Crash course: Voltage, amperage, and resistance basics for solar installers (part 1)

Why these three matter on a solar job

Voltage, amperage, and resistance drive every decision you make on a PV system, from conductor sizing to OCPD selection to grounding. Get them wrong and you cook conductors, trip inverters, or fail inspection under NEC Article 690. Get them right and the system runs at spec for 25 years.

Solar adds wrinkles you do not see on a standard service call. DC strings can sit at 600V or 1000V with no neutral, current rises with irradiance, and resistance shifts with temperature across a hot rooftop. The fundamentals do not change, but the conditions punish sloppy math.

This part covers the three quantities, how they relate, and where the NEC pins specific numbers to them. Part 2 will cover voltage drop, conductor ampacity, and array sizing calculations.

Voltage: the pressure

Voltage is electrical pressure measured in volts. It does not flow, it pushes. On the AC side you are dealing with 120/240V single phase or 208/480V three phase. On the DC side, a PV source circuit can run anywhere from roughly 30V on a microinverter branch to 1000V on a commercial string array.

NEC 690.7 governs maximum PV system voltage and requires you to calculate Voc adjusted for the lowest expected ambient temperature, not the STC nameplate value. Cold mornings spike Voc above the rated number, and that is the value that matters for conductor insulation rating, disconnect rating, and inverter MPPT window.

Field tip: Pull the record low temperature for the site's ASHRAE zone, apply the module's temperature coefficient of Voc, and document the corrected number on the plan set. Inspectors in cold climates ask for it.

Dwelling unit DC PV circuits are limited to 600V under NEC 690.7(C) unless specific conditions are met. One- and two-family dwellings with rapid shutdown compliance per 690.12 still need attention to accessible voltage limits.

Amperage: the flow

Amperage, or current, is the rate of electron flow measured in amps. This is what heats conductors and trips breakers. On a PV source circuit, the maximum current is Isc, and NEC 690.8(A)(1) requires you to multiply Isc by 1.25 to get the circuit current used for ampacity and OCPD calculations.

Then NEC 690.8(B) adds a second 1.25 multiplier for continuous operation when sizing the OCPD and conductors without temperature or fill adjustments. Stack them and you are at 156% of Isc. Miss either factor and the conductors are undersized for code.

• Source circuit current: Isc x 1.25 (690.8(A)(1))
• OCPD and conductor minimum: that result x 1.25 again (690.8(B))
• Inverter output circuit: rated continuous output current, also x 1.25
• Apply ampacity correction factors per 310.15 after the 156% calculation

For a 10A Isc module string, the ampacity-driving number is 15.625A, not 10A. Size your USE-2 or PV wire and your fuse holders accordingly.

Resistance: the pushback

Resistance is opposition to current flow, measured in ohms. In a PV system it shows up as conductor resistance, connector resistance, and module internal resistance. Resistance turns voltage into heat, which is exactly what you do not want in a junction box on a 140 degree roof.

Conductor resistance scales with length and inversely with cross-sectional area. NEC Chapter 9 Table 8 gives DC resistance per 1000 feet for copper and aluminum at 75C. For long DC home runs, this is the table you live in when calculating voltage drop.

Connector resistance is the silent killer. A loose MC4, a corroded lug, or a mismatched-brand connector pair can sit at fractions of an ohm but dissipate real watts at 10 to 15 amps. NEC 690.33(C) requires connectors of different types to not be intermateable, and field experience says even same-brand connectors need a torque check.

Ohm's law on the roof

V = I x R is the relationship that ties them together. Voltage equals current times resistance. Rearranged: I = V/R, R = V/I. Power follows: P = V x I, or P = I squared x R for resistive losses.

The I squared term is why voltage drop matters disproportionately on high-current circuits. Double the current, quadruple the heat dissipated in the conductor. This is also why high-voltage strings are more efficient over long runs, lower current for the same power means lower I squared R losses.

Field tip: When a string is underperforming, measure Voc open-circuit and Isc short-circuit at the combiner with the inverter off. Compare to the corrected nameplate values. A low Voc points to a shaded or failed module, a low Isc with normal Voc points to soiling or a connector issue.

What to carry in your head

Three numbers, three NEC hooks, one equation. Voc corrected for cold per 690.7. Isc multiplied by 1.25 twice per 690.8. Resistance from Chapter 9 Table 8 for voltage drop work. Ohm's law to sanity-check anything that looks off.

1. Voltage: pressure, governed by 690.7, watch cold-temperature Voc
2. Amperage: flow, governed by 690.8, apply both 1.25 factors
3. Resistance: pushback, Chapter 9 Table 8, check connectors and torque
4. Ohm's law: V = I x R, P = I squared x R for loss calculations

Part 2 will work through voltage drop calculations on a real string layout, ampacity adjustments for rooftop conduit per 310.15(B), and how these three quantities drive your OCPD selection at the combiner and the inverter output.