Service entrance and panelboards

The service is where the utility's system ends and the building's wiring begins. It brings power to the house, measures it, provides a means of disconnecting everything at once, protects the building wiring against overcurrent, and establishes the connection to earth and the bond between the neutral and the equipment grounding system. Mistakes at the service affect every circuit in the house and, because service conductors usually have no overcurrent protection on their supply side other than the utility's transformer fuse, they can carry very large fault currents. This lesson covers the parts of a residential service, overhead and underground supplies, sizing the service conductors, panelboards, grounding and bonding at the service, and the location and working space for equipment.

The parts of a residential service

North American homes are supplied with single-phase, three-wire 120/240 V power from a utility transformer whose secondary winding is center-tapped: the two ends (the ungrounded or "hot" conductors, often called L1 and L2) are 240 V apart, and each is 120 V from the grounded center tap, the neutral. A typical service has these parts, in order from the utility:

• Service drop (overhead) or service lateral (underground): the utility's conductors to the building. Ownership and responsibility boundaries vary by utility.
• Service entrance conductors: from the point of connection to the service equipment, often run through a mast or conduit.
• Meter base (meter socket): supplied and installed by the electrician, to the utility's specification, holding the utility's revenue meter.
• Service equipment: the main disconnecting means and overcurrent protection, usually a main breaker in the panelboard ("main-breaker panel"), or a separate disconnect where the panel is remote or the code requires it.
• Grounding electrode system and grounding electrode conductor, and the main bonding connection between the neutral and the service enclosure.

Overhead and underground supply

An overhead service uses a service drop from the utility pole to an attachment point on the house, often a mast that passes through the roof eave to gain height. The code and the utility set minimum clearances for the drop above roofs, ground, driveways and streets, and from windows, decks and swimming pools, and the mast must be strong enough to resist the drop's tension (masts are typically rigid metal conduit of at least the size the utility specifies, braced if tall).

In the US, NEC 230.24 sets minimum clearances for service drops: generally at least 8 ft above roofs (with reductions for low-slope and overhang conditions), 10 ft above finished grade and sidewalks for drops of 150 V to ground or less, 12 ft over residential property and driveways, and 18 ft over public streets and roads. Utilities publish service requirements that add details such as meter heights and mast specifications.

Underground services run the lateral in conduit or as direct-buried cable from a pad-mounted transformer or a pedestal, entering the meter base from below. They avoid the clearance and storm problems of overhead drops and are standard in new subdivisions. Conduits must be deep enough to protect the conductors, sealed where they enter the building to keep out water and gas, and arranged so that frost heave does not damage the meter base (expansion fittings are common in cold climates).

Sizing service conductors

Service conductors must carry the calculated load from Lesson 1 and be protected by the service overcurrent device. In general they are sized from the ampacity tables used in ELEC-120, at the 75 °C column for typical terminations. Because the diversity of household loads means a dwelling's service conductors rarely carry their rated current for long, both codes allow smaller conductors for dwelling services than the general tables would give.

NEC 310.12 permits the service and main feeder conductors of a single-family dwelling (or each unit of a two-family or multi-family dwelling) to have an ampacity of not less than 83% of the service rating. For a 200 A service, conductors with an ampacity of at least 166 A may be used: 2/0 AWG copper (175 A at 75 °C) or 4/0 AWG aluminum (180 A). For 100 A, 83 A: 4 AWG copper (85 A) or 2 AWG aluminum (90 A). Aluminum is common for services and must be terminated in equipment marked for it, torqued to the specified value.

Panelboards

The panelboard (load center) distributes power to branch circuits. A typical residential panel has two bus bars, one for each ungrounded conductor, arranged so that alternate breaker positions connect to alternate buses: a single-pole breaker connects to one bus and supplies 120 V circuits; a two-pole breaker spans both and supplies 240 V circuits. A neutral bar collects the neutrals, and a separate equipment grounding (bonding) bar collects the equipment grounding conductors.

Choosing a panel involves its ampere rating (at least the service or feeder rating), its number of circuit positions (allow plenty of spare spaces: heat pumps, EV chargers and solar are often added later), its short-circuit current rating (which must be at least the fault current available from the utility, commonly 10 kA for residential, but larger near big transformers), and the breaker types it accepts. Breakers must be those listed for the panel; mixing brands or using "classified" substitutes only where specifically permitted.

The NEC requires multiwire branch circuits to have a means to disconnect all ungrounded conductors simultaneously (a two-pole breaker or identified handle tie), panelboards to have a short-circuit current rating at least equal to the available fault current, and overcurrent devices not to be located in bathrooms or clothes closets (240.24). Since the 2020 edition, dwelling services must have a surge protective device (230.67), and one- and two-family dwellings need an emergency disconnect at a readily accessible outdoor location (230.85).

Open neutrals

If the service neutral is lost (from a corroded connection at the meter base or a broken drop), 120 V loads on the two buses are left in series across 240 V. The bus with the lighter load sees more than 120 V, sometimes nearly 240 V, and the heavier one less, which can destroy electronics and appliances and cause lights to flicker bright and dim as loads change. Symptoms such as lights brightening when a large 120 V load starts are a warning that should be investigated at once. Neutral connections at the service must be tight, correctly torqued and protected from corrosion.

Grounding and bonding at the service

ELEC-120 explained the two separate jobs of grounding (connecting the system to earth to limit voltages from lightning, line surges and contact with higher-voltage lines) and bonding (connecting metal parts so that a fault clears quickly and no dangerous touch voltages remain). At a residential service:

• The neutral is connected to the grounding electrode system and bonded to the service enclosure at one point only, the service equipment. Downstream of the service, the neutral and the equipment grounding conductors are kept separate, so that normal neutral current does not flow on metal enclosures and grounding conductors. A subpanel in the house or a detached garage therefore has an isolated neutral bar and a separate bonded ground bar, fed by a four-wire feeder.
• The grounding electrode system includes every electrode present at the building (250.50): metal underground water pipe in contact with earth for 10 ft or more (which must be supplemented by another electrode), concrete-encased electrode (20 ft of rebar or 4 AWG copper in the footing), ground ring and rod or pipe electrodes. A single rod must be supplemented by a second electrode unless it has a resistance of 25 Ω or less; two rods must be at least 6 ft apart (250.53). The grounding electrode conductor is sized from Table 250.66 by the service conductor size: for example, 4 AWG copper for 2/0 copper or 4/0 aluminum service conductors, and connections to rods need not be larger than 6 AWG copper.
• Metal water piping and other metal piping systems, such as gas piping where required, are bonded to the service so that they cannot become energized at a different potential from the electrical system.

Location and working space

Service equipment and panelboards must be readily accessible and have clear working space in front of them so that a person can operate and maintain them safely.

NEC 110.26 requires working space in front of panelboards at least 30 in wide (or the width of the equipment, if greater), 36 in deep for 120/240 V residential equipment, and 6½ ft high, kept clear of storage, with illumination. The service disconnect must be at a readily accessible location nearest the point of entrance of the service conductors (230.70), and overcurrent devices may not be located in bathrooms or clothes closets (240.24).

Lesson 10 returns to the service when solar, batteries, generators and EV chargers are added, and Lesson 11 covers service upgrades and panel replacements in existing homes.