Sizing service-entrance conductors
Service-entrance conductors must be sized for the full calculated load: noncontinuous load plus 125% of continuous load. The neutral must carry the maximum unbalanced load and be at least the size 250.24(D) requires.
What the code requires
Service-entrance conductors must have an ampacity at least equal to the maximum load they serve. The load comes from an Article 220 calculation (Part III, IV or V). Ampacity comes from 310.14 and must also meet the terminal temperature limits in 110.14(C).
Under 230.42(A)(1), the conductors are sized for the noncontinuous load plus 125% of the continuous load. Under (A)(2), their ampacity after any adjustment or correction factors must still cover the load. The conductors must satisfy whichever of the two is larger.
Under 230.42(B), the ungrounded conductors also need an ampacity at least equal to the minimum service disconnect rating in 230.79(A) through (D), for example 100 A for a one-family dwelling.
Under 230.42(C), the grounded (neutral) conductor must be at least the size 250.24(D) requires. Its load is the maximum unbalanced load calculated under 220.61.
On the job
- Continuous loads must be counted at 125%. Leaving that factor out undersizes the service-entrance conductors.
- For a single-phase 120/240 V dwelling service of 100 to 400 A that carries the whole dwelling load, conductors may be sized using 310.12, and new 240.4(H) lets them be protected at those ampacities.
Exceptions that matter
- Grounded (neutral) conductors that do not connect to an overcurrent device may be sized at 100% of the load.
- Where the conductors end in an overcurrent device, and the device and its assembly are both listed for 100% operation, the conductors may be sized at 100% of the load.
What changed in 2023
The main 2023 change is the neutral cross-reference in (C), which became 250.24(D) after 250.24 was re-lettered (it was 250.24(C) in 2020). Items (A)(1) and (A)(2) already existed in 2020 and only gained titles in 2023.
Related sections
- NEC 220.61
- NEC 230.79: Minimum service size, including 100 A for homes
- NEC 240.4(H)
- NEC 250.24: Grounding and bonding at the service
- NEC 310.12: Sizing service wires for a house: the 83% rule
- NEC 310.14
- NEC 110.14(C)
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