2023 NEC Article 240
Overcurrent protection does two jobs. It protects each conductor at a rating tied to that conductor's ampacity, and it sits at the point where the conductor receives its supply. Article 240 sets both rules and then lists the exceptions, and the exceptions are where inspectors spend their time: the next-size-up rule, the small-conductor limits, and the tap rules. This lesson covers all three, plus where the devices may be mounted. Ampacity itself is Lesson 8, feeder sizing is Lesson 4, and the service overcurrent device is Lesson 5. Local adoption governs which NEC edition applies (see Lesson 1).
The starting rule is that conductors, other than flexible cords, flexible cables, and fixture wires, are protected against overcurrent at their ampacity from Section 310.14, unless 240.4(A) through (H) say otherwise (2023 NEC 240.4). One exception is a circuit where opening would create a hazard, such as a fire pump circuit, which needs short-circuit protection but not overload protection (240.4(A)). Tap conductors follow 240.21 (240.4(E)), and the transformer secondary rule is covered below (240.4(F)).
Fuses and inverse time circuit breakers come in the standard ratings of Table 240.6(A): 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, 1000, 1200, 1600, and 2000 A, continuing upward. Fuses also come in 1, 3, 6, and 601 A. Notice what is not on the list: 65 A and 75 A. A conductor whose ampacity is 65 A does not line up with any device, and that gap is exactly what the next-size-up rule fills.
Where the overcurrent device is rated 800 A or less, the next higher standard rating above the conductor ampacity may be used if all three conditions are met (240.4(B)). First, the conductors are not part of a branch circuit supplying more than one receptacle for cord-and-plug portable loads. Second, the conductor ampacity does not match a standard rating. Third, the next standard rating does not exceed 800 A. Ampacity here is the value after the adjustments covered in Lesson 8.
| Copper conductor at 75 C | Ampacity | Largest device permitted by 240.4(B) |
|---|---|---|
| 6 AWG | 65 A | 70 A |
| 4 AWG | 85 A | 90 A |
| 3 AWG | 100 A | 100 A (already standard) |
| 2 AWG | 115 A | 125 A |
| 1 AWG | 130 A | 150 A |
The rule is a permission, not a requirement, and it does not lift other limits. A 65 A conductor on a row of receptacles for bench tools may not go on a 70 A device, because that circuit supplies more than one receptacle for portable loads, and without the round-up the device may not exceed the 65 A conductor ampacity, so the largest standard rating is 60 A. Above 800 A, there is no round-up: the conductor ampacity must be at least the device rating (240.4(C)). A 1,000 A breaker needs conductors that carry 1,000 A, not 920 A.
For small conductors, the overcurrent protection may not exceed the limits in 240.4(D), after any correction and adjustment factors have been applied. For copper: 18 AWG at 7 A, 16 AWG at 10 A, 14 AWG at 15 A, 12 AWG at 20 A, and 10 AWG at 30 A. For aluminum and copper-clad aluminum: 12 AWG at 15 A and 10 AWG at 25 A. The 18 and 16 AWG limits carry extra conditions on continuous load and on the listing of the device. These limits explain why a 12 AWG copper branch circuit tops out at a 20 A breaker even where its table ampacity at 90 C is higher.
Overcurrent protection goes in each ungrounded conductor at the point where the conductors receive their supply, except in the cases 240.21(A) through (H) list (240.21). A tap is a conductor connected to a feeder without its own overcurrent device at the connection. Taps are allowed only in specific forms, and 240.4(B) may not be used for tap conductors (240.21(B)).
The 10 ft tap: if the tap is 10 ft or less, its ampacity must be at least the calculated load it serves and at least the rating of the equipment or device it ends in. It must not extend beyond the equipment it supplies. It must be enclosed in a raceway from the tap to the enclosure. If it leaves the enclosure or vault where the tap is made, its ampacity must be at least one-tenth of the rating of the feeder overcurrent device (240.21(B)(1)).
The 25 ft tap: if the tap is 25 ft or less, its ampacity must be at least one-third of the rating of the feeder overcurrent device. It must end in a single circuit breaker or set of fuses that limits the load to the ampacity of the tap conductors, and that device may feed any number of other devices. The conductors must be protected from physical damage (240.21(B)(2)). A transformer tap allows one primary plus one secondary conductor of up to 25 ft combined (not counting any primary portion protected at its ampacity), with a primary ampacity of one-third and a secondary of at least the voltage ratio times one-third of the feeder device (240.21(B)(3)). Outside taps of unlimited length must end in a single device at a disconnect that is readily accessible outdoors or nearest the entrance (240.21(B)(5)). Taps longer than 25 ft indoors are limited to high-bay manufacturing buildings over 35 ft high (240.21(B)(4)).
A 400 A feeder passes a new 125 A panelboard 8 ft away in the same electrical room. A 10 ft tap can feed it. The tap must carry the 125 A load and the 125 A rating of the panel's main, so 1 AWG copper at 75 C (130 A) works. If the tap leaves the enclosure where it is made, the one-tenth test gives 400 / 10 = 40 A, which 130 A easily exceeds. Now a second panelboard is 22 ft away, so only a 25 ft tap can reach it, and the one-third test gives 400 / 3 = 133 A. The same 1 AWG conductor at 130 A now falls short, so the smallest choice is 1/0 AWG copper at 150 A. That tap ends at a single 150 A breaker ahead of the panel. A designer who keeps 1 AWG and adds a 150 A breaker cites 240.4(B), but taps may not use the next-size-up rule, and 130 A is below the one-third minimum in any case.
Secondary conductors of a transformer, other than single-phase 2-wire secondaries and 3-wire delta-delta secondaries, are not protected by the primary device (240.4(F)). The 10 ft secondary rule in 240.21(C)(2) has the same style of conditions as the 10 ft tap, and 240.4(B) may not be used there either. Industrial 25 ft secondaries are limited to switchgear or switchboards in industrial installations (240.21(C)(3)). Under Section 240.24, breakers and fused switches must be readily accessible with the center of the operating handle grip, in its highest position, not over 6 ft 7 in above the floor or working platform (240.24(A)). Each occupant needs ready access to the devices protecting that occupancy, with limited management exceptions (240.24(B)). Devices must not be exposed to physical damage (240.24(C)), must not be near easily ignitible material such as in clothes closets (240.24(D)), may not be in bathrooms, showering facilities, or locker rooms with showers (240.24(E)), and may not be over steps of a stairway (240.24(F)).
A device must have an interrupting rating at least equal to the available fault current at its line terminals (110.9), and Lesson 2 covers the required marking of that current. Series-rated combinations must be selected by a licensed professional engineer, documented, and field marked in existing installations (240.86(A)), or be a tested combination marked on the equipment (240.86(B)). Where a circuit breaker's highest continuous trip setting can be 1200 A or higher, 240.87 requires documentation of the breaker locations and a method to reduce clearing time, such as zone-selective interlocking or an arc-energy reduction maintenance switch.
In the field
A GCHS inspector walks a small church's fellowship hall addition. A 400 A feeder panel is on the wall, and a new sub-panel sits about 20 ft away, fed by conductors spliced into the feeder lugs with no breaker at the splice. That is a tap, so the inspector asks for the tap length, the ampacity of the conductors, and what protects them at the far end. Conductors that are shorter than 25 ft but carry only 115 A fail the one-third rule of 133 A. The same visit turns up a panel in a coat closet and a breaker whose handle is 7 ft off the floor, both violations of 240.24.
1. A feeder to a panelboard in a Group B building uses 6 AWG copper conductors with a 65 A ampacity at 75 C. It is not a branch circuit. What is the largest standard overcurrent device 240.4(B) allows?
2. A feeder uses 1 AWG copper conductors with a 130 A ampacity at 75 C, and the ampacity does not match a standard device rating. What is the largest overcurrent device the next-size-up rule allows?
3. An electrician wires a row of receptacle outlets for bench tools with 6 AWG copper (65 A) and protects it with a 70 A breaker. Under 240.4(B)(1), what is the problem?
4. 12 AWG aluminum conductors are used for a 120 V branch circuit in a small office. What is the maximum overcurrent protection under 240.4(D)?
5. A circuit in a community center kitchen uses 10 AWG copper conductors, with no correction or adjustment factors needed. What is the maximum overcurrent protection under 240.4(D)?
6. A 1,000 A overcurrent device on a commercial feeder is proposed with two parallel sets that together have a 920 A ampacity. What does 240.4(C) say?
7. A 400 A feeder has a 10 ft tap to a panel, and the tap conductors leave the enclosure where the tap is made. What minimum ampacity does the one-tenth rule of 240.21(B)(1) set for the tap, apart from the load?
8. A 25 ft tap is made from a 400 A feeder. What is the smallest copper conductor, using the 75 C column, that meets the one-third rule of 240.21(B)(2)?
9. An electrician wants to tap a 400 A feeder with conductors 30 ft long inside an office building and end them at a panelboard. Does 240.21(B) allow it?
10. A 300 A feeder has a 25 ft tap of 2 AWG copper (115 A), which meets the one-third minimum of 100 A. The designer ends the tap in a 125 A breaker, citing 240.4(B). What is the problem?
11. Which condition is required for an outside tap of unlimited length under 240.21(B)(5)?
12. An inspector finds the center of the grip of a circuit breaker handle, in its highest position, at 7 ft 2 in above the floor of a commercial kitchen panelboard. Which rule is violated?
13. During a rehab of a community center, an electrician mounts a new panelboard inside the coat closet of the front office. Which rule does this conflict with?
14. A panelboard in a church annex has 22,000 A of available fault current at its line terminals, and its breakers are marked 10,000 A interrupting rating. What do 110.9 and 240.86 require?