How to Size Service-Entrance Conductors and Feeder Cables?

Electrical Technology

Sizing of Service-Entrance Conductors, Feeders and Service Disconnecting Means

Service-entrance conductors for the service disconnect and main panel, as well as feeder cables for subpanels, must be properly sized in accordance with the National Electrical Code (NEC). Correctly sized service conductors in electrical wiring installations ensure system efficiency and protection for both personnel and equipment.

Below is a step-by-step guide to sizing these conductors based on NEC guidelines, including references to relevant codes and solved examples based on total calculated load.

Service-Entrance Conductors

According to NEC 100 – Definitions, service-entrance conductors are conductors that run between the utility transformer and main service equipment (such as the main panel or load center). These conductors may be service drop (overhead service conductors) or service-lateral (underground service conductors) wired from the utility transformer to the electric energy meter, main service disconnect and finally to the service point i.e. main panel.

In simple terms, the underground or overhead service conductor that runs from the utility pole to supply the main panel and associated load circuits in a dwelling or non-dwelling unit is known as the main service conductor.

Good to Know:

Service Entrance Conductors → Runs between transformer on utility pole to the main panel in distribution system.
Feeder Conductors → Runs between main panel to the subpanel (main lug). Feeder supplies power to a branch circuit overcurrent protection devices (OCPD).
Branch Circuit Conductors → Runs between breaker in the main or subpanel to the final load point.

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According to the NEC 230.42, the ampacity of service-entrance conductors must not less than the rating of the service disconnect. Similarly, according to the 215.2, feeder conductors shall have an ampacity not less than the larger of 215.2(A)(1) or (A)(2) and shall comply with 110.14(C). Moreover, the size of the service conductors must be sufficient to carry the load safely as calculated complying Article 220.

The rating of minimum-size main service disconnecting means for a residential one family dwelling is 100-amp (3-wires) – 230.79(C). The rating of service-conductors shall not be lower than specified in 230.79(A) to (D).

Service size for residential applications can be determined using two methods viz conventional method (Article 220 – Part III) and the optional method (Article 220 – Part IV. As we have already calculated the load size in amperes for load center, panelboards and subpanel in the previous articles (see ref links), we will stick to determine the size of service conductors for 120/240V main panel supplying load appliances in a home.

Related Post: How to Size Feeder Conductors with Overcurrent Protection

Service-Entrance Conductors Sizing

Service-entrance conductors are the cables that run between the utility connection and the main service panel. The size of these conductors is determined by the minimum ampacity requirement and factors like voltage drop, temperature rating, and conductor material.

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The minimum size of service conductors shall be based on NEC – 310.12 and Table 310.12(A). Service-entrance conductors must be sized based on the calculated load and meet the ampacity requirements of NEC Table 310.16 in accordance with 230.42.

For instance:

Single-Family Dwelling Main Service Conductors (NEC 310.12):

100A Service → Copper: #4 AWG, Aluminum: #2 AWG
125A Service → Copper: #2 AWG, Aluminum: #1/0 AWG
150A Service → Copper: #1 AWG, Aluminum: #2/0 AWG
200A Service → Copper: #2/0 AWG, Aluminum: #4/0 AWG
400A Service → Copper: 400 kcmil, Aluminum: 600 kcmil

Note:

⚡NEC 310.12 allows the use of these reduced conductor sizes for single-phase residential service entrance conductors due to the nature of residential loads.
⚡For service conductors, follow NEC 250.24(A) and table 250.66. for grounding electrode conductor sizing.

Example 1:

The total calculated load of a 1500 ft² (139.35 m²) residential unit is 150-amp. What is the required service entrance cable for copper and aluminum?

Solution:

The step by step load calculation for 120V/240V load center/main panel for a dwelling unit is given which is 150-amp. The size of the service entrance can be determined using Table 310.12(A) which is #1 AWG copper or #2/0 AWG aluminum or Copper-clad aluminum.

For this 3-wire supplying 150-amp load, we have to find the wire size of equipment grounding conductor (EGC) and grounding electrode conductor (GEC). To do that, refer to the Table 250.66 to determine the size of GEC which is is #6 AWG copper or #4 AWG aluminum or copper-clade aluminum for #1 AWG copper or #2/0 AWG aluminum service supplying a 150-amp main panel. Similarly, the size of equipment grounding conductor (EGC) can be calculated based on OCPD rating based on Table 250.122 – NEC.

Related Post: How to Find the Right Wire Size for 100A Service 120V/240V Panel?

Example 2:

The required size of main breaker in the main panel is 200-amp supplying a 60-amp subpanel. What will be the suitable size of service entrance and feeder conductors?

Solution:

Based on the determined size of 200-amp main breaker for a dwelling unit, the required size of service entrance is 2/0 AWG copper THWN or 4/0 AWG aluminum (NEC – Table 310.12(A) for Sizing Service/Feeders).

Similarly, the size of feeder to the 60-amp subpanel is #6 AWG copper (Table – 310.16) with 10 AWG ground (Table 250.66 for GEC). If the distance between main and subpanel exceeds 140 feet (42 meters), use 4 AWG wire for feeder instead of 6 AWG. When distance involved, the recommended ground wire size would be #8 AWG.

In addition, the required size of grounding electrode conductor (GEC) for the 200-amp panel feeding 60-amp main lug in the same dwelling unit is #4 AWG copper or #2 AWG aluminum or copper-clade aluminum – Table 250.66 for GEC.

Related Post: How to Find the Proper Size of Wire & Cable in Metric & Imperial Systems

Warning:

Table 310.12(A) shall only be applicable for service-entrance conductors used for single phase 120/240V 3-wire supply in a residential unit. This may also be used for underground service conductors, and feeder conductors that serve as the main power feeder to a dwelling unit.
Table 310.12(A) should not be used for feeder from main panel to the subpanel due to the diversity factor because the specific feeder carries only part of the load in the dwelling unit. Instead, use NEC Table 310.16 for sizing feeder conductors.
Apply the correction factors in NEC Tables 310.15(B)(1)(1) and Table 310.15(8)(1)(2) where the conductors will likely to expose to temperatures in excess of 86°F (30°C).
Apply the temperature correction, demand factor and conduit fill when sizing feeder and Service entrance conductors. If the ambient temperature exceeds 30°C (86°F), adjust ampacity using Table 310.15(B)(1)(1). Similarly, If multiple conductors are in a conduit, apply 310.15(C)(1) adjustment factors.
NEC recommends a maximum voltage drop of 3% for feeders and 5% total for service + feeder. Similarly, if distance involved, add 20% additional ampacity for every 100 feet to counter voltage drop in long-distance runs (over 50 feet or 15.25 meters) – Table – 310-16.

Sizing of Service-Entrance Conductors, Feeders and Service Disconnecting Means

Service-Entrance Conductors

Service-Entrance Conductors Sizing

Example 1:

Example 2:

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