Crash course: Voltage, amperage, and resistance basics for solar installers (part 4)

Why Ohm's law still runs the job

Solar work looks different from service work, but the physics is identical. Voltage pushes, current flows, resistance fights back. Get the three straight in your head and every PV calc, from string sizing to voltage drop, falls out of one equation: V = I x R.

On a residential rooftop, you're typically dealing with 300-600V DC strings feeding an inverter, then 240V AC out. The DC side is where most installers get bit, because the rules differ from what you learned bending pipe on commercial AC jobs. NEC Article 690 governs PV systems specifically, and 690.7 and 690.8 are the ones you'll reach for most.

Voltage: open circuit vs operating

A PV module has two voltage values on its nameplate: Voc (open circuit) and Vmp (max power). Voc is what you measure with nothing connected. Vmp is what you'll see under load at standard test conditions. Voc is always higher, and Voc rises as temperature drops.

That cold-weather rise is what kills inverters and trips AHJ inspections. NEC 690.7 requires you to correct Voc for the lowest expected ambient temperature using either Table 690.7(A) or the manufacturer's coefficient. A string that reads 580V at 77F can hit 650V or more at -10F. If your inverter maxes at 600V DC input, you've cooked it.

• Voc: measured terminal to terminal, no load
• Vmp: measured under load at the maximum power point
• Vsystem: corrected Voc x number of modules in series, per 690.7
• Always size to the corrected cold value, not the nameplate

Amperage: Isc, Imp, and the 1.25 factor (twice)

Current on the DC side has two nameplate values too: Isc (short circuit) and Imp (max power). For conductor and OCPD sizing, NEC 690.8(A) tells you to use Isc, then multiply by 1.25 to account for irradiance above 1000 W/m^2. Then 690.8(B) makes you apply another 1.25 for continuous duty. That's the famous 156% factor (1.25 x 1.25 = 1.5625).

Skip either factor and your conductors run hot, your fuses nuisance trip, and your inspector hands the job back. The math isn't optional.

Field tip: Tape a sticky note inside your meter case with "Isc x 1.56" written on it. You will forget the second 1.25 at least once on a long day, and that note saves a callback.

Resistance: where your losses live

Resistance in a PV circuit comes from three places: the conductors themselves, every connection point, and the modules' internal resistance. You can't change the modules, but you absolutely control the other two.

Voltage drop on a long DC home run is the silent profit killer. NEC doesn't mandate a specific Vd limit for PV (690.45 covers EGC sizing, not Vd), but the informational note in 210.19 and good practice keep total Vd under 3% from array to inverter, 5% end to end. On a 400-foot home run at 10A and 400V DC, #10 copper drops about 4%. Bump to #8 and you're under 2%.

1. Calculate one-way distance from the combiner to the inverter
2. Use Vd = (2 x L x I x R) / 1000 for single-phase DC, where R is ohms per 1000 ft from Chapter 9 Table 8
3. If Vd exceeds 2%, upsize one wire gauge and recalculate
4. Verify the larger conductor still fits the terminal lugs on both ends

Putting it together on a real string

Say you've got 12 modules in series, each rated Voc 41.5V, Isc 10.2A, with a temperature coefficient of -0.29%/C. Lowest expected temp is -15C (a 40C delta from STC's 25C reference).

Corrected Voc per module: 41.5 x (1 + 0.0029 x 40) = 46.3V. String Voc: 46.3 x 12 = 555.6V. That fits a 600V inverter, but barely. Add one more module and you're over. For current, design Isc is 10.2 x 1.25 x 1.25 = 15.94A, so your string conductor needs at least 15.94A ampacity after temperature and conduit fill derating per 310.15.

Field tip: When you're string-sizing on the truck, round Voc correction up and ampacity derating down. You want headroom, not a fight with the inspector.

Common mistakes that fail inspection

Most red tags on PV jobs trace back to one of three errors: ignoring the cold-weather Voc correction, using Imp instead of Isc for OCPD sizing, or undersizing the EGC per 690.45. None of these are exotic. They're the basics, and they're the basics inspectors check first.

• Cold Voc not documented on the plan set
• OCPD calculated from Imp instead of Isc x 1.56
• EGC sized from 250.122 instead of 690.45 (PV uses a different rule)
• DC disconnect not within sight of the inverter per 690.15
• Rapid shutdown initiation device not labeled per 690.56(C)

Get voltage, current, and resistance right at the design stage and the rest of the install follows. Get one wrong and you're pulling wire twice.