Branch circuits, feeders, overcurrent protection and voltage drop

Branch circuits and feeders are the backbone of every installation, and questions about them make up a large share of any electrical exam. They test the definitions that distinguish branch circuits from feeders and services, how to size conductors and overcurrent devices for continuous and non-continuous loads, standard device ratings, protection against ground faults and arc faults, receptacle layout rules in dwellings, and voltage drop. This lesson reviews each topic with worked examples in the style of exam questions.

Services, feeders and branch circuits

The supply to a building passes through three levels of circuit. The service consists of the conductors and equipment that deliver energy from the utility to the building's wiring system, ending at the service disconnecting means and overcurrent protection. Feeders run from the service equipment, or from a separately derived system such as a transformer, to panelboards and other distribution equipment. Branch circuits run from the final overcurrent device, typically a breaker in a panelboard, to the outlets, such as receptacles, luminaires and appliances. Exam questions frequently depend on recognizing which of these a set of conductors is, because different rules apply to each.

Continuous loads

A continuous load is one where the maximum current is expected to continue for three hours or more, such as store lighting, office lighting, electric heating and many industrial processes. Conductors and overcurrent devices are tested at their ratings under controlled conditions, and in real enclosures continuous operation at full rating can overheat them, so codes require extra capacity for continuous loads.

In the NEC, 210.19(A) and 210.20(A) for branch circuits, and 215.2 and 215.3 for feeders, require conductors and overcurrent devices to be rated at least 125 % of the continuous load plus 100 % of the non-continuous load, unless the assembly including the overcurrent device is listed for operation at 100 % of its rating. Article 100 defines a continuous load as one where the maximum current is expected to continue for three hours or more.

Standard overcurrent device ratings

Section 240.6(A) lists standard ampere ratings for fuses and inverse-time breakers: 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800 A and upward. Where a conductor's ampacity does not match a standard rating, 240.4(B) permits the next higher standard rating up to 800 A, provided the circuit does not supply multiple receptacles for cord-and-plug-connected portable loads and the conductor is not otherwise restricted; above 800 A, 240.4(C) requires the conductor ampacity to be at least the device rating.

Overcurrent devices also have an interrupting rating, the maximum fault current they can safely interrupt, which must be at least the available fault current at their location. Exam questions may present a short-circuit current and ask whether a device with a stated interrupting rating is suitable, or ask about series-rated combinations of breakers tested together.

Ground-fault and arc-fault protection

Ground-fault circuit interrupters (GFCIs) protect people from shock by opening the circuit when the current leaving on the line conductor differs from the current returning on the neutral by more than about 5 mA (4 to 6 mA for Class A devices), indicating that current is flowing through an unintended path, possibly a person. They are required in locations where people are likely to contact water or earth. Arc-fault circuit interrupters (AFCIs) detect the electrical signatures of arcing, from damaged cords, loose connections or nails through cables, and open the circuit to prevent fires. Ground-fault protection of equipment (GFPE), set at much higher levels, protects equipment from destructive arcing ground faults on large solidly grounded wye services.

In the NEC, 210.8 lists where GFCI protection is required: in dwelling units it now covers bathrooms, kitchens, laundry areas, garages, accessory buildings, outdoors, crawl spaces, unfinished basements, within 1.8 m (6 ft) of sinks, bathtubs and showers, boathouses and other areas, and it has expanded with each recent edition, including to many 250 V receptacles. 210.12 requires AFCI protection for most 120 V, 15 and 20 A branch circuits supplying outlets and devices in dwelling unit living areas, and 230.95 requires ground-fault protection of equipment for solidly grounded wye services of more than 150 V to ground and 1,000 A or more.

Receptacle layout in dwellings

Dwelling receptacle rules in 210.52 aim to put a receptacle within reach of a typical cord. In general, receptacles are placed so that no point along the floor line of any wall space is more than 1.8 m (6 ft) from a receptacle, which gives the familiar maximum spacing of 3.7 m (12 ft). Kitchen countertop receptacles are placed so that no point along the wall line is more than 600 mm (24 in) from a receptacle, and at least two 20 A small-appliance branch circuits serve kitchen and dining areas. Bathrooms need at least one 20 A circuit, laundry areas a 20 A circuit, and receptacles are required outdoors at the front and back of one-family dwellings and in garages. Tamper-resistant receptacles are required in dwelling units by 406.12.

Voltage drop

Long circuits lose voltage along their conductors, reducing the voltage at the load, which can make motors run hot, lights dim and electronic equipment malfunction. Voltage drop in a two-wire circuit is the current times the resistance of both conductors, the outgoing and the return:

Here \(R\) is the conductor resistance per unit length and \(L\) the one-way length. For AC circuits with large conductors, reactance also matters, and more exact methods use effective impedance. For exam purposes, the resistance method is usually sufficient.

The NEC treats voltage drop mainly as a recommendation: informational notes to 210.19 and 215.2 suggest no more than 3 % in a branch circuit or feeder and 5 % overall. Specific articles do impose limits, for example for sensitive electronic equipment, fire pumps and some health care systems, and energy codes may require limits. Exam questions may ask for the conductor size needed to stay within 3 %.

Multi-wire branch circuits

A multi-wire branch circuit shares one neutral between two or three ungrounded conductors from different phases, saving a conductor. When loads are balanced, the neutral carries little current. Because opening the neutral of a multi-wire circuit can put loads in series across 240 V and damage equipment, codes require that the neutral not depend on device terminals for continuity, usually by splicing neutrals with pigtails, and that the ungrounded conductors be disconnected simultaneously, typically by a two-pole or handle-tied breaker, where they supply devices on the same yoke or in other specified situations. Multi-wire circuits supplying non-linear loads can overload the shared neutral with harmonic currents.

Feeder sizing and taps

Feeders are sized from the calculated load using the demand factors of the relevant calculation rules (Lesson 5), plus the continuous-load multiplier where it applies. Conductors smaller than the feeder's overcurrent device, called taps, are permitted under specific conditions of length, ampacity and termination, such as short taps to a single overcurrent device. Tap rules are detailed and frequently examined; read every condition. Feeder neutrals are sized for the maximum unbalanced load, with demand factors permitted for some loads but not for non-linear loads or electric discharge lighting.

Branch circuit ratings and receptacles

A branch circuit's rating is set by its overcurrent device, not by its conductors, and the receptacles on it must suit that rating. A 15 A receptacle may be installed on a 20 A circuit where there are two or more receptacles, while a single receptacle on an individual branch circuit must have a rating at least equal to the circuit. Cord-and-plug-connected loads on a circuit are generally limited to a percentage of the circuit rating, and fixed equipment supplied by a circuit that also serves lighting or receptacles is limited in the same way, so that the remaining capacity is available for other loads. Exam questions combine these rules with continuous-load requirements.

Selective coordination and protection

In large systems, overcurrent devices are coordinated so that a fault is cleared by the device nearest to it, leaving the rest of the system in service. Coordination requires comparing time-current curves of upstream and downstream devices. Codes require selective coordination for some critical systems, such as emergency systems, legally required standby systems and elevators. Other protective concepts that appear in questions include current-limiting fuses, which open within the first half cycle of a large fault and limit the energy let through, and series ratings, which allow a downstream breaker with a lower interrupting rating to be protected by an upstream device when the combination has been tested.

Feeder neutrals

Feeder neutral conductors carry the maximum unbalanced load between the neutral and any ungrounded conductor. In a balanced three-phase four-wire system, linear loads produce little neutral current, but codes require the neutral to be sized for the calculated maximum unbalance, with demand factors permitted for some large loads but not for non-linear loads such as electronic equipment and discharge lighting. Where large non-linear loads are present, the neutral may need to be larger than the phase conductors, and it counts as a current-carrying conductor for derating.

Branch circuits for specific loads

Some loads require dedicated or specifically sized branch circuits: fixed space heating, which is usually treated as continuous; water heaters, which some codes require to be treated as continuous; central heating equipment, which typically needs an individual circuit; laundry equipment; and kitchen appliances such as dishwashers, ranges and built-in microwave ovens. Questions may ask for the minimum circuit rating for a specific appliance, combining the appliance nameplate with continuous-load rules and the next standard device rating.