Solar Panel Grounding & EGC Size Calculator (NEC)

How to use this calculator

Enter the rating of the overcurrent device protecting the circuit, pick copper or aluminum, and the calculator returns the minimum equipment grounding conductor (EGC) size from NEC Table 250.122. The same value is also used for the module-frame bonding jumper that links each row of panels back to the array EGC.

Inputs:

1. DC overcurrent device rating (A) — this is the fuse or breaker that protects the source or output circuit. For a typical 600 V residential string it’s the touch-safe DC fuse in the combiner (often 15 A or 20 A). For the array-to-inverter run on the AC side of a microinverter system, it’s the 20 A or 60 A branch breaker at the main panel.
2. EGC material — copper is standard for solar work. Aluminum is allowed at 6 AWG and larger but only used on long ground-mount runs where the cost savings on big trenches justify the extra size.

The result follows NEC Table 250.122 — the canonical EGC sizing table referenced by 690.45 for PV systems.

How the math works (NEC method)

Table 250.122 is a direct lookup, not a calculation. The threshold ampere ratings are:

OCPD ≤ 15 A      → 14 AWG Cu / 12 AWG Al
OCPD ≤ 20 A      → 12 AWG Cu / 10 AWG Al
OCPD ≤ 60 A      → 10 AWG Cu /  8 AWG Al
OCPD ≤ 100 A     →  8 AWG Cu /  6 AWG Al
OCPD ≤ 200 A     →  6 AWG Cu /  4 AWG Al
OCPD ≤ 300 A     →  4 AWG Cu /  2 AWG Al
OCPD ≤ 400 A     →  3 AWG Cu /  1 AWG Al
OCPD ≤ 500 A     →  2 AWG Cu / 1/0 AWG Al
OCPD ≤ 600 A     →  1 AWG Cu / 2/0 AWG Al

The exception in 250.122(B): if the ungrounded conductor is upsized for voltage drop or any other reason, scale the EGC by the same circular-mil ratio.

Worked example. A 7.6 kW string inverter has two MPPT inputs at 12 A Imp each, protected at 20 A. Table 250.122 says 12 AWG Cu. But the array is on a detached garage 150 ft from the main panel, and the AC output (40 A back-fed breaker) was upsized from 8 AWG to 6 AWG copper to keep voltage drop under 2% on the AC run. Circular-mil ratio: 26,240 / 16,510 = 1.59. The EGC starts as 10 AWG Cu (10,380 cmil) for the 40 A breaker per Table 250.122, then upsizes proportionally to 10,380 × 1.59 = 16,500 cmil. That’s at least 8 AWG Cu (16,510 cmil) — round up to the next standard size.

NEC 690 section walkthrough

NEC Article 690 covers PV systems. The grounding sub-sections you actually use:

• 690.41 — at least one functional ground or PV ground-fault device for any system over 50 V (effectively all residential).
• 690.43 — equipment grounding for module frames, racks, combiner boxes, inverters and the grounded conductor (if any) of a 2-wire DC source.
• 690.45 — sizing of the EGC. Same as 250.122 except no extra “in-kind reduction” for parallel conductors.
• 690.47 — connection to grounding electrode system. Since the 2017 cycle there’s no separate auxiliary electrode required (former 690.47(D) deleted).
• 690.50 — module frame and rack bonding hardware must be listed to UL 2703.

Most jurisdictions are on the 2020 or 2023 NEC. Check with your AHJ before ordering hardware — wildfire-prone CA counties sometimes add local amendments requiring a #6 Cu bare conductor visible from the meter to the array regardless of OCPD.

Module-frame bonding hardware

Anodized aluminum module frames are not conductive end-to-end out of the box — the oxide layer is an insulator. To get a UL-listed continuous ground path you have to bite through it with one of three methods:

1. Toothed washers / bonding clips (WEEB, ILSCO GBL-4DBT, S-5! Mini Clamp) — stainless teeth bite into the aluminum frame. Cheapest option at $0.40-$0.80 per module. Use WEEB-9.5 for typical 40-mm frames; the WEEB number is the frame thickness in mm × 10.
2. Integrated grounding rail (IronRidge XR1000, Unirac SolarMount, K2 Speedrail) — the rail itself has a serrated grounding pin built in. No extra hardware per module, but you’re locked to that manufacturer.
3. Discrete lugs and bare 6 AWG copper jumper (ILSCO GBL, Burndy BGBL) — old-school but still UL-listed. Required when mixing brands of rails and modules without a verified WEEB compatibility test.

Whatever you pick, the part has to be listed to UL 2703 for the exact frame profile and rail you’re using. Most module manufacturers publish a compatibility matrix — Canadian Solar HiKu7, JinkoSolar Tiger Neo, REC Alpha Pure, Q.PEAK DUO ML-G11 all have published WEEB compatibility lists.

Voltage-drop interaction with EGC sizing

The 250.122(B) upsize rule trips up most DIY solar installs. The rule of thumb:

If you upsize current-carrying conductors → upsize EGC by same area ratio

A 1.4× area ratio (one full AWG size larger) bumps the EGC one full size in almost all cases. If you’ve already upsized for VD, you’ve probably already done the math; just round the EGC up to the next AWG and you’ll always be at or above compliance. The solar panel voltage calculator on this site handles the VD math.

Bonding the inverter, combiner and DC disconnect

The full chain of bonded enclosures from rooftop array to service equipment looks like this:

Module frames
   └─ bonded via WEEB / lug to rack
        └─ rack bonded to EGC at one end (UL 2703 listed lay-in lug, e.g. Wiley WEEB-Lay-IN-3)
             └─ EGC into combiner box ground bar
                  └─ EGC into DC disconnect ground bar
                       └─ EGC into inverter chassis ground (DC side)
                            └─ EGC into AC disconnect / inverter AC chassis
                                 └─ EGC into main panel ground bar
                                      └─ bonded to GEC and service grounding electrode

Every transition point gets a listed lug, torqued to spec, and verified with a continuity test before energizing. Don’t rely on raceway threads or set-screw connectors as the only ground path — code requires a bonding bushing or grounding-type fitting at every transition to and from non-metallic enclosures.

When to use a ground rod at the array

Pre-2017 NEC required an auxiliary ground rod within 8 ft of the array. The 2017 cycle removed that (690.47(D) deleted because it created two separated grounding electrodes that could carry surge current). The current best practice:

• Roof arrays: no array-side rod. EGC runs back to the existing service grounding electrode system at the main panel.
• Ground-mount arrays close to the service (< 50 ft): no separate rod. Single EGC back to the service.
• Detached structures or remote ground-mounts (> 50 ft, or on a sub-panel): NEC 250.32 applies — the subpanel grounding electrode rule kicks in. Drive a ground rod and bond it to the subpanel ground bar, but the EGC still runs back to the main service.

In wildfire AHJ zones (Cal Fire jurisdictions, Maui County after 2023) — some local amendments still call for an array-side rod regardless of NEC. Check the local solar permit checklist.

Common code violations

The five most-common grounding write-ups from rooftop solar inspections (per a 2024 survey of California municipal AHJs):

1. EGC not upsized after ungrounded conductor was upsized for VD (NEC 250.122(B)).
2. Module-frame bonding hardware not UL 2703 listed for the specific frame profile.
3. Mid-clamp / end-clamp used for bonding without a listed grounding mid-clamp.
4. Discontinuous EGC at a metallic raceway-to-non-metallic transition (no bonding bushing).
5. Aluminum EGC terminated in a copper-only lug (NEC 110.14).

All five are caught by a continuity test from the farthest module frame back to the service ground — under 1.0 Ω end-to-end is the field-acceptance threshold most installers use.

Related calculators

• Solar panel wire size calculator — sizes the ungrounded conductors that determine your OCPD
• Solar panel voltage calculator — voltage drop on the same conductors that drives 250.122(B)
• Solar string sizing calculator — sets the OCPD that drives EGC size

Sources

• NFPA NEC 2023 Article 250 + Article 690 — primary code reference
• UL 2703 Outline of Investigation — module-frame bonding hardware listing
• NREL — Best Practices for Grounding in Photovoltaic Systems — design guidance
• SEIA / SunSpec — Rapid Shutdown and Grounding Compliance Guide 2024 — industry interpretation of 690.45 and 690.12
• Mike Holt — Understanding the NEC 250.122 Updates — code educator reference