NEC EGC Sizing Calculator (Equipment Grounding Conductor)

Quickly determine the minimum equipment grounding conductor (EGC) size based on NEC-style tables using either the overcurrent device rating or the phase conductor size.

Design aid only – always verify against the latest adopted NEC and local amendments.

EGC sizing tool

Mode 1 – From OCPD rating

Enter the breaker/fuse rating in amperes. The tool returns the base EGC size from a NEC-style table.

Mode 2 – From conductor upsizing

Compare minimum vs. actual phase conductor size to get a proportional increase factor for upsizing the EGC (NEC 250.122(B)).

Values are based on a NEC-style Table 250.122 structure. Always confirm with the actual Code table for your edition.

How this NEC EGC sizing calculator works

This tool is designed as a fast design aid for sizing equipment grounding conductors (EGCs) in accordance with the structure of NEC 250.122. It does not replace the Code; always verify results against the latest NEC edition and local amendments.

1. Sizing from the overcurrent device (Table 250.122 concept)

The primary method in the NEC is to size the EGC from the rating of the overcurrent protective device (OCPD) that protects the circuit conductors. The calculator:

  • Takes the OCPD rating in amperes (breaker or fuse).
  • Finds the next higher value in a NEC-style table range.
  • Returns the minimum copper or aluminum EGC size in AWG or kcmil.

Conceptual rule (simplified):

If OCPD rating ≤ 15 A → EGC ≥ 14 AWG Cu (or 12 AWG Al)

If 15 < OCPD ≤ 20 A → EGC ≥ 12 AWG Cu (or 10 AWG Al)

If 20 < OCPD ≤ 60 A → EGC ≥ 10 AWG Cu (or 8 AWG Al)

…and so on, following the structure of NEC Table 250.122.

2. Proportional increase when phase conductors are upsized

NEC 250.122(B) requires that if ungrounded (phase) conductors are increased in size for any reason other than correction or adjustment factors, the EGC must be increased in size proportionally.

The calculator helps you compute a simple ratio:

Proportional increase factor

Let \( A_\text{min} \) be the cross-sectional area of the minimum phase conductor (for the load), and \( A_\text{act} \) be the area of the actual upsized phase conductor.

Then the upsizing factor is: \[ k = \frac{A_\text{act}}{A_\text{min}} \]

The EGC cross-sectional area should be multiplied by \( k \). In practice, you select the next larger standard conductor size that meets or exceeds this area.

The tool uses typical cross-sectional areas for common AWG/kcmil sizes to estimate this factor and shows you the required increase relative to your base EGC size.

3. Typical use cases

  • Branch circuits and feeders in commercial and industrial facilities.
  • Service equipment bonding jumpers and feeder EGCs (with appropriate NEC articles).
  • Checking field designs or shop drawings for compliance with 250.122 and 250.122(B).

Worked examples

Example 1 – 225 A breaker, copper EGC

  1. Select Size from OCPD mode.
  2. Set EGC material to Copper.
  3. Enter OCPD rating = 225 A.
  4. Click Calculate EGC size.

The calculator will map 225 A into the appropriate table range and return a minimum copper EGC size (for example, 4 AWG Cu in a typical NEC table structure). Confirm this against your actual NEC table.

Example 2 – Upsizing phase conductors for voltage drop

  1. Switch to Conductor upsizing factor mode.
  2. Set phase material to Copper.
  3. Select minimum phase size = 3 AWG Cu (for the load).
  4. Select actual phase size = 1/0 AWG Cu (upsized for voltage drop).
  5. Set base EGC size from table (e.g., 6 AWG Cu).
  6. Click Apply factor.

The tool computes the ratio of areas between 1/0 and 3 AWG and shows the proportional increase factor. You then choose the next larger standard EGC size whose area meets or exceeds that requirement.

Limitations and assumptions

  • The table values are representative and simplified for calculator use.
  • Parallel raceways, special conditions, and specific equipment rules are not fully modeled.
  • Ambient temperature, conductor bundling, and derating are not applied to the EGC itself.
  • Local codes, standards, and AHJ interpretations may differ.

Always consult the actual NEC articles (especially 250.4, 250.118, 250.122, 250.134, 250.146) and any local amendments, and coordinate with the authority having jurisdiction (AHJ) for final approval.

Frequently asked questions

What is an equipment grounding conductor (EGC)?

An equipment grounding conductor is the conductor that connects non–current-carrying metal parts of equipment, raceways, and enclosures to the grounding system. Its purpose is to provide a low-impedance path for ground-fault current so that the overcurrent device opens quickly and clears the fault.

Which NEC table does this calculator follow?

The sizing logic follows the structure of NEC Table 250.122 for copper and aluminum EGCs. Because jurisdictions adopt different NEC editions and may have amendments, the values here are for design guidance only. Always verify against the actual table in the Code book you are required to use.

Do I always need to upsize the EGC when I upsize the phase conductors?

Only when the phase conductors are increased in size for reasons other than correction or adjustment factors (for example, for voltage drop). In that case, NEC 250.122(B) requires the EGC to be increased proportionally. If you increase conductor size solely to meet ampacity after applying correction/adjustment factors, the EGC does not have to be upsized beyond what Table 250.122 requires for the OCPD rating.

Can I use this calculator for service-entrance conductors?

The general principles are similar, but service-entrance bonding jumpers and grounding electrode conductors have their own specific NEC tables and rules (e.g., 250.66). This tool is focused on equipment grounding conductors sized from 250.122 and should not be used directly for grounding electrode conductors.

Does the calculator consider fault current levels?

No. NEC EGC sizing by Table 250.122 is based on the OCPD rating, not on a detailed fault-current calculation. For critical systems or very high available fault currents, engineers may perform additional checks to ensure the EGC withstands thermal and mechanical stresses during faults.