Green retrofits add new continuous loads in old envelopes: heat pumps where resistance heat lived, EV chargers where a table lamp once sat, battery inverters in garages built in 1978. Breaker space is easy to see; copper length is hidden until something underperforms.
Length × current sets the project
Two projects with the same amp rating can need different wire:
| Project | Typical sustained A | Length risk |
|---|---|---|
| Heat pump (mini-split) | 15–30 | Outdoor line set + electrical whip |
| Central heat pump | 30–60 | Panel to outdoor unit |
| Level 2 EVSE | 32–48 | Garage feed dominates |
| Solar inverter subpanel | 20–40 | Roof to garage run |
Ampacity tables tell you the minimum gauge that will not melt. Voltage-drop math tells you the practical gauge that will not starve the load.
Workflow for electricians and DIY planners
- Convert power to amps — Watts to Amps with correct voltage (240 V vs. 120 V)
- Measure one-way length — panel → disconnect → load
- Pick trial AWG or mm² from supply house stock
- Run calculator — aim ≤3% on dedicated motor/EV circuits
- Step up if borderline—copper cost is smaller than a second trench
Metric vs. imperial installs
European and UK projects size in mm²; North American branches list AWG. The calculator includes both—select the label on the spool you are buying, not a rough conversion from memory.
HVAC and heat pumps
Compressors draw LRA on startup but run hours at FLA. Undervoltage from thin feeders shows up as:
- Brownouts on other circuits when the compressor kicks
- Manufacturer fault codes for low supply
- Reduced COP because the system works harder on less voltage
Size the electrical run to the outdoor disconnect, not just the indoor air handler circuit if they share a marginal subfeed.
EV charging runs
Continuous load rules mean:
- Use nameplate maximum current, not “average”
- Account for derating in hot attic conduit
- Long garage feeds often need 6 AWG / 10 mm² where intuition says 10 AWG / 2.5 mm²
Run drop at 40 A and 35 m once—you will thank yourself when the car takes full power instead of 24 A.
Solar and battery subpanels
Inverter AC output circuits still obey drop. A 7.6 kW inverter at 240 V is ~32 A—fine on paper with 10 AWG until the combiner is 45 m from the main panel. Model it.
Pair DC runs with DC Cable Size and AC runs with Residential Voltage Drop—hybrid systems need both.
Mistakes that pass inspection but fail physics
- “Breaker is 40 A, so wire must be fine” — load may be continuous
- Reusing a 1960s garage feeder for a 2026 EVSE
- Mixing aluminum old feed with copper extension without proper rating
Related reading
- Residential Voltage Drop Guide — targets and checklist
- Why Home Voltage Drop Matters — motors and heat
Green home projects fail quietly on voltage, not loudly on carbon math. Pull the right gauge the first time—the calculator is cheaper than opening finished drywall.