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Solar Grounding Calculator

Enter system size, voltage, inverter type, and soil conditions — get EGC size (NEC 250.122), GEC size (NEC 250.166), bonding jumper, ground fault protection, and soil resistance pass/fail check.

System parameters
kW
V DC
circuits
ft
Site conditions
Solar Grounding Analysis (NEC 250 & 690)
System current (est.)16.7 A
OCPD rating20 A
EGC size (NEC 250.122)12 AWG copper
GEC size (NEC 250.166)8 AWG copper
Bonding jumper size (NEC 250.102)12 AWG copper
Ground fault protection1A (transformerless inverter GFDI per NEC 690.41)
Est. soil resistance17.7 Ω (PASS ≤ 25Ω)
NEC 250.52 electrode requirements
  • Metal underground water pipe (if available, first 10 ft in earth)
  • Ground rod — min 8 ft long, 5/8" copper or 3/4" galvanized
  • Ground ring (if installed) — min 2 AWG copper, min 20 ft
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How to Use This Calculator

Enter system size and voltage

Start with your PV system's DC capacity in kW and maximum system voltage. The calculator estimates system current, determines the appropriate overcurrent protection device (OCPD) rating, and sizes the equipment grounding conductor (EGC) per NEC Table 250.122. System voltage matters because the same kW at higher voltage means lower current — which can change EGC sizing.

Select inverter type and soil conditions

Inverter type determines the ground fault protection threshold per NEC 690.41. Transformerless string inverters require a 1-ampere Ground Fault Detection and Interruption (GFDI) device — this is built into modern transformerless inverters. Soil type significantly affects grounding electrode resistance; rocky or sandy soil may require multiple ground rods, a ground ring, or a Ufer (concrete-encased) electrode to meet the NEC 25-ohm requirement.

Read grounding conductor sizes

The results show EGC size per NEC 250.122, GEC size per NEC 250.166, bonding jumper size per NEC 250.102, estimated soil resistance, and whether a single electrode passes the 25-ohm requirement. Use these values directly in your permit drawings.

The Formula

System Current = (System kW × 1000 ÷ Vmax) × 1.25 safety factor OCPD = next standard size above System Current EGC Size = NEC Table 250.122 based on OCPD rating (copper) GEC Size = NEC 250.166 based on largest ungrounded conductor Bonding Jumper = same size as EGC (NEC 250.102) Ground Resistance (rod) = (ρ ÷ 2πL) × (ln(4L/d) − 1) where ρ = soil resistivity (Ω·m), L = rod length (m), d = rod diameter (m) Pass/Fail = Estimated resistance ≤ 25Ω (NEC 250.53(A)(2))

The 25-ohm resistance requirement comes from NEC 250.53(A)(2) — if a single ground rod cannot achieve 25 ohms, a supplemental electrode must be added. The Dwight formula used here is an engineering estimate; actual soil resistance should be measured with a fall-of-potential (3-point) test using a ground resistance tester before finalizing electrode design.

Example

Commercial 50kW system — concrete building, NEC 2020

A 50kW commercial rooftop system uses a 1000V transformerless string inverter with 8 source circuits and an EGC run of 150 feet. The building is new construction (Ufer ground available) with typical loam soil.

System current50kW ÷ 1000V × 1.25 = 62.5A
OCPD80A (next standard size above 62.5A)
EGC size (NEC 250.122)8 AWG copper (for 80A OCPD)

Result

EGC8 AWG copper
GEC (NEC 250.166)6 AWG copper
Bonding jumper8 AWG copper
Ground fault protection1A GFDI (transformerless)
Ufer ground resistance~3Ω — PASS (under 25Ω)

The Ufer ground (concrete-encased electrode) provides excellent performance at ~3 ohms — far below the 25-ohm requirement. For new commercial construction, specifying a Ufer ground during the concrete pour costs virtually nothing and provides a far superior, maintenance-free grounding electrode compared to ground rods driven into soil.

FAQ

Solar PV systems have unique grounding requirements because: (1) They operate at high DC voltages (600-1500V) which behave differently than AC for ground faults; (2) DC ground faults can sustain arcs that AC systems automatically extinguish at zero-crossing; (3) Module frames, racking, and DC equipment must all be bonded together to prevent floating voltages; (4) Transformerless inverters are more efficient but require Ground Fault Detection and Interruption (GFDI) because they don't have galvanic isolation between AC and DC circuits. NEC Article 690 supplements NEC Article 250 for PV-specific requirements.
A Ufer ground (named after Herbert Ufer) is a concrete-encased electrode — copper wire or rebar cast into the concrete foundation. It's preferred because: (1) Resistance is typically 1-5 ohms regardless of surface soil conditions; (2) Concrete retains moisture, maintaining low resistance even in drought; (3) Large surface area of the foundation provides excellent ground contact; (4) Once poured, it's maintenance-free for the building's life. NEC 250.52(A)(3) recognizes Ufer grounds as a required electrode type when concrete with embedded metal exists. If you're doing new construction, always install a Ufer ground — it's essentially free (just some wire and labor before the pour).
NEC 250.53(A)(2) requires that if a single ground rod cannot achieve a resistance to earth of 25 ohms or less, a supplemental electrode must be added. This is a maximum resistance threshold — lower is always better. Note: the NEC does not require you to measure resistance; it only requires a supplemental electrode if the single rod doesn't meet the standard. In practice, many installers simply add a second rod spaced 8 feet away (minimum) to be safe. If you're in rocky soil where rods can't achieve 25 ohms even with multiple rods, a ground ring, chemical ground rod, or Ufer is the solution.
Three distinct conductors: EGC (Equipment Grounding Conductor) — connects equipment frames, racking, and non-current-carrying metal parts to the grounding system. Sized per NEC 250.122 based on OCPD. Carries fault current if insulation fails. GEC (Grounding Electrode Conductor) — connects the system's grounding point to the grounding electrode (ground rod, Ufer, water pipe). Sized per NEC 250.166 for DC systems. Does not normally carry current. Bonding jumper — connects separate equipment sections together to ensure they're at the same potential. Sized same as EGC per NEC 250.102. A common installation error is using undersized bare copper wire for EGC when the OCPD requires a larger size — always size per Table 250.122.
Yes. Transformerless inverters (most modern string inverters are transformerless for efficiency) lack galvanic isolation between the DC PV circuit and the AC grid circuit. This means: (1) GFDI is required — NEC 690.41 mandates Ground Fault Detection and Interruption. Modern transformerless inverters have this built in, typically tripping at 300mA-1A; (2) Grounded DC circuits may not be used with transformerless inverters unless the inverter is specifically listed for use with a grounded conductor; (3) Module-frame grounding is critical because a frame-to-ground fault creates a direct connection to the AC system. Most modern UL 1741-listed transformerless inverters meet all these requirements — verify your inverter's listing before installation.

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