Crash course: Voltage, amperage, and resistance basics for solar installers (part 3)

Why Part 3 Focuses on Solar

Solar circuits behave differently than the 120/240V branch work most sparkies cut their teeth on. DC strings climb past 600V on residential and 1000V+ on commercial, current stays roughly constant once the sun hits the panel, and resistance shows up in places you do not expect, like undersized MC4 stubs and corroded lugs in a hot rooftop combiner.

If you do not have Ohm's law wired into muscle memory by now, solar will punish you. Voltage drop calcs get unforgiving when your home run is 180 feet across a flat roof, and a sloppy torque spec on a 30A string fuse can cook a combiner before the inverter even logs a fault.

Voltage: DC Strings vs AC Output

PV module nameplate gives you Voc (open circuit) and Vmp (max power). Voc is what NEC 690.7 makes you use for conductor and equipment ratings, adjusted for the coldest expected ambient using either Table 690.7(A) or the manufacturer's temperature coefficient. Cold mornings push Voc up, sometimes 15 to 20 percent over STC, and that is what trips inverter input limits or violates the 600V dwelling cap in 690.7(C).

On the AC side, you are usually landing at 240V single phase or 208/480V three phase. The interconnection rules in 705.12 dictate where you can land, and the 120 percent busbar rule still bites people on load side ties. Verify the main breaker rating, the busbar rating, and the proposed backfeed breaker before you ever bend a piece of EMT.

• Voc cold-corrected per 690.7(A) for conductor sizing and inverter input
• Vmp for production estimates and MPPT window verification
• AC nominal at the point of interconnection per 705.12
• Grounded vs ungrounded array per 690.41, most modern systems are functionally grounded

Amperage: Why the 1.25 Factors Matter

PV source and output circuit current is not just Isc off the label. NEC 690.8(A)(1) makes you take Isc times 125 percent for continuous duty, then 690.8(B) layers another 125 percent on top for conductor and OCPD sizing. That is the infamous 156 percent multiplier (1.25 x 1.25) for the wire and breaker, even though the inverter never actually pulls that.

On the AC inverter output, 690.8(A)(3) uses the inverter's rated continuous output current, again multiplied by 125 percent in 690.8(B). Do not back into this from kW divided by voltage, use the spec sheet number. Microinverter trunk cables have published max counts per branch based on this exact math, follow the manufacturer table and stop trying to be clever.

Field tip: when a string fuse keeps blowing on one combiner input but not the others, suspect a partially shorted module or a backfed string before you upsize the fuse. Upsizing past the module series fuse rating in 690.9(B) will void the listing and burn the panel.

Resistance: Voltage Drop and Connections

NEC 690.45 and Chapter 9 Table 8 are your friends. The unofficial industry target is 2 percent VD on DC strings and 1 percent on AC output, total system under 3 percent. On long roof runs, that often pushes you from 10 AWG PV wire to 8 or even 6 AWG before you ever leave the array.

Resistance also shows up at every termination. A loose MC4, an under-torqued lay-in lug at the combiner, or aluminum-to-copper without the right antiox compound will all show up as a hot spot on a thermal scan and a dead string by year three. Torque every connection to the manufacturer spec, mark it with a paint pen, and log it.

1. Calculate one-way distance from array to inverter, double it for round trip
2. Use Vmp and Imp at STC for the DC drop calc, not Voc/Isc
3. Check inverter MPPT low end after voltage drop is subtracted
4. Re-torque all field terminations at commissioning, not just at rough-in

Putting V, I, and R Together on a Real Job

Take a 20 module string of 400W panels at Vmp 33.5V, Imp 11.94A, Voc 40.2V, Isc 12.65A. String Voc cold-corrected at 14F ambient comes out around 46V per module, so 920V string Voc. That is a 1000V system, fine for commercial, dead on arrival for a one or two family dwelling under 690.7(C).

Conductor sizing on the source circuit: 12.65A x 1.56 = 19.7A minimum ampacity after derates. Run that through 690.31(C) conduit fill and 310.15(B) ambient correction for a rooftop in conduit less than 7/8 inch above the surface, and your 10 AWG USE-2 might not make it. Bump to 8 AWG or move the conduit off the roof deck.

Field tip: rooftop ambient adders from 310.15(B)(3)(c) were removed in the 2017 NEC, but most AHJs still want to see you account for actual rooftop temperatures. Check your local amendment before assuming the easy path.

Quick Reference Before You Pull Wire

Solar math is not hard, but it is unforgiving. The codes that bite hardest on inspections are 690.7 (voltage), 690.8 and 690.9 (current and OCPD), 690.45 (EGC sizing), and 705.12 (interconnection). Get those four right and most of the rest falls into place.

• Cold-correct Voc every time, no exceptions
• Use 156 percent for DC source and output conductor sizing
• Treat voltage drop as a design constraint, not an afterthought
• Torque, mark, and log every termination
• Verify 705.12 compliance before ordering the backfeed breaker