Conduit Ampacity and Fuse Sizing on a Hot Commercial Roof
A 22 kWdc commercial rooftop system has four source circuits, each with Isc 13.9 A, combined into two combiner outputs. Each output is a 2-wire circuit, so four current-carrying conductors share one 1 in. EMT run supported 3 ft above a dark membrane roof. Roof ambient at the time of installation is 38°C. The installer pulled 8 AWG RHW-2 (90°C insulation) conductors, terminated them on 75°C-rated lugs, and the combiner is fitted with 15 A fuses. The inverter MPPT input is rated 30 A maximum. The installer's justification is: "8 AWG is good for 55 A and each circuit only carries 27.8 A, so we have plenty of headroom." The electrical inspector has not yet visited, and the crew is ready to close the combiner and energize.
1. Calculate the required conductor ampacity and the adjusted ampacity of the installed 8 AWG, and determine whether it is acceptable.
Read the worked answer
Required ampacity. Each combiner output carries two strings: 2 × 13.9 A = 27.8 A. The continuous-current factor is 1.25, so the minimum conductor ampacity is 27.8 × 1.25 = 34.8 A.
Adjusted ampacity of 8 AWG. The quoted 55 A is the 90°C base ampacity in free air at 30°C, before any correction - it is not the ampacity of this installation.
- Rooftop ambient adder for conduit 3 ft above the roof is about 25°C, so the design ambient is roughly 38 + 25 = 63°C.
- The 90°C correction factor at 63°C is approximately 0.67 (interpolated from the ambient correction table).
- Four current-carrying conductors in one conduit requires an 80% adjustment.
55 × 0.67 × 0.80 ≈ 29.5 A, which is below the required 34.8 A. 8 AWG fails.
Upsize. 6 AWG at 90°C is 75 A: 75 × 0.67 × 0.80 ≈ 40.2 A, above 34.8 A. Check the termination limit separately: 6 AWG at the 75°C rating of the lugs is 65 A, so terminations are not the weak point here. The run should be upsized to 6 AWG, or the circuits split or rerouted to reduce the rooftop adder.
2. Evaluate the 15 A combiner fuses and state the correct rating, including the checks that bracket it.
Read the worked answer
Minimum rating. Overcurrent protection for a PV source circuit is sized at not less than 1.25 × Isc: 1.25 × 13.9 A = 17.4 A. A 15 A fuse is below that, so it is undersized and will nuisance-blow on cold, bright, edge-of-cloud days when current exceeds Isc - an intermittent production complaint waiting to happen.
Correct rating. The next standard size above 17.4 A is 20 A; 25 A is also common. The chosen fuse must satisfy two brackets:
- It must not exceed the module's maximum series fuse rating on the datasheet (commonly 20 A or 25 A - verify against the actual module).
- It must be dc-rated at the system's maximum voltage (600 or 1000 Vdc as applicable) and installed in a holder rated for that voltage. AC fuses or automotive fuses are not acceptable.
Conductor interaction. The fuse also protects the conductor, so the conductor's adjusted ampacity must be at least the fuse rating in the relevant section of the circuit; that is another reason the 8 AWG result cannot stand. Document the module maximum series fuse rating and the fuse model number on the as-built, because this is a common inspection finding.
3. The installer insists the 55 A figure proves the design is fine. Write the technical response you would give, and note any other checks before energizing.
Read the worked answer
The 55 A number is real, but it is the wrong number. It is the base ampacity of 8 AWG with 90°C insulation in free air at 30°C, with no conduit and no rooftop adder. The correct path is: start with the 90°C column, apply the rooftop ambient correction for the actual mounting height, apply the conduit-fill adjustment for four current-carrying conductors, then cap the result at the termination temperature rating (75°C lugs). Doing that gives 29.5 A - below the 34.8 A required - so the conductor is undersized regardless of how the claim is phrased.
Other checks before energizing:
- Inverter input current: 27.8 A against a 30 A MPPT maximum leaves only about 8% headroom; confirm the combiner-to-inverter configuration matches the inverter's per-MPPT limits and that no future string addition is implied.
- Voltage drop: a long rooftop run at 22 kW can lose several tenths of a percent per 100 ft; confirm the design target is still met after upsizing.
- Terminations and torque: verify lugs are listed for the conductor class, torqued to spec, and that the combiner is rated for the ambient inside its enclosure, which on a dark roof often exceeds the outdoor air temperature.
Fix the conductor and the fuses, then request inspection - not the reverse.