6.1 Branch-Circuit Ratings, Conductor Sizing & Multiwire Circuits

Branch-Circuit Ratings — 210.3

Branch circuits recognized by this article shall be rated in accordance with the maximum permitted ampere rating or setting of the overcurrent device. The rating for other than individual branch circuits shall be 15, 20, 30, 40, and 50 amperes. Where conductors of higher ampacity are used for any reason, the ampere rating or setting of the specified overcurrent device shall determine the circuit rating.

A circuit supplying more than one outlet must be one of those five ratings. A 25-ampere multi-outlet branch circuit does not exist in the NEC. An individual branch circuit — one supplying only a single utilization equipment — may be any rating.

Voltage Limitations — 210.6

210.6(A) Occupancy Limitation. In dwelling units and guest rooms or guest suites, the voltage shall not exceed 120 volts, nominal, between conductors that supply the terminals of luminaires and cord-and-plug-connected loads 1440 volt-amperes or less or less than 1/4 horsepower.

210.6(B) 120 Volts Between Conductors. Permitted to supply terminals of luminaires, auxiliary equipment of electric-discharge lamps, and cord-and-plug-connected or permanently connected utilization equipment.

210.6(C) 277 Volts to Ground. Permitted to supply listed electric-discharge or LED luminaires, luminaires with mogul-base screwshell lampholders, and similar equipment, subject to the mounting-height conditions in that subsection.

210.6(D) 600 Volts Between Conductors is permitted for specific applications, and 210.6(E) covers over 600 volts.

Conductor Sizing — 210.19

210.19(A)(1)(a) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

Exception: Where the assembly, including the overcurrent devices protecting the branch circuits, is listed for operation at 100 percent of its rating, the ampacity of the branch-circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.

210.19(A)(3) Household Ranges and Cooking Appliances. Branch-circuit conductors supplying household ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. For ranges of 8.75 kW or more rating, the minimum branch-circuit rating shall be 40 amperes.

210.19(A)(4) Other Loads. Branch-circuit conductors that supply loads other than those specified in 210.19(A)(3) and other than cooking appliances shall have an ampacity sufficient for the loads served and shall not be smaller than 14 AWG.

The Informational Note to 210.19(A) contains the familiar voltage-drop recommendation: conductors for branch circuits as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, provide reasonable efficiency of operation. Being an Informational Note, it is not enforceable under 90.5(C).

Overcurrent Protection — 210.20

210.20(A) Continuous and Noncontinuous Loads. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.

210.20(B) Conductor Protection. Conductors shall be protected in accordance with 240.4, and flexible cords and fixture wires per 240.5.

Worked Example — Continuous and Noncontinuous

A 120 V branch circuit supplies an 18 A continuous lighting load plus a 6 A noncontinuous convenience load. What overcurrent device is required?

(18 x 1.25) + 6 = 22.5 + 6 = 28.5 A
Next standard size from 240.6(A) ......... 30 A

The conductor must also have an allowable ampacity of at least 28.5 A after any correction and adjustment, and 240.4(D) must be satisfied for the conductor chosen.

Receptacle and Cord-Connected Load Ratings — 210.21

210.21(B)(1) Single Receptacle on an Individual Branch Circuit. A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit. A 20 A individual circuit therefore requires a 20 A receptacle, not a 15 A device.

210.21(B)(2) Total Cord-and-Plug-Connected Load. Where connected to a branch circuit supplying two or more receptacles or outlets, a receptacle shall not supply a total cord-and-plug-connected load in excess of the maximum specified in Table 210.21(B)(2) — 12 A on a 15 A circuit with a 15 A receptacle, 16 A on a 20 A circuit with a 20 A receptacle, 24 A on a 30 A circuit with a 30 A receptacle.

210.21(B)(3) Receptacle Ratings. Where connected to a branch circuit supplying two or more receptacles, receptacle ratings shall conform to Table 210.21(B)(3):

The 20 A row is the one to know: 15 A receptacles are permitted on a 20 A multi-outlet circuit. This is legal because no single 15 A device can draw more than 12 A per Table 210.21(B)(2), and the circuit serves several of them.

210.21(B)(4) requires receptacles rated 15 or 20 A at 125 V supplying loads such as those in Table 210.21(B)(2) to comply with the tamper-resistant and other requirements of Article 406.

Permissible Loads — 210.23

210.23(A) 15- and 20-Ampere Branch Circuits. A branch circuit supplying two or more outlets or receptacles shall supply only the loads specified: cord-and-plug-connected equipment not fastened in place, and utilization equipment fastened in place. (A)(1) limits cord-and-plug-connected equipment not fastened in place to 80 percent of the branch-circuit ampere rating. (A)(2) limits utilization equipment fastened in place, other than luminaires, to 50 percent of the branch-circuit rating where lighting units, cord-and-plug-connected equipment not fastened in place, or both are also supplied.

210.23(B) 30-Ampere Branch Circuits. Shall supply fixed lighting units with heavy-duty lampholders in other than a dwelling unit, or utilization equipment. Cord-and-plug-connected utilization equipment not fastened in place shall not exceed 24 amperes.

210.23(C) 40- and 50-Ampere Branch Circuits. Shall supply cooking appliances that are fastened in place in any occupancy, or fixed lighting units with heavy-duty lampholders, infrared heating units, or other utilization equipment in other than a dwelling unit.

Multiwire Branch Circuits — 210.4

A multiwire branch circuit shares one neutral among two or three ungrounded conductors taken from different phases of the same system. Done correctly it saves copper; done incorrectly it is dangerous.

210.4(A) General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire circuit shall be permitted to be considered as multiple circuits. All conductors of a multiwire branch circuit shall originate from the same panelboard or similar distribution equipment.

210.4(B) Disconnecting Means. "Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates." In practice this is a two- or three-pole common-trip breaker, or single-pole breakers with an identified handle tie. Handle ties are permitted for the disconnecting means requirement; common trip is not required by 210.4(B) itself.

210.4(C) Line-to-Neutral Loads. Multiwire branch circuits shall supply only line-to-neutral loads, with two exceptions: a multiwire branch circuit that supplies only one utilization equipment, and where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device.

210.4(D) Grouping. The ungrounded and grounded circuit conductors of each multiwire branch circuit shall be grouped by cable ties or similar means in at least one location within the panelboard or other point of origination. An exception applies where the circuit enters from a cable or raceway unique to the circuit that makes the grouping obvious.

Why the Neutral Must Not Be Broken

On a 120/240 V multiwire circuit, the two ungrounded conductors are on opposite legs, so the neutral carries only the difference between the two loads. Break that neutral while the circuit is energized and the two loads become a series circuit across 240 volts. The voltage divides in proportion to impedance: a small load in series with a large one sees far more than 120 V and is destroyed.

This is also why a multiwire branch circuit must never be created by taking two ungrounded conductors from the same phase. Then the neutral carries the sum of both loads rather than the difference, and it can be badly overloaded while both breakers remain happy.

Counting the Neutral

Remember from Chapter 4 of this guide: the neutral of a three-wire circuit from a single-phase three-wire system is not a current-carrying conductor for adjustment purposes under 310.15(E)(1). The neutral of a four-wire three-phase wye circuit is not counted either — unless the major portion of the load is nonlinear, in which case 310.15(E)(3) counts it.

Ground-Fault and Arc-Fault Rules Preview

210.8 (GFCI) and 210.12 (AFCI) are large enough to warrant their own sections and are covered in 6.3 and 6.4 of this chapter. 210.11(C) and the required-outlet rules of 210.52 are covered in 6.2.