7.1 Overcurrent Principles and OCPD Types
Key Takeaways
- NEC Article 100 defines overcurrent as any current in excess of rated current, and splits it into three forms: overload, short-circuit, and ground fault.
- NEC 240.6(A) standard ampere ratings run 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, 2000, 2500, 3000, 4000, 5000, and 6000 amperes.
- Series ratings under NEC 240.8 are only valid for specific listed combinations of upstream and downstream devices; any modification requires re-evaluation of the series rating.
- Dual-element (time-delay) fuses hold a large inrush temporarily then clear on sustained overload, which is why they protect motor branch circuits without nuisance tripping.
- NEC 240.12 requires equipment rated over 1000 A at 600 V or less to be field-marked with available fault current, and the marking must be updated when modifications affect that value.
Why Overcurrent Protection Matters for the TX Journeyman Exam
Overcurrent protection is the backbone of NEC Article 240 and shows up in both halves of the Texas Journeyman exam. The NEC Knowledge part (59 questions) tests definitions, standard ampere ratings, device types, and series-combination rules. The Calculations part (26 questions) uses these same ratings when sizing branch-circuit and motor overcurrent devices. Getting 240.6(A) standard ratings and the 240.4(D) small-conductor caps wrong costs points in both sections, so this chapter is high-leverage.
The Three Forms of Overcurrent (NEC Article 100)
Overcurrent is defined in Article 100 as any current in excess of the rated current of equipment or the ampacity of a conductor. The NEC recognizes three forms, each with a different cause and clearing-time requirement.
| Form | Definition (Art. 100) | Typical Cause | Magnitude | Clearing Time |
|---|---|---|---|---|
| Overload | Operation in excess of normal, full-load current that persists long enough to damage equipment | Too many loads on a circuit; a stalled motor | 1× to 6× FLC | Seconds to minutes |
| Short-circuit | An unintended connection between ungrounded conductors of different phases/polarities | Insulation failure phase-to-phase | Hundreds to thousands of A | Must clear in cycles |
| Ground fault | An unintended connection between an ungrounded conductor and a grounded surface | Insulation nick touching conduit or a box | Tens to thousands of A | Must clear in cycles |
A motor stalling and drawing 3× full-load current is an overload. A phase conductor welding to another phase in a J-box is a short-circuit. That same conductor touching the grounded raceway is a ground fault. All three are overcurrent, but a device that handles one well (a motor overload relay) is not necessarily rated to interrupt another (a short-circuit).
OCPD Types — Fuses vs Circuit Breakers
NEC 240.2 defines an overcurrent protective device (OCPD) as a device capable of opening a circuit at a predetermined overcurrent. Two families are recognized.
Fuses
A fuse is a single-use device that contains a calibrated element designed to melt and open the circuit under overcurrent. The NEC recognizes several varieties.
- Plug fuses (Edison-base and Type S) — used on circuits of 125 V or less to ground, 30 A or less. Governed by NEC 240.5. Type S plugs are rejection-base so the wrong size cannot be installed.
- Cartridge fuses — ferrule (up to 60 A) and knife-blade (60 A and larger), used in disconnect switches and panelboards.
- Non-time-delay (one-time) fuses — a single element that opens quickly; good for incandescent lighting and resistive loads but they blow on motor inrush.
- Dual-element (time-delay) fuses — two elements in one tube: a short-circuit element that clears fast on high fault current, and an overload element that holds a temporary inrush (typically 5× rating for up to 10 seconds) before opening. These are the standard choice for motor branch circuits per 430.52.
Circuit Breakers
A circuit breaker is a mechanical switching device that opens automatically on overcurrent and can be reset and reused.
- Thermal-magnetic breakers — a bimetallic strip responds to sustained overload (the thermal part), and a magnetic solenoid responds instantaneously to short-circuit/ground-fault current. The standard residential and commercial breaker.
- Magnetic-only (instantaneous-trip) breakers — no thermal element; used as the short-circuit/ground-fault element in a motor combination starter where a separate overload relay (430.32) handles the sustained-overload function. Listed under 430.52 as "instantaneous trip breaker."
- Intermittent-duty / adjustable-trip breakers — allow setting the instantaneous pickup, common on larger motor circuits.
Fuse vs Circuit Breaker at a Glance
| Attribute | Fuse | Circuit Breaker |
|---|---|---|
| Operation | Sacrificial — must be replaced after opening | Resettable — re-close after clearing |
| Reset time | Requires stocking spares | Re-close immediately |
| Interrupting rating | Commonly 200 kA+; check label | Commonly 10–65 kA; check label |
| Selectivity | Easy to coordinate by ampere rating alone | Requires selective coordination study (240.12) |
| Initial cost | Lower | Higher |
| Maintenance | None (passive) | Periodic exercise/testing |
NEC 240.5 — Lamp Holders
NEC 240.5 limits how OCPDs protect luminaires and lampholders. 240.5(B)(2) sets the maximum branch-circuit OCPD for circuits supplying medium-base lampholders at 20 A for circuits with a mix of luminaires, and at 15 A or 20 A depending on the lampholder rating. The takeaway for the exam: a 30 A or 50 A branch circuit cannot supply standard medium-base screw-shell lampholders.
NEC 240.6(A) — Standard Ampere Ratings
240.6(A) lists the standard ampere ratings used for fuses and fixed-trip circuit breakers. The exam expects you to know the list, including which values are NOT available as standard so you can size correctly under 240.4(B).
| 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 |
Ratings continue at 1600, 2000, 2500, 3000, 4000, 5000, and 6000 A.
Exam tip: 25 A and 35 A ARE standard ratings under 240.6(A), but the small-conductor rule in 240.4(D) caps 14, 12, and 10 AWG copper at 15, 20, and 30 A respectively — so you will never size a 25 A or 35 A OCPD for a 14/12/10 AWG small-conductor branch circuit. Treat 25 and 35 as "standard but rarely used for small conductors."
NEC 240.8 — Series Ratings
A series combination rating is the short-circuit current that a specific upstream-plus-downstream pair can clear together, even though the downstream device alone has a lower interrupting rating. NEC 240.8 imposes strict rules.
- Only listed series combinations are permitted. You cannot pair an arbitrary 65 kAIC upstream breaker with a 10 kAIC downstream breaker and call the result a series system — the pair must be tested and listed together.
- The series rating is valid only in unmodified equipment. If the installation is modified, the series combination must be re-evaluated by a qualified engineer or the equipment must be re-marked.
- Series ratings are not permitted where the downstream device feeds a motor in a way that could let motor short-circuit contribution defeat the coordination (240.8(C)).
The exam will try to bait you with "any upstream breaker with a high enough kAIC protects a downstream low-kAIC breaker." That is false. The pair must be a listed combination.
NEC 240.12 — Available Fault Current and Selective Coordination
NEC 240.12 has two prongs.
- Available fault current marking. Equipment rated over 1000 A at 600 V or less must be field-marked with the available fault current at the line terminals. The marking must be re-evaluated whenever modifications change the available fault current.
- Selective coordination. Where an orderly shutdown is required (690, 700, 701, 702 systems; life safety; critical operations), OCPDs must be selectively coordinated so that only the device closest to the fault opens — upstream devices stay closed. Coordination is typically studied on a time-current curve overlay and may require setting adjustments or fuse-only solutions in the lower range.
A phase conductor in a 480 V three-phase feeder contacts the grounded metal raceway. By NEC Article 100, what form of overcurrent is this?
Which statement about NEC 240.8 series ratings is correct?
Per NEC 240.6(A), which of the following is a standard ampere rating for a fixed-trip circuit breaker?