5.3 Service Drop, Service Lateral & Service-Entrance Sizing
Key Takeaways
- The service point marks the exact boundary where the serving utility's jurisdiction ends and customer premises wiring begins under NEC Article 100 and Article 230.
- NEC 230.2 enforces the general mandate that a building or other structure shall be supplied by only one service, subject to specific exceptions including fire pumps, emergency systems, capacity exceeding 2,000 amperes, and multi-tenant occupancies.
- Under NEC 230.79(C), the minimum service rating for a single-family dwelling unit is 100 amperes, 3-wire, while general commercial and other installations must have a minimum rating of 60 amperes.
- Service-entrance conductors must have an ampacity sufficient to carry the calculated load per Article 220 and not less than 125% of continuous load plus 100% of noncontinuous load (NEC 230.42(A)).
- NEC 230.28 strictly permits only power service-drop conductors to be attached to a service mast; attaching telephone, CATV, or broadband communication lines to an electrical service mast is a direct code violation.
5.3 Service Drop, Service Lateral & Service-Entrance Sizing
The electrical service is the vital link between the electric utility supplier and the premises electrical wiring system. NEC Article 230 governs the installation, conductor sizing, overcurrent protection, and physical support of all service conductors and service equipment.
Mastering Article 230 is crucial for the Idaho Journeyman examination because service installations involve strict demarcation lines, mechanical mast strain requirements, minimum conductor sizing thresholds, and distinct rules for overhead versus underground supplies.
1. Anatomy & Demarcation of an Electrical Service
To apply the code correctly, electricians must distinguish between utility-owned supply conductors and customer-owned premises wiring.
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| ANATOMY OF AN ELECTRICAL SERVICE |
| |
| [OVERHEAD UTILITY SERVICE] |
| Utility Pole ---> [SERVICE DROP] ---> [SERVICE POINT (Splice)] |
| | |
| v |
| [WEATHERHEAD / SERVICE MAST] ---> [SERVICE-ENTRANCE CONDUCTORS (Overhead)]|
| | |
| v |
| [METER SOCKET] ---> [SERVICE DISCONNECT] ---> [PREMISES FEEDERS/BRANCHES] |
| |
| ======================================================================= |
| |
| [UNDERGROUND UTILITY SERVICE] |
| Utility Pad Transformer ---> [SERVICE LATERAL] ---> [SERVICE POINT] |
| | |
| v |
| [METER BASE / ENCLOSURE] ---> [SERVICE-ENTRANCE CONDUCTORS (Underground)] |
| | |
| v |
| [MAIN SERVICE PANEL] ---> [SERVICE EQUIPMENT & OVERCURRENT PROTECTION] |
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Key Article 100 Definitions
- Service Point: The point of connection between the facilities of the serving utility and the premises wiring. The NEC only applies on the customer side of the service point.
- Service Drop: The overhead conductors between the utility electric supply system and the service point.
- Service Lateral: The underground conductors between the utility electric supply system and the service point.
- Service-Entrance Conductors, Overhead System: The service conductors between the terminals of the service equipment and a point usually outside the building, clear of building walls, where joined by tap or splice to the service drop or overhead service conductors.
- Service Equipment: The necessary equipment (circuit breakers, switches, fuses, and their enclosures) connected to the load end of service conductors intended to constitute the main control and cutoff of the supply.
2. Number of Services Allowed per Building (NEC 230.2)
NEC 230.2 establishes the core safety principle that a building or other structure shall be supplied by only ONE service, unless specifically permitted under subsections (A) through (D).
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| NEC 230.2 EXCEPTIONS TO SINGLE SERVICE |
| |
| [SPECIAL CONDITIONS (230.2(A))] |
| * Fire Pumps (dedicated isolated service to prevent loss of life safety) |
| * Emergency / Legally Required Standby Systems |
| * Solar Photovoltaic / Energy Storage Systems |
| |
| [SPECIAL OCCUPANCIES (230.2(B))] |
| * Multiple-occupancy buildings with no available space for service |
| equipment accessible to all occupants (requires special permission) |
| * Exceptionally large buildings or structures |
| |
| [CAPACITY REQUIREMENTS (230.2(C))] |
| * Capacity requirements exceeding 2,000 Amperes at 600V or less |
| * Supply requirements greater than utility single-phase capacity |
| |
| [DIFFERENT CHARACTERISTICS (230.2(D))] |
| * Different voltages, frequencies, or phases (e.g., 480V 3-phase and |
| 120/240V single-phase supplies to the same commercial industrial plant) |
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[!IMPORTANT] Plaque or Directory Mandate (NEC 230.2(E)): Where a building is served by more than one service, or any combination of services, feeders, and branch circuits, a permanent directory or plaque must be installed at each service disconnect location denoting all other services, feeders, and branch circuits supplying that building and the area served by each.
3. Minimum Service Ratings (NEC 230.79)
The service disconnecting means must have an ampere rating not less than the calculated load determined in accordance with Article 220, and shall in no case be less than the ratings specified in NEC 230.79.
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| MINIMUM SERVICE DISCONNECT RATINGS (230.79) |
| |
| [100 AMPERES, 3-WIRE] ---> Single-Family Dwelling Units (230.79(C)) |
| [60 AMPERES] ---> All other general commercial/shop (230.79(D))|
| [30 AMPERES] ---> Installations of not more than two 2-wire |
| branch circuits (230.79(B)) |
| [15 AMPERES] ---> Single branch circuit installations (230.79A)|
+-----------------------------------------------------------------------------+
Detailed Service Rating Table
| Installation Type | Minimum Disconnect Rating | Minimum Conductor Sizing | Code Section |
|---|---|---|---|
| Single-Family Dwelling Unit | 100A, 3-Wire | Sized per Table 310.12 or 310.16 | NEC 230.79(C) |
| General Commercial / Multifamily / Other | 60A | Sized per calculated load (Art. 220) | NEC 230.79(D) |
| Two 2-Wire Branch Circuits | 30A | Sized for load (min 8 AWG Cu) | NEC 230.79(B) |
| Single Branch Circuit Only (e.g., Sign/Pump) | 15A | Sized for branch circuit load | NEC 230.79(A) |
4. Service-Entrance Conductor Sizing (NEC 230.42 & Table 310.12)
Service-entrance conductors must have an ampacity sufficient to carry the loads calculated in accordance with Article 220, before any temperature or fill derating adjustments.
General Sizing Formula (NEC 230.42(A))
Service Conductor Ampacity >= (Continuous Load * 1.25) + (Noncontinuous Load * 1.00)
Dwelling Unit Service Sizing Table (NEC 310.12)
For single-phase dwelling unit 120/240V 3-wire services and feeders that supply the entire dwelling electrical load, NEC 310.12 permits the service conductors to be sized using Table 310.12 (or the 83% demand factor calculation under 310.12(B)).
| Service / Feeder Rating | Copper Conductor Size (75°C) | Aluminum / Copper-Clad Al (75°C) |
|---|---|---|
| 100 Amperes | 4 AWG Copper | 2 AWG Aluminum |
| 125 Amperes | 2 AWG Copper | 1/0 AWG Aluminum |
| 150 Amperes | 1 AWG Copper | 2/0 AWG Aluminum |
| 200 Amperes | 2/0 AWG Copper | 4/0 AWG Aluminum |
| 225 Amperes | 3/0 AWG Copper | 250 kcmil Aluminum |
| 300 Amperes | 250 kcmil Copper | 350 kcmil Aluminum |
| 400 Amperes | 400 kcmil Copper | 600 kcmil Aluminum |
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| 83% DWELLING SERVICE CALCULATION METHOD (310.12(B)) |
| |
| For a 200A Dwelling Service: |
| Required Conductor Ampacity = 200A * 0.83 = 166 Amperes |
| - Table 310.16 (75°C Cu) -> 2/0 AWG Copper is rated 175A (Complies >=166A)|
| - Table 310.16 (75°C Al) -> 4/0 AWG Aluminum is rated 180A (Complies) |
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5. Service Mast Engineering, Guying & Physical Support (NEC 230.28)
Where an overhead service mast is used for the support of service-drop conductors, it must be designed and installed to withstand the severe mechanical tension imposed by utility spans, wind loading, and winter ice accretion.
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| SERVICE MAST SUPPORT REQUIREMENTS |
| |
| Weatherhead |
| | |
| v |
| [=== MAST ===] <--- Power Conductors ONLY |
| | |
| | <--- Guy Wire / Bracing (if required) |
| Roof Overhang ---------------|------------------------------------------- |
| | |
| v |
| [METER SOCKET] |
| |
| [PROHIBITED ATTACHMENTS (NEC 230.28(A))] |
| * Telephone / Telecommunications drop wires |
| * Cable Television (CATV) coaxial drops |
| * Broadband / Satellite internet lead-ins |
| * Radio antennas or clotheslines |
+-----------------------------------------------------------------------------+
Mast Rules & Mechanical Specifications
- Permitted Conduit Types: Only rigid metal conduit (RMC) or intermediate metal conduit (IMC) is permitted for service mast support. Electrical metallic tubing (EMT) and PVC conduit are strictly prohibited from supporting overhead service drops.
- Guying & Bracing: If the service mast extends more than 36 to 48 inches above the roof line (or as mandated by local utility engineering rules), it must be securely guyed with galvanized steel aircraft cable or braced with rigid steel pipe back to structural roof rafters.
- Prohibited Attachments (NEC 230.28(A)): Service masts shall never be used for the attachment of telephone, CATV, broadband communication cables, or aerial radio antennas. Only electric power service conductors are permitted.
6. Point of Attachment & Service Head (Weatherhead) Standards
- Point of Attachment Height (NEC 230.26): The point of attachment for service-drop conductors must be placed at a height that ensures the overhead span clearances required by NEC 230.24 are satisfied, and shall in no case be less than 10 feet (3.0 m) above finished grade.
- Service Head Location (NEC 230.54): Service raceways must be equipped with a listed, raintight service head (weatherhead). The service head must be located above the point of attachment of the service drop conductors.
- Drip Loop Requirement (NEC 230.54(F)): Drip loops must be formed on individual conductors below the level of the service head to prevent water from following the conductors directly into the raceway and meter cabinet.
Under NEC 230.79(C), what is the minimum required ampere rating for the service disconnecting means of a single-family dwelling unit?
Which of the following attachments to a through-the-roof electrical service mast is strictly prohibited under NEC 230.28(A)?
Under NEC 310.12 and Table 310.12, what is the minimum size 75°C Copper conductor permitted for a 200-ampere single-phase 120/240V 3-wire service supplying an entire single-family dwelling unit?