Services and feeders
A service brings power from the utility to the building's service disconnect, and feeders carry it from there to panelboards. NEC Article 230 covers services, including overhead clearances (230.24), service disconnects and conductor sizing. Article 215 covers feeders. Both size conductors at 125% of the continuous load plus the noncontinuous load.
What services and feeders are, and why they matter on the exam
The difference is where the conductors sit relative to the overcurrent protection. Article 100 defines the pieces:
- Service point: where the utility's facilities connect to the premises wiring.
- Service conductors: from the service point to the service disconnecting means.
- Feeder: conductors between the service equipment (or another source such as a transformer or generator) and the final branch-circuit overcurrent device.
- Branch circuit: conductors from the final overcurrent device to the outlets. See branch circuits.
Service conductors have no overcurrent protection on their supply end, which is why Article 230 is strict about how they run and how quickly they reach a disconnect.
Overhead service clearances: 230.24
Above roofs, 230.24(A) sets a general minimum of 8 ft 6 in, maintained for 3 ft in all directions beyond the roof edge. Exceptions for 300 volts or less between conductors reduce it:
- 3 ft where the roof slope is at least 4 in 12
- 18 in where no more than 6 ft of conductors pass over no more than 4 ft of roof overhang and end at a through-the-roof raceway or approved support
Above final grade, 230.24(B) sets these minimums:
- 10 ft at the service entrance, at the drip loop, and over sidewalks or areas reached only by pedestrians, for messenger-supported cable at 150 volts or less to ground
- 12 ft over residential property and driveways, and commercial areas not subject to truck traffic, at 300 volts or less to ground
- 15 ft over those same areas where the voltage exceeds 300 volts to ground
- 18 ft over public streets, alleys, roads, parking areas subject to truck traffic, and non-residential driveways
- 24.5 ft over railroad tracks
Conductors must also stay 3 ft from openable windows, doors, porches and balconies (230.9(A)), and the point of attachment must be at least 10 ft above grade (230.26).
Service disconnects: 230.70 through 230.85
The service disconnecting means must be readily accessible, outside or inside nearest the point where the service conductors enter (230.70(A)), and never in a bathroom. Each must be marked as a service disconnect.
Each service generally gets one disconnect. Under 230.71, up to six are allowed when each is in its own enclosure or in separate compartments of approved equipment, grouped at one location. Since 2020, a single panelboard with six breakers and no main does not count as six service disconnects, and 2023 keeps that rule.
Minimum disconnect ratings under 230.79:
- 100 A, 3-wire for a one-family dwelling
- 60 A for other installations
- 30 A for a service with only two 2-wire branch circuits
- 15 A for a service with a single branch circuit
Dwelling services need a Type 1 or Type 2 surge protective device (230.67). One- and two-family dwellings need a marked, readily accessible outdoor emergency disconnect for first responders (230.85). Ground-fault protection of equipment is required for each disconnect rated 1,000 A or more on solidly grounded wye services over 150 volts to ground and not over 1,000 volts phase to phase (230.95).
Service conductor sizing
Service-entrance conductors must have an ampacity of at least 125% of the continuous load plus 100% of the noncontinuous load under 230.42(A). They must also be no smaller than the disconnect rating required by 230.79. Overhead and underground service conductors have a minimum size of 8 AWG copper or 6 AWG aluminum, except for limited loads.
Dwellings get an allowance in 310.12. For a 120/240-volt, single-phase service of 100 A through 400 A that supplies the entire load of a dwelling unit, the service conductors may have an ampacity of 83% of the service rating. If correction or adjustment factors apply, apply them to the conductor's table ampacity, and the result must still be at least 83% of the service rating.
Service or feeder ampacity = (continuous × 1.25) + noncontinuousDwelling service ampacity (310.12) = service rating × 0.83
Feeders: Article 215
Feeder conductors are sized like service conductors: at least 125% of the continuous load plus the noncontinuous load, per 215.2(A)(1). The feeder overcurrent device follows the same math under 215.3, unless the assembly is listed for 100% operation.
The feeder neutral may be no smaller than the equipment grounding conductor required by 250.122. Feeders to a separate building fall under Article 225, which requires a disconnect at that building nearest the point of entrance. For grounding at the second building, see grounding and bonding.
Worked examples
Example 1: feeder to a subpanel. A feeder supplies 80 A of continuous load and 20 A of noncontinuous load, with 75°C terminations. Minimum ampacity = (80 × 1.25) + 20 = 120 A. Table 310.16 gives 130 A for 1 AWG copper at 75°C, so 1 AWG copper works. The overcurrent device math under 215.3 also gives 120 A, so the next standard size, 125 A (240.6(A)), protects it.
Example 2: 200 A dwelling service. A 200 A, 120/240-volt service supplies the entire load of a one-family dwelling. Minimum ampacity = 200 × 0.83 = 166 A. Table 310.16 gives 175 A for 2/0 AWG copper at 75°C, so 2/0 copper is the smallest copper conductor that qualifies.
Example 3: commercial service. A store has 160 A of continuous lighting load and 75 A of noncontinuous load. Minimum service-entrance ampacity = (160 × 1.25) + 75 = 275 A. Table 310.16 gives 285 A for 300 kcmil copper at 75°C, so 300 kcmil copper meets it. The 83% allowance does not apply, because this is not a dwelling.
For more conductor sizing practice, see conductor ampacity and the exam calculations guide.
Common exam mistakes
- Using 83% for commercial buildings, or for dwelling services under 100 A or over 400 A.
- Applying 125% to the whole load instead of to the continuous part only.
- Confusing roof clearances (8 ft 6 in, 3 ft, 18 in) with ground clearances (10, 12, 15, 18, 24.5 ft).
- Using voltage to ground for the roof exceptions. Those use voltage between conductors.
- Counting six breakers in one panelboard as six service disconnects on a new installation.
Practice questions
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Frequently asked questions
What is the difference between a service and a feeder?
Service conductors run from the service point to the service disconnecting means (Article 230). Feeders run from the service equipment, or another source like a transformer, to the final branch-circuit overcurrent device (Article 215).
How high must a service drop be over a driveway?
For a residential driveway at 300 volts or less to ground, overhead service conductors need at least 12 ft of clearance under 230.24(B). Over public streets, areas subject to truck traffic and non-residential driveways, the minimum is 18 ft.
What size wire do I need for a 200 amp service?
For a 200 A, 120/240-volt service supplying an entire dwelling, 310.12 allows 83% of 200 A, which is 166 A. That makes 2/0 copper at 75°C the smallest copper size.
How many service disconnects are allowed?
Generally one per service. Under 230.71, up to six are permitted when each is in its own enclosure or compartment, grouped at one location.
Is a main breaker required in a subpanel?
Not always. The feeder breaker in the main panel can protect it if it is no larger than the panelboard rating. A subpanel in a separate building does need a disconnecting means there under Article 225.