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Services & Distribution

The service and distribution system delivers utility power to the building and splits it into feeders and branch circuits. This chapter covers service equipment, feeders, panelboards, grounding and bonding, overcurrent protection, conductor ampacity, and transformers.

Services (Article 230) & Feeders (Article 215)

The service is the conductors and equipment that deliver electricity from the utility to the premises wiring. Article 230 limits the number of service disconnects and requires them to be readily accessible. Feeders, covered by Article 215, carry power from the service equipment to remote panels and must be sized for the calculated load plus continuous-load factors.

Service disconnect location
The service disconnecting means must be installed at a readily accessible location outside or nearest the point of entrance of the service conductors.
2023 NEC §230.70
Number of disconnects
Each service is limited to a maximum of six disconnects grouped in one location, unless a single main is provided.
2023 NEC §230.71
Feeder sizing
Feeder conductors must have an ampacity not less than the noncontinuous load plus 125% of the continuous load before applying adjustment factors.
2023 NEC §215.2
Service conductor size
Ungrounded service conductors must be sized to carry the calculated load and be no smaller than the code-specified minimum for the service rating.
2023 NEC §230.42

Panelboards & Overcurrent Protection (Articles 408 & 240)

A panelboard distributes power to branch circuits through overcurrent devices. Article 240 requires that conductors be protected against overcurrent at the point they receive their supply, with standard ampere ratings and specific rules for taps and location. Overcurrent protection guards conductors and equipment from the heat of excessive current.

Overcurrent at supply point
Conductors must be protected against overcurrent per their ampacity at the point where they receive their supply, with limited exceptions for taps.
2023 NEC §240.21
Standard ampere ratings
Standard fuse and breaker ratings begin at 15, 20, 25, 30, 35, 40, 45, 50 amperes and continue upward as listed in the code.
2023 NEC §240.6
Next-size-up rule
Where conductor ampacity does not match a standard device, the next higher standard rating up to 800 A is permitted if the conductor is not part of a receptacle branch circuit.
2023 NEC §240.4(B)
Panelboard overcurrent protection
Each panelboard must be protected by an overcurrent device rated no greater than the panelboard's busbar rating.
2023 NEC §408.36

Grounding & Bonding (Article 250)

Grounding connects the system and equipment to earth, while bonding ties metal parts together to create a low-impedance fault path. Article 250 specifies how to size the grounding electrode conductor (GEC), equipment grounding conductor (EGC), and bonding jumpers so fault current returns safely and opens the overcurrent device.

Grounding electrode conductor
The GEC is sized from Table 250.66 based on the size of the largest ungrounded service-entrance conductor.
2023 NEC §250.66
Equipment grounding conductor
The EGC is sized from Table 250.122 according to the rating of the overcurrent device protecting the circuit.
2023 NEC §250.122
Main bonding jumper
The main bonding jumper connects the grounded (neutral) conductor to the equipment grounding system at the service, sized like the GEC.
2023 NEC §250.28
Grounded conductor brought to service
For a grounded system, a grounded conductor must be run to each service disconnect and bonded to the enclosure.
2023 NEC §250.24(C)

Conductor Ampacity (Article 310)

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. Article 310 provides ampacity tables based on insulation temperature and installation conditions, then requires correction for ambient temperature and adjustment for the number of current-carrying conductors bundled together.

Ampacity table
Conductor ampacities for common raceway installations are taken from Table 310.16 at the 60, 75, or 90 degree Celsius column.
2023 NEC §310.16
Termination temperature limit
Ampacity is limited by the lowest temperature rating of any connected terminal, commonly 60 C for circuits 100 A or less.
2023 NEC §110.14(C)
Temperature correction
Ampacity must be corrected downward when ambient temperature exceeds the table basis, using the correction factors in the code.
2023 NEC §310.15(B)
Conductor adjustment
When more than three current-carrying conductors share a raceway, ampacity is adjusted by the percentages in Table 310.15(C)(1).
2023 NEC §310.15(C)

Transformers (Article 450)

Transformers change voltage between the utility supply and utilization equipment. Article 450 governs their overcurrent protection, and the secondary conductors must be protected and sized for the load they serve. Proper ventilation and working clearances protect the transformer and personnel.

Primary overcurrent protection
A transformer with primary protection only must have that device rated per the percentages in Table 450.3(B) based on primary current.
2023 NEC §450.3
Secondary conductor protection
Transformer secondary conductors must be protected against overcurrent, generally at their ampacity, subject to the tap and transformer rules.
2023 NEC §240.21(C)
Ventilation
Transformers must be installed so that required ventilation openings are not blocked, allowing heat to dissipate.
2023 NEC §450.9
Working space
Adequate working clearances must be maintained around transformers and their overcurrent devices for safe operation and maintenance.
2023 NEC §110.26
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Last updated: July 2026

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