EGC Proportional Upsize for Increased Conductors (NEC 250.122(B))
When ungrounded conductors are increased in size for voltage drop or future capacity.
Example
You enter
- Base EGC area (cmil, per 250.122) 26240
- Base (minimum) phase area (cmil) 167800
- Installed phase area (cmil) 250000
You get
- Upsize ratio 1.4899
- Upsized EGC 39094
Checked against published worked examples from: IAEI Magazine (Keith Lofland); Mike Holt Enterprises.
Details, formula, and sources
ScopeThe equipment grounding conductor increases in the same circular-mil proportion per NEC 250.122(B). Base EGC and base phase area are user-supplied table values; the EGC need not exceed the ungrounded conductors. The AHJ governs.
Formula or tableratio = max(1, installed_phase_cmil / base_phase_cmil); upsized_EGC_cmil = base_EGC_cmil x ratio, then select the next standard AWG/kcmil at or above this. A ratio below 1 is clamped to 1 (the EGC is never reduced below its table size).
Source and editionNEC 2023 (NFPA 70). 250.122(B) (increase in size of equipment grounding conductors).
Free accessFree read-only access at nfpa.org/freeaccess.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesbase_egc_cmil, base_phase_cmil, installed_phase_cmil, ratio, upsized_egc_cmil
- User-supplied tables the base EGC size (from the 250.122 table for the OCPD) and the minimum-required phase area are user-supplied; the tile bundles neither tableNEC 2023 Table 250.122
- Upper bound the EGC need never exceed the ungrounded conductors it runs with; the AHJ governs the final sizeNEC 2023 250.122(B); the AHJ governs