Size the system from the loads backward.

Twelve AWG copper, protected by a 20 A breaker. NEC Table 310.16 gives 12 AWG a 60 °C ampacity of 20 A, and NEC 240.4(D) caps its overcurrent protection at 20 A no matter what the table says in the warmer columns. Fourteen AWG is 15 A and is not permitted on a 20 A circuit. On runs longer than about 100 feet at 120 V, voltage drop can push you to 10 AWG even though the ampacity is fine.

The one the terminations are rated for, not the one the wire insulation is rated for. NEC 110.14(C) says that for equipment rated 100 A or less — which is most residential work — you use the 60 °C column, and above 100 A the 75 °C column, unless the equipment is specifically listed for a higher temperature. This surprises people: THHN insulation is rated 90 °C, but you almost never get to use the 90 °C column. That column is mostly there as the starting point for derating calculations.

A load that runs for three hours or more at a stretch: lighting circuits in a commercial space, EV chargers, some HVAC equipment. NEC 210.19(A) requires the conductor and 210.20(A) the breaker to be sized at 125% of that load. The reason is thermal: breakers and terminations are tested for a short duration, and a circuit sitting at its rating for hours reaches a higher steady-state temperature than the test assumed. A 40 A EV charger is therefore a 50 A circuit.

Voltage drop equals 2 × K × I × L ÷ CM, where K is 12.9 for stranded copper at 75 °C, I is the current in amps, L is the one-way length in feet, and CM is the conductor area in circular mils. Twenty-four amps over 100 feet on 10 AWG (10,380 cmil) is 2 × 12.9 × 24 × 100 ÷ 10,380, or 5.96 volts — 2.5% on a 240 V circuit and a rather unpleasant 5% on a 120 V one. That asymmetry is why long runs are worth doing at 240 V wherever the load allows.

It is in the NEC as an informational note to 210.19(A) and 215.2(A), which means it is a recommendation rather than an enforceable requirement — a note is not a rule. In practice, treat it as one. Below the recommended voltage, incandescent lighting dims noticeably, electronics run their supplies harder, and motors draw more current to deliver the same shaft power, which heats the windings. Well pumps and compressors on undersized long runs are one of the most common causes of premature motor failure.

The breaker protects the conductor, so it must not exceed the conductor ampacity in the applicable column, and for 14, 12 and 10 AWG it is hard-capped by NEC 240.4(D) at 15, 20 and 30 A. Above 10 AWG, standard sizes from 240.6(A) apply, and 240.4(B) lets you round up to the next standard size when the ampacity does not land on one, for circuits of 800 A or less that do not supply receptacles. Never install a bigger breaker to stop nuisance tripping — that is exactly the failure mode the breaker exists to prevent.

Divide your monthly kWh by what one panel produces in a month. One 400 W panel at 4.5 peak sun hours and 80% system efficiency yields 0.4 × 4.5 × 30 × 0.8 = 43.2 kWh a month. A 900 kWh bill therefore needs 21 panels, which is about 8.4 kW of array and roughly 500 square feet of unshaded south-facing roof. Peak sun hours are the variable that moves the answer most: the same house in Phoenix needs about a third fewer panels than in Seattle.

South, at a tilt roughly equal to your latitude. This is worth stating because sizing tools written for the southern hemisphere say north, and the guidance flips exactly. West-facing arrays produce less total energy but shift it into the late afternoon, which pays better under time-of-use rates in some markets. East-west split arrays flatten the production curve. Any of these beats a shaded south roof.

The Residential Clean Energy Credit under IRC section 25D is 30% of the total installed cost, including equipment, labor, permitting and battery storage, claimed against your federal income tax liability in the year the system is placed in service. It is a credit, not a deduction, and it is non-refundable but can be carried forward. State, utility and local incentives stack on top of it and vary enormously. Tax rules change, so confirm the current terms with the IRS or a tax professional before you count on a number.

It lowers the apparent power, in kVA, that the service has to carry for the same real work in kW. A 50 kW plant at 0.75 power factor pulls 66.7 kVA; corrected to 0.95 it pulls 52.6 kVA. That 14 kVA is transformer, cable and breaker capacity freed up without touching a conductor, and it removes any utility power factor penalty. What it does not do is reduce your kWh consumption — the real power is unchanged, which is why this never pays back on a residential meter.

Because capacitive reactance depends on it. The capacitance needed is the reactive power divided by two pi times frequency times voltage squared, so at the US 60 Hz you need about 17% less capacitance than the same correction at 50 Hz. A capacitor sized from a European or Latin American table and installed on a 60 Hz service will overcorrect. The kVAR rating stamped on a capacitor is also frequency-specific for the same reason.

Not cleanly, because AWG is a geometric series and metric sizes are round numbers of square millimeters. The near equivalents are: 14 AWG is 2.08 mm² against a 2.5 mm² metric size, 12 AWG is 3.31 mm² against 4 mm², 10 AWG is 5.26 mm² against 6 mm², 8 AWG is 8.37 mm² against 10 mm², and 6 AWG is 13.3 mm² against 16 mm². In every case the metric size is the larger one, so substituting AWG for a metric spec without checking ampacity undersizes the conductor.