How to Use the Grounding Conductor Sizer

1. Choose the Grounding Conductor Type — select Equipment Ground (EGC) if you are grounding metal enclosures, racking, or frames. Select Electrode Ground (GEC) if you are running a wire to a ground rod or concrete-encased electrode.
2. Select the Input parameters — for EGC calculations, pick the rating of the overcurrent protective device (OCPD) in Amps. For GEC calculations, select the size and material of the service entrance conductors.
3. Select the Ground Conductor Material — choose Copper (Cu) or Aluminum (Al). Since copper conducts better than aluminum, the required wire size for a copper ground wire will be smaller.
4. Verify the Output Sizing — the calculator displays the minimum required AWG wire size, references the specific NEC Table utilized, and details the input parameters.

Why Grounding Conductor Sizing Matters for Solar Installers

Grounding and bonding are the cornerstones of electrical safety in solar PV systems. A properly sized equipment grounding conductor (EGC) provides a safe, low-impedance path for fault currents to return to the source, tripping the overcurrent protective device (breaker or fuse) and clearing the fault. In rooftop solar arrays, metal racking, PV module frames, and metal conduits are all exposed to the elements and potential physical damage. If a hot conductor faults to a metal rack that is poorly grounded or lacks a properly sized EGC, the entire racking system can become energized, posing a fatal shock hazard.

NEC Article 250 governs grounding and bonding requirements. EGC sizing is dictated by Table 250.122, while Grounding Electrode Conductor (GEC) sizing is governed by Table 250.66. Grounding electrode conductors connect the main electrical panel (or solar service disconnect) to the earth via a ground rod, water pipe, or Ufer ground, protecting the entire system from lightning surges and high-voltage line spikes.

One critical rule solar designers often overlook is NEC 250.122(B). If the ungrounded circuit conductors are upsized to compensate for voltage drop on long conduit runs (common in ground mounts or remote arrays), the equipment grounding conductor must also be upsized proportionally. Failing to verify grounding sizes leads to immediate inspection failures and delays solar permission-to-operate (PTO).

Worked Example: Grounding a Commercial Solar Subpanel

A solar contractor is installing a commercial solar subpanel. The subpanel is fed from a main distribution board with a 200A circuit breaker. The contractor needs to size the copper Equipment Grounding Conductor (EGC) that will be routed inside the conduit feeding the subpanel.

Inputs: Grounding type = EGC, OCPD Rating = 200 Amps, Ground conductor material = Copper.

Calculation: Using NEC Table 250.122, we find the row corresponding to a 200 Amp overcurrent device rating. For a 200A OCPD, the minimum size copper equipment grounding conductor required is #6 AWG. (If aluminum was used, the minimum size would be #4 AWG).

Result: The installer must run a #6 AWG copper EGC inside the conduit. If the distance between the main panel and the solar subpanel was long and required upsizing the phase conductors to #3/0 AWG instead of the standard #2/0 AWG to combat voltage drop, the #6 AWG EGC would also have to be upsized proportionally using the circular mil calculation method under NEC 250.122(B).

Frequently Asked Questions