Feeders and services under Articles 215 and 230

Lesson 1 worked at the outlet end of the system. This lesson moves upstream to the feeders that carry power between panelboards and the service that brings power into the building from the utility. Article 215 covers feeders and Article 230 covers services, and together they produce some of the most heavily tested rules on master exams: feeder sizing, ground-fault protection of equipment, service conductor sizing, overhead clearances, the location and number of service disconnects, and the minimum service rating for a dwelling. Feeder and service load calculations, which feed into these rules, are covered in depth in MELU-102; here the focus is on what Articles 215 and 230 require once you have the load.

Delgado Fabrication's upgrade

Brightwater has won the contract for the service upgrade at Delgado Fabrication, a small machine shop moving from an old 200-ampere, 208-volt service to a new 400-ampere, 480Y/277-volt service with a step-down transformer for the office and lighting loads. Ray wants Dana to lay out the service equipment and the feeders. 'This job has a service, three feeders, a transformer and a panel in a mezzanine,' he says. 'If you can explain every conductor on it, you can pass the service and feeder questions.' Gloria Haynes, the city's chief electrical inspector, has already asked to see the plan before the utility schedules the new drop.

Feeder or service?

The first exam skill is naming the conductor. Article 100 defines the service point as the point of connection between the utility's facilities and the premises wiring. Everything from the service point to the service disconnecting means is service conductors: overhead service conductors and service drops on overhead systems, underground service conductors and service laterals on underground systems, and service-entrance conductors within or on the building. Everything from the service equipment, or from a separately derived system such as a transformer, to the final branch-circuit overcurrent device is a feeder. The distinction matters because services have rules feeders do not, such as limited routes inside buildings and no overcurrent protection at the supply end, and feeders have rules services do not.

Feeder sizing under 215.2

Section 215.2 uses the same structure as branch circuits. The feeder conductors must have an ampacity not less than the noncontinuous load plus 125 out of 100 of the continuous load, before any adjustment or correction factors, and not less than the maximum load to be served after them. The feeder's overcurrent protection, under 215.3, must likewise be rated not less than the noncontinuous load plus 125 out of 100 of the continuous load, unless the whole assembly is listed for 100 out of 100 operation. A feeder must also have an ampacity not less than the service conductors when it carries the total load supplied by service conductors with an ampacity of 55 amperes or less, a small-system rule that rarely applies to commercial work.

Feeders that supply two-wire or multiwire branch circuits must have a grounded conductor if any of those branch circuits need one, and the feeder neutral is sized from the calculated neutral load in Article 220, which can be less than the ungrounded conductors when loads are balanced or are line-to-line. Feeders to a building or structure other than the one with the service also bring in the rules of Article 225, including a disconnect at each building.

# Feeder minimum ampacity and device (illustrative list of standard ratings)
standard = [100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600]

def feeder(noncontinuous, continuous):
    need = noncontinuous + 1.25 * continuous
    return need, next(r for r in standard if r >= need)

for name, nc, c in [("Shop distribution", 260, 64), ("Mezzanine panel", 70, 40), ("Office transformer primary", 40, 12)]:
    need, dev = feeder(nc, c)
    print(f"{name}: at least {need:.0f} A ampacity, device at least {dev} A")

Shop distribution: at least 340 A ampacity, device at least 350 A
Mezzanine panel: at least 120 A ampacity, device at least 125 A
Office transformer primary: at least 55 A ampacity, device at least 100 A

Ground-fault protection of equipment

Large solidly grounded wye systems at 480Y/277 volts can sustain low-level arcing ground faults that do not draw enough current to trip a large breaker quickly, and those arcs can destroy switchgear. Sections 230.95 for services and 215.10 for feeders therefore require ground-fault protection of equipment for each service or feeder disconnect rated 1000 amperes or more on a solidly grounded wye system with more than 150 volts to ground but not more than 600 volts phase to phase in older editions, with the voltage limit worded as 1000 volts in recent ones. The protection must operate to open all ungrounded conductors, with a maximum setting and time delay set by the Code, and it must be performance tested when first installed. Fire pumps and continuous industrial processes where a sudden shutdown would create more hazard are exempt.

Delgado's 400-ampere service is below the 1000-ampere threshold, so ground-fault protection of equipment is not required there. On the exam, check three facts before you apply these sections: the disconnect rating, whether the system is a solidly grounded wye, and the voltage to ground. A 208Y/120-volt system is only 120 volts to ground and never triggers the requirement.

Service conductors and clearances

Article 230 starts with a basic rule in 230.2: a building or structure is supplied by only one service, with exceptions for fire pumps, emergency and standby systems, parallel power production, multiple-occupancy buildings, large capacity requirements and different voltages or uses. Service conductors from one building may not pass through the interior of another, and service conductors inside a building are treated as outside the building only when they are under at least 2 inches of concrete, in a raceway encased in brick or concrete, or in a vault, under 230.6. That rule exists because service conductors have no overcurrent protection on their supply end.

Overhead service conductors need clearances: from building openings such as windows and doors, from roofs, and above grade, driveways and roads. The vertical clearances above ground step up with the risk: lower values over areas accessible only to pedestrians, higher values over residential driveways and commercial areas not subject to truck traffic, and higher still over roads, alleys and parking lots where trucks travel. Rather than memorize every value, know the structure of the rule, which section holds it and how the voltage to ground changes the lowest values, and look the numbers up in your book. Service-entrance conductors must have sufficient ampacity for the load calculated under Article 220 and must not be smaller than the minimum service rating.

The service disconnect

Section 230.70 requires a means to disconnect all service conductors from the building wiring, installed at a readily accessible location either outside the building or inside nearest the point of entrance of the service conductors. Many jurisdictions set a maximum length of service-entrance conductor inside the building before the disconnect, so check local amendments. Each disconnect must be permanently marked as a service disconnect, and where there is more than one, a plaque or directory must identify the others.

Section 230.71 limits the number of service disconnects. Under editions since 2020, each service may have up to six disconnects, but they must be in separate enclosures, in separate vertical sections of switchgear or switchboards with barriers, or in separate compartments; the old practice of six main breakers in one panelboard with no main is no longer permitted. Section 230.72 requires the disconnects to be grouped. Section 230.79 sets minimum ratings: 100 amperes for a one-family dwelling, 60 amperes for other installations, with smaller values for installations of one or two circuits.

For one- and two-family dwellings, 230.85 now requires an emergency disconnect at a readily accessible outdoor location, so firefighters can shut off power without entering the building. It can be the service disconnect itself, a meter disconnect or another listed device, and it must be marked. And under 230.67, services supplying dwelling units, and in recent editions dormitory units, guest rooms and some patient rooms, must have a surge-protective device, a rule lesson 11 returns to.

Dwelling services and feeders: the 83 out of 100 rule

For 120/240-volt, three-wire single-phase dwelling services and the feeders that carry the entire load of a dwelling unit, 310.12 allows the service-entrance or feeder conductors to be sized at 83 out of 100 of the service rating, before any adjustment and correction factors. A 200-ampere service may therefore use conductors with an ampacity of at least 166 amperes. Older editions had a separate table of conductor sizes for this; since 2020 the rule is the 83 out of 100 factor. It does not apply to a feeder that does not carry the whole dwelling load, to three-phase services, or to a 208Y/120-volt service in an apartment building where each unit is fed by two phases and a neutral.

2026 notes

In the 2026 edition, the service and feeder sizing rules stay in Articles 230 and 215, but the load calculations they refer to move from Article 220 to Article 120, and medium-voltage service and feeder material is gathered in new articles in the 260s. Section numbers you have tabbed for services and feeders under 1000 volts mostly stay put. Always check which edition your exam uses.