Lesson 3: Overcurrent Protective Device Ratings (2023)

As generally required in the NEC, match the overcurrent device ampere rating for the following small conductor sizes.

10 AWG Aluminum → 25 amperes 12 AWG Aluminum → 15 amperes 10 AWG Copper → 30 amperes 12 AWG Copper → 20 amperes 16 AWG Copper → 10 amperes 18 AWG Copper → 7 amperes Note: See NEC Reference 240.4(D).

If a branch-circuit rated fuse is not marked with an interrupting rating, then the interrupting rating for that fuse is ? .

10,000 A NEC Reference: 240.60(C) Note: This pertains to branch-circuit rated fuses but not to supplementary fuses. Of the branch-circuit fuse types, only Class H fuses and plug fuses are commercially available with a 10 kA interrupting rating (IR) and do not need to be marked with their IR. All other commercially-available branch-circuit rated fuses will have an interrupting rating greater than 10 kA and therefore will be marked with their IR. For instance, all Class J, L, T, CF, CC, and R fuses have at least a 200kA IR.

If a conductor's ampacity does not correspond to a standard circuit breaker or fuse ampere rating found in ? , ? permits selecting the next standard ampere rating if specific conditions in ? are met.

240.6 / 240.4(B) / 240.4(B)

A panelboard that is located downstream from a service disconnect is calculated to have 45,000 amperes of available fault current. A circuit breaker installed in this downstream panel must have an interrupting rating equal to or greater than ? .

45,000 A Note: This question assumes using fully rated circuit breakers. Series rating is covered later. If the circuit breaker in the downstream panelboard were part of a series rating of at least 45 kA, then this circuit breaker could have a full rating of less than 45 kA.

If a branch-circuit circuit breaker is not marked with an interrupting rating, then the interrupting rating for that circuit breaker is ? .

5,000 A NEC Reference: 240.83(C) Note: This pertains to branch-circuit circuit breakers and not to supplementary protectors. Of the branch-circuit circuit breaker types that are commercially available, the lowest interrupting ratings are 5 kA, and these circuit breakers do not have to be marked with their IR. All other commercially-available branch-circuit circuit breakers will have interrupting ratings greater than 5 kA, and therefore will be marked with their IR.

Straight voltage rated circuit breakers of the proper voltage rating (where the voltage rating is greater than the system line-line voltage) are allowed to be used on which of the following voltage systems? I.) 120 V, single-phase, 2 W, solidly grounded II.) 120/240 V, single-phase, 3 W, solidly grounded III.) 208 Y/120 V, 3-phase, 4 W, solidly groundedIV. 240 V, 3-phase, 3 W, ungrounded or corner grounded delta V.) 480 Y/277 V, 3-phase, 4 W, solidly grounded VI.) 480 V, 3-phase, 3 W, ungrounded or high resistance grounded VII.) 600 Y/347 V, 3-phase, 4 W, solidly grounded VIII.) 600 V, 3-phase, 3 W, solidly grounded

All voltage systems above apply. NEC Reference: 240.85

What is/are the most common interrupting ratings for current-limiting fuses?

C.) 200,000 A D.) 300,000 A Both c. and d.

The voltage of an electrical system must be equal to or greater than voltage rating of the OCPD.

False

Per the second paragraph of Section 110.9, devices such as motor starters and switches must have interrupting ratings at applied voltage sufficient for the available fault current at its line terminals.

False NEC 110.9 (First paragraph, not second): Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the available fault current at the line terminals of the equipment.

Fuses have straight and slash voltage ratings.

False Note: By their design, fuses are full voltage rated devices. Slash voltage ratings are not an issue. For example, Class J fuses are rated at 600 volts AC and are utilized on systems rated 600 volts AC or less.

A fuse or circuit breaker with a 600-volt AC rating is permitted to be used on a 300-volt DC application by the half voltage rule in NEC Article 240.

False Note: For DC applications, a fuse or circuit breaker should be marked with a DC voltage rating equal or greater than the system DC voltage.

When a fuse or circuit breaker interrupts a fault current exceeding its interrupting rating, UL product standards require that an indication pin on the fuse or circuit breaker actuate so that the Electrical Worker knows to replace the device with a new OCPD with an adequate interrupting rating.

False Note: Review the still photos from Figure 11 and the associated videos available.

All circuit breakers are required to have their ampere ratings marked on the circuit breaker handle or escutcheon.

False Note: See 240.83(B) and 240.83(A).

Interrupting ratings are required to be marked on supplemental protective devices.

False NEC Reference: 240.83(C) Note: Supplemental OCPDs are not required to be marked with an interrupting rating.

Which of the following is not one of three OCPD ratings that are important and must be applied properly in order to not cause an unsafe condition?

Frame size rating

Slash voltage rated circuit breakers of the proper voltage rating are allowed to be used on which of the following voltage systems? I.) 120 V, single-phase, 2 W, solidly grounded II.) 120/240 V, single-phase, 3 W, solidly grounded III.) 208 Y/120 V, 3-phase, 4 W, solidly groundedIV. 240 V, 3-phase, 3 W, ungrounded or corner grounded delta V.) 480 Y/277 V, 3-phase, 4 W, solidly grounded VI.) 480 V, 3-phase, 3 W, ungrounded or high resistance grounded VII.) 600 Y/347 V, 3-phase, 4 W, solidly grounded VIII.) 600 V, 3-phase, 3 W, solidly grounded

I., II., III., VI., and VIII. NEC Reference: 240.85

The ? rating is the highest current at rated voltage that a device is identified to interrupt under standard test conditions.

Interrupting NEC Reference: 100

Typically, what is the single most influencing factor on the magnitude of the available fault current at the service?

Transformer supplying the service

Fuses and circuit breakers that are intended to interrupt current at fault current levels must have an interrupting rating at applied voltage sufficient for the available fault current at its line terminals.

True

New service equipment in commercial buildings must be field marked with the available fault current and the date the calculations were performed.

True

When installed in commercial installations, service equipment is required to be field marked with the available fault current and the date of fault current calculations.

True NEC Reference: 110.24(A)

Generally, conductors shall be protected at their ampacities by branch-circuit overcurrent devices.

True NEC Reference: 240.4

The ampere rating, voltage rating, interrupting rating (where other than 10,000 amperes), "current-limiting" (where applicable), and the name or trademark of the manufacturer are all items that must be plainly marked on a fuse (this pertains to branch circuit rated fuses and not supplementary fuses).

True NEC Reference: 240.60(C)