Short answer: Article 230 covers the service: the conductors and equipment that bring utility power into a building and the main disconnect. Article 240 covers overcurrent protection: standard breaker and fuse sizes, the next-size-up rule, and the 15, 20 and 30 amp limits for 14, 12 and 10 AWG copper. Continuous loads are sized at 125%, unless the overcurrent device and its assembly are listed for operation at 100% of their rating.
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Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the method in our own words and do not reproduce NEC text. Check every rule in your own 2026 NEC. Service equipment carries serious shock and arc-flash hazards. Apprentices work on services only under supervision, follow the electrical safety program of their employer, and stay de-energized unless that program and the qualified-person rules allow otherwise.
Where services and protection live in the 2026 NEC
Article 230 covers services and Article 240 covers overcurrent protection, both for systems up to 1000 volts. Rules for higher voltages have been moved to separate articles. The load calculation that tells you how big a service must be is now in Article 120 (it was Article 220 before 2026), according to the City of Denison’s summary of 2026 changes (Denison 2026 NEC summary). Texas uses the 2026 NEC for installations and exams from September 1, 2026 (TDLR rule adoption notice).
Trace a service on a one-line
| Part | What it is | Article 230 topic |
|---|---|---|
| Service drop or service lateral | Utility conductors overhead (drop) or underground (lateral) to the service point | Clearances and installation |
| Service point | The point of connection between the serving utility’s facilities and the premises wiring | Definition in Article 100 |
| Service-entrance conductors | From the service point to the service disconnect | Size, protection, splices and taps |
| Service disconnect | The means to disconnect the electrical service; other power sources may require separate disconnecting means | Location, marking, number of disconnects |
| Service equipment overcurrent protection | The main breaker or fuses | Rating and location |
| Grounding electrode conductor and bonding | The link from the service to the grounding electrodes | Article 250 |
Rules worth knowing by name:
- A building normally has one service, with listed exceptions (Article 230, number of services).
- The service disconnect goes at a readily accessible location, either outside or inside nearest the point where the conductors enter (230.70). Recent editions added outdoor emergency-disconnect rules for one- and two-family dwellings; read the 2026 text and ask about local amendments.
- A one-family dwelling service must be rated at least 100 amps (230.79(C)).
- Line-side connection devices must be marked as suitable for that use; the 2026 code allows a short “SVC” style marking (230.46), per the Denison summary.
Article 240 rules in plain words
| Rule | What it says in plain words | Where to look |
|---|---|---|
| Protect at ampacity | A conductor is protected at no more than its ampacity, with listed exceptions. | 240.4 |
| Next size up | If the ampacity does not match a standard size, you may use the next standard size up, as long as it is 800 A or less and the circuit is not a multi-outlet branch circuit for cord-and-plug receptacle loads. | 240.4(B) |
| Small conductors | Copper 14 AWG is limited to 15 A, 12 AWG to 20 A and 10 AWG to 30 A protection, unless a specific rule allows otherwise. | 240.4(D) |
| Standard sizes | The list of standard breaker and fuse ratings. | 240.6(A) |
| Location | Overcurrent protection generally sits where the conductor receives its supply. | 240.21 |
Standard ratings you will use most often: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175 and 200 amps. Note the gaps: there is no 55, 65 or 120.
Worked example 1: service rating from a calculated load
A one-family dwelling has a calculated load of 39,600 volt-amperes at 120/240 volts, from the Article 120 method.
- Current: 39,600 ÷ 240 = 165 A.
- The smallest standard rating that is at least 165 A is 175 A.
- 175 A is above the 100 A minimum for a one-family dwelling, so 175 A is the smallest standard rating that meets this example load. Many contractors install 200 A for spare capacity.
Check: 175 × 240 = 42,000 VA, which covers 39,600. The next lower size, 150 × 240 = 36,000 VA, does not.
Worked example 2: next size up
A feeder’s conductors have an ampacity of 118 A after all corrections. The load is 105 A, and the feeder supplies a subpanel, not a receptacle circuit.
- 118 A is not a standard size.
- The next standard size above 118 is 125 A.
- 125 A is not more than 800 A, and this is not a multi-outlet receptacle branch circuit, so a 125 A breaker is permitted.
- The load (105 A) is still within the conductor ampacity (118 A).
Check: the next standard size below 118 is 110 A, which would also be allowed. The rule gives you the option to go up one size, not a requirement.
Worked example 3: continuous load
A commercial lighting feeder carries 64 A of continuous load and 20 A of noncontinuous load. For this example, assume the overcurrent device and its assembly are not listed for operation at 100% of their rating. Under the general feeder rule in Article 215, the overcurrent device must be rated at least 125% of the continuous load plus 100% of the noncontinuous load.
- Continuous at 125%: 64 × 1.25 = 80 A.
- Add the noncontinuous load: 80 + 20 = 100 A.
- 100 A is a standard size, so a 100 A overcurrent device is the minimum under the assumptions of this example.
Check: work backward. 100 − 20 = 80, and 80 ÷ 1.25 = 64. Same continuous load.
Worked example 4: the small-conductor limit
12 AWG copper THHN has a 90°C ampacity of 30 A in Table 310.16. Can it go on a 25 A breaker for a general-purpose circuit?
- The small-conductor rule limits 12 AWG copper to 20 A protection unless a specific rule allows more.
- A general-purpose branch circuit has no such rule, so the maximum is 20 A.
- Answer: no. Use a 20 A breaker, or a larger conductor.
Motor circuits, air-conditioning equipment and some other loads have their own rules that can allow larger protection. That is why motor circuit questions send you to Article 430.
Check yourself
Our own practice questions. Answers below each.
- Calculated dwelling load: 30,000 VA at 240 V. Minimum standard service rating? Answer: 30,000 ÷ 240 = 125 A, which is a standard size. 125 A.
- Conductor ampacity 65 A, not a receptacle circuit. Largest breaker by the next-size-up rule? Answer: 70 A.
- 40 A continuous load, nothing else. Minimum overcurrent device? Answer: 40 × 1.25 = 50 A.
- Maximum breaker for 10 AWG copper on a general-purpose circuit? Answer: 30 A.
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The code book for services and overcurrent protection
NFPA 70, National Electrical Code (NEC), 2026 Edition, softbound. Articles 230 and 240 are in it. Read each section in your own copy before you rely on it.
FAQ
What is the difference between NEC Article 230 and Article 240?
Article 230 covers services: the conductors and equipment that bring power from the utility into the building, including the service disconnect. Article 240 covers overcurrent protection for conductors and equipment throughout the system.
What is the next size up rule?
When a conductor’s ampacity does not match a standard breaker or fuse size, the NEC allows the next standard size up, if it is 800 amps or less and the circuit is not a multi-outlet branch circuit supplying receptacles for cord-and-plug-connected loads.
What size breaker for 12 AWG copper wire?
For general-purpose circuits, 20 amps maximum. The small-conductor rule limits 14, 12 and 10 AWG copper to 15, 20 and 30 amps unless a specific NEC rule, such as for motors, allows otherwise.
What is the minimum service size for a house?
A one-family dwelling service must be rated at least 100 amps under the NEC. The actual size comes from the load calculation, which is in Article 120 in the 2026 NEC.
Sources checked October 2026
- TDLR rule adoption notice, 2026 NEC: https://www.tdlr.texas.gov/news/rulemaking/2026/09/01/commission-adopts-rules-12/
- PSI Candidate Information Bulletin for TDLR (updated 9/15/2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- City of Denison, 2026 NEC significant changes: https://denisontx.gov/DocumentCenter/View/4131/2026-National-Electrical-Code-Significant-Changes
- IAEI Magazine, NEC 2026 significant code changes (article moves for over 1000 volts): https://iaeimagazine.org/standards/nec-2026-significant-code-changes/
- NFPA 70 (NEC), 2026 edition, Articles 230 and 240, read in your own copy: https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
