Power Cabling

NEC-compliant conductor sizing for all system cable runs. Wire gauge selection balances code compliance, voltage drop, future expansion, and cost.

Building Layout

Panels: South-facing roof (4× Q.TRON BLK M-G2+ 430W, 4S1P string)First floor: ~10 ft ceiling heightBasement: 12 ft walls, ~30 ft wide (south to north)Main panel: North wall of basementVertical run: ~25 ft (roof penetration → basement floor)Horizontal run: ~30 ft (across basement)

Design decision: Where you place the inverter + batteries determines whether the long cable run is DC (lower voltage, more loss) or AC (higher voltage, less loss). Both options are analyzed below.

Inverter Placement Comparison

Placement	DC Run	AC Run	Battery	Notes
Option A: South Wall	32 ft	37 ft	5 ft	Inverter + batteries on south basement wall, near roof cable entry point
Option B: North Wall	62 ft	5 ft	5 ft	Inverter + batteries on north wall, adjacent to main electrical panel

DC String Cables (Panels → Inverter)

4S1P string: 131.76 V at 13.05 A (Vmp/Imp). Isc = 13.74 A. NEC 690.8(B) requires conductor ampacity >= 17.2 A (13.74 A × 1.25). Target voltage drop < 2%. Cable type: USE-2 / PV Wire (UL 4703), rated 90°C wet/dry.

Option A: South Wall — DC Run: 32 ft one-way

AWG	Ampacity	V Drop	V Drop %	Power Loss	Cable Cost	Status
14 AWGmin	25 A	2.62 V	1.99%	34.2 W	$19	PASS
12 AWGexpansion	30 A	1.65 V	1.26%	21.6 W	$29	PASS
10 AWG	40 A	1.04 V	0.79%	13.5 W	$51	PASS
8 AWG	55 A	0.65 V	0.49%	8.5 W	$67	PASS
6 AWG	75 A	0.41 V	0.31%	5.4 W	$96	PASS

Option B: North Wall — DC Run: 62 ft one-way

AWG	Ampacity	V Drop	V Drop %	Power Loss	Cable Cost	Status
14 AWG	25 A	5.08 V	3.86%	66.3 W	$37	FAIL
12 AWG	30 A	3.20 V	2.43%	41.8 W	$56	FAIL
10 AWGmin	40 A	2.01 V	1.52%	26.2 W	$99	PASS
8 AWGexpansion	55 A	1.26 V	0.96%	16.4 W	$129	PASS
6 AWG	75 A	0.79 V	0.60%	10.4 W	$186	PASS

DC recommendation: 10 AWG USE-2 PV Wire is the minimum code-compliant size for either placement. For future expansion headroom (second string, longer runs), step up to 8 AWG — the cost difference is ~$0.24/ft more. WindyNation 8 AWG PV Wire — 40 ft pair: $82.88

AC Output Cables (Inverter → Main Panel)

EG4 6000XP output: 240 V split-phase, 6000 W = 25.0 A. NEC continuous: 31.3 A. Target voltage drop < 3%. Cable type: THWN-2 in conduit. Requires 2 hots + 1 neutral + 1 ground (4-wire for 240V split-phase).

Option A: South Wall — AC Run: 37 ft one-way

AWG	Ampacity	V Drop	V Drop %	Power Loss	Cable Cost	Status
12 AWG	30 A	3.66 V	1.53%	91.6 W	$30	FAIL
10 AWG	40 A	2.29 V	0.96%	57.4 W	$44	PASS
8 AWG	55 A	1.44 V	0.60%	36.0 W	$74	PASS
6 AWG	75 A	0.91 V	0.38%	22.7 W	$104	PASS

Option B: North Wall — AC Run: 5 ft one-way

AWG	Ampacity	V Drop	V Drop %	Power Loss	Cable Cost	Status
12 AWG	30 A	0.49 V	0.21%	12.4 W	$4	FAIL
10 AWG	40 A	0.31 V	0.13%	7.8 W	$6	PASS
8 AWG	55 A	0.19 V	0.08%	4.9 W	$10	PASS
6 AWG	75 A	0.12 V	0.05%	3.1 W	$14	PASS

AC recommendation: 8 AWG THWN-2 handles either placement with ample ampacity (55 A vs 31.25 A required) and minimal voltage drop. For the short Option B run, even 10 AWG works fine. 240V split-phase needs 4 conductors (2 hot + neutral + ground). Amazon — 8 AWG THHN 4-color × 50 ft bundle: ~$226 or Home Depot Cerrowire — $48–53 per 50 ft roll

Battery Cables (Inverter → Battery Bank)

2× ECO-WORTHY Cubix 100 at 51.2 V, 100 A max combined (50 A each). NEC continuous: 125 A. Short run (~5 ft). Target voltage drop < 1% (low-voltage DC is very sensitive to loss).

Current Config: 2 Batteries (100 A, 125 A NEC)

AWG	Ampacity	V Drop	V Drop %	Power Loss	Cable Cost	Status
4 AWG	95 A	0.308 V	0.60%	30.8 W	$25	FAIL
2 AWG	130 A	0.194 V	0.38%	19.4 W	$40	PASS
1/0 AWG	170 A	0.122 V	0.24%	12.2 W	$55	PASS
2/0 AWG	195 A	0.097 V	0.19%	9.7 W	$69	PASS

Expansion: 3 Batteries (150 A, 188 A NEC)

AWG	Ampacity	V Drop	V Drop %	Power Loss	Cable Cost	Status
4 AWG	95 A	0.462 V	0.90%	69.3 W	$25	FAIL
2 AWG	130 A	0.291 V	0.57%	43.6 W	$40	FAIL
1/0 AWG	170 A	0.183 V	0.36%	27.4 W	$55	FAIL
2/0 AWG	195 A	0.145 V	0.28%	21.8 W	$69	PASS

Battery recommendation: 2/0 AWG battery cable. It handles the current 2-battery setup (100 A) with excellent headroom and remains code-compliant if you add a 3rd battery (150 A). WindyNation 2/0 AWG 5 ft pair with lugs: $69.38 — the cost premium over smaller gauges is negligible compared to the risk of undersizing these high-current, low-voltage runs.

Grounding

NEC 690.43 requires an equipment grounding conductor (EGC) for all exposed metal. NEC 250.122 sizes the EGC based on the overcurrent protection device (OCPD) rating.

Circuit	OCPD	Min EGC (Cu)	Notes
DC string	20 A fuse	12 AWG bare or green	NEC 250.122 Table — 20 A OCPD → 12 AWG
AC output	40 A breaker	10 AWG bare or green	NEC 250.122 Table — 40 A OCPD → 10 AWG
Panel frames	—	6 AWG bare copper	Grounding electrode conductor per NEC 250.66
Racking	—	6 AWG bare copper	Bond to building grounding system

Cable Bill of Materials

Recommended cable list assuming Option A (south wall placement) with one gauge up from minimum for DC expansion headroom.

Circuit	Cable	Color	Length	$/ft	Cost	Source
DC String (+)	8 AWG USE-2 PV Wire	Red	36 ft	$1.04	$37	WindyNation
DC String (−)	8 AWG USE-2 PV Wire	Black	36 ft	$1.04	$37	WindyNation
AC Hot 1 (L1)	8 AWG THWN-2	Black	41 ft	$1.00	$41	Home Depot
AC Hot 2 (L2)	8 AWG THWN-2	Red	41 ft	$1.00	$41	Home Depot
AC Neutral	8 AWG THWN-2	White	41 ft	$1.00	$41	Home Depot
AC Ground	10 AWG THWN-2	Green	41 ft	$0.60	$25	Home Depot
DC Ground	12 AWG bare Cu	Bare	36 ft	$0.30	$11	Home Depot
Battery + / −	2/0 AWG battery cable w/ lugs	Red / Black	10 ft	$6.94	$69	WindyNation
GEC (panels/racking)	6 AWG bare Cu	Bare	25 ft	$2.00	$50	Home Depot
Total estimated cable cost					$353

Prices sourced from WindyNation and Home Depot/Cerrowire as of March 2026. Does not include conduit, fittings, connectors, junction boxes, breakers, fuses, or disconnect switches.

Cable Types Reference

Type	Use	Rating	Where
USE-2 / PV Wire (UL 4703)	DC string cables	90°C, 600V/1000V/2000V	Outdoor, exposed, conduit, direct burial
THWN-2	AC circuits in conduit	90°C dry / 75°C wet, 600V	Interior conduit runs
Battery cable (SGT/SGX)	Inverter ↔ battery bank	105°C, flexible fine-strand	Short runs, high current, tight bends
Bare copper	Grounding electrode conductor	—	Panel frames, racking, ground rods

Placement Recommendation

Option A (south wall) total power loss: 44.5 W (8.5 W DC + 36.0 W AC)Option B (north wall) total power loss: 21.3 W (16.4 W DC + 4.9 W AC)

Option A saves ~-23 W in cable losses. The DC cable run is shorter (lower-voltage DC is more sensitive to distance), while the longer AC run at 240 V tolerates the distance easily. However, Option B is simpler to wire if the main panel is the natural termination point. Either works — the annual energy difference is about -42 kWh/year (assuming ~5 peak sun hours/day).

Methodology & Sources

Voltage drop formula: V_drop = 2 × L(ft) × I(A) × R(Ω/kft) / 1000 where L = one-way distance, I = operating current (Imp for DC, rated for AC).Ampacity: NEC 690.8(B) — conductor ampacity > 1.25 × Isc for PV source circuits. Based on NEC Table 310.16 at 90°C insulation rating.Resistance values: Copper at 75°C per NEC Chapter 9, Table 8.Voltage drop targets: < 2% DC (NEC 210.19 informational note),< 3% AC, < 1% battery (industry best practice for low-voltage DC).

Reference:

Mayfield Renewables — NEC 690.8 Wire SizingExpertCE — Sizing PV Conductors & OCPD per NEC 690Southwire — Voltage Drop CalculatorUnbound Solar — Voltage Drop Calculator

Where to buy (priced March 2026):

WindyNation — 8 AWG PV Wire (paired red/black, $82.88 for 40 ft pair)Amazon/WindyNation — 8 AWG PV Wire 50 ft pair (~$97)Home Depot/Cerrowire — 8 AWG THHN-2 50 ft roll (~$48–53)Amazon — 8 AWG THHN 4-color × 50 ft bundle (~$226)WindyNation — 2/0 AWG Battery Cable with lugs ($69.38 for 5 ft pair)Amazon/WindyNation — 2/0 AWG Battery Cable 5 ft pairHome Depot/Southwire — 6 AWG Bare Copper Ground Wire 25 ft (~$50)Wire & Cable Your Way — 8 AWG PV Wire (cut to length)