWG copper at 75 °C = 150 A (insufficient); 2/0 AWG copper at 75 °C = 175 A ✔
- Feeder overcurrent device per 215.3: not less than 152.5 A, so a 175 A standard device.
Feeder Overcurrent Protection — NEC 215.3
The feeder overcurrent protective device must have a rating not less than the noncontinuous load plus 125 percent of the continuous load. Where the assembly and the overcurrent device are listed for operation at 100 percent of their rating, the 125% multiplier is not applied to the continuous portion — a listing that in practice is limited to certain equipment rated 400 A and above.
Feeder taps that omit overcurrent protection at the point of supply are governed by 240.21(B), covered in detail in Section 6.3.
Feeder Grounded Conductor and Equipment Grounding Conductor
| Conductor | Sizing Rule |
|---|---|
| Grounded (neutral) feeder conductor | Sized to the maximum unbalanced load under 220.61, and never smaller than required by 250.102(C) where it also serves as the effective ground-fault current path for the feeder |
| Equipment grounding conductor | Sized from Table 250.122 based on the rating of the feeder overcurrent device, and increased proportionally where the ungrounded conductors are increased in size for voltage drop (250.122(B)) |
Section 7.3 covers Table 250.122 and the proportional-increase rule in depth.
Feeder Ground-Fault Protection — NEC 215.10
GFPE is required for each feeder disconnect rated 1,000 amperes or more on a solidly grounded wye system of more than 150 volts to ground but not exceeding 1,000 volts phase-to-phase, mirroring the service rule at 230.95.
The important exception: GFPE is not required for a feeder disconnect where ground-fault protection is provided on the supply side of the feeder — you do not stack redundant GFPE at the service and again at the feeder. Continuous industrial processes and fire pumps are also excepted.
Identification and Documentation
| Rule | Requirement |
|---|---|
| 215.12(A) | Where a premises has feeders supplied from more than one nominal voltage system, each ungrounded feeder conductor must be identified by phase or line and system, using separate color coding, marking tape, tagging, or other approved means, and the method must be documented and readily available or permanently posted at each feeder distribution equipment location |
| 215.12(B) | Grounded conductors of more than one system in the same raceway or enclosure must be identified per 200.6 |
| 215.5 | Feeder diagrams may be required by the AHJ prior to installation |
| 215.6 | Where a feeder supplies branch circuits requiring equipment grounding conductors, the feeder must include or provide an equipment grounding conductor |
| 215.9 | Feeders may be protected by a GFCI device in lieu of the provisions for GFCI protection specified in 210.8 and 590.6(A) |
Michigan Overlay
Two Michigan-relevant feeder rules are covered elsewhere in this guide and should not be answered from the plain NEC text:
- 215.18 surge protection — required for feeders supplying dwelling units, dormitory units, hotel and motel guest rooms and guest suites, and nursing home and limited-care facility patient sleeping rooms. See Section 10.4.
- 225.41 feeder emergency disconnect — for a one- or two-family dwelling supplied by a feeder, the disconnect must be on or within sight of the dwelling unit. See Section 10.4.
A feeder supplies 120 amperes of continuous load and 30 amperes of noncontinuous load to a panelboard with 75-degree-Celsius terminals at both ends. What minimum conductor ampacity does NEC 215.2(A)(1) require before adjustment or correction factors?
A 1,200-ampere feeder disconnect is supplied from a 480Y/277-volt solidly grounded wye service that already has ground-fault protection of equipment installed at the service disconnect. What does NEC 215.10 require at the feeder?
Under Article 100, at what point do feeder conductors end and branch-circuit conductors begin?
A building has feeders supplied from both a 208Y/120-volt system and a 480Y/277-volt system. What does NEC 215.12(A) require?