Introduction
Every voltage drop calculation asks for the conductor material before it asks for anything else — and for good reason. Copper and aluminum don’t just cost different amounts; they behave differently in the formula, at the termination, and over decades of service. Picking the material is really picking a package deal: resistivity, price, weight, termination practice, and where each metal is welcome.
The Voltage Drop Calculator handles both materials (K = 12.9 for copper, 21.2 for aluminum) so you can compare them head-to-head on the same run. This guide explains what those numbers mean in practice, when aluminum makes sense, and the termination rules that keep aluminum installations safe.
Copper vs. Aluminum: The Core Tradeoff
Aluminum’s resistivity constant (K = 21.2) is about 64% higher than copper’s (K = 12.9). That single fact drives everything else: for the same gauge, aluminum drops about 64% more voltage than copper. To match copper’s voltage-drop performance, aluminum needs roughly two AWG sizes larger — 6 AWG aluminum performs similarly to 8 AWG copper, 4 AWG aluminum to 6 AWG copper, and so on.
| Property | Copper | Aluminum |
|---|---|---|
| Resistivity constant (K) | 12.9 | 21.2 |
| Relative drop, same gauge | Baseline (100%) | ~164% |
| Gauge needed to match copper’s drop | — | ~2 AWG sizes larger |
| Material cost per amp of capacity | Higher | Substantially lower |
| Weight | Heavier (~3× denser) | Lighter |
| Termination care required | Standard | Higher — see below |
| Typical residential use | Branch circuits, most wiring | Feeders, service entrances |
The economic logic: aluminum costs much less per pound and you need more pounds of it — but the net cost per amp of capacity still favors aluminum substantially, especially at larger sizes. That’s why aluminum dominates service entrances and long feeders while copper owns branch circuits, where the small gauges make the price difference trivial and the termination simplicity matters more.
When Aluminum Makes Sense
Aluminum earns its place in specific situations:
- Long feeders and service entrances. This is aluminum’s home turf — 100+ ft (30+ m) runs at 60–200 A, where the copper premium for upsized gauges runs into hundreds of dollars. Utilities have used aluminum for service drops for decades.
- Subpanel feeders to outbuildings. The classic voltage-drop scenario (long run, significant current) is also the classic aluminum scenario. A 6 AWG aluminum feeder performing like 8 AWG copper typically costs far less than the copper equivalent.
- Weight-sensitive or long-span runs. Aluminum’s light weight matters in overhead spans and very long pulls.
And situations where copper is the clear choice:
- Branch circuits (15–20 A). At 14 and 12 AWG the price gap is small, terminations are simpler, and most devices are terminated for copper.
- Small-device terminations. Receptacles, switches, and many breakers are rated copper-only; landing aluminum on them requires special devices or pigtailing.
- Corrosive or damp environments where connection integrity is hardest to maintain — copper’s more forgiving nature pays off.
Cost comparisons for a specific run belong in the cost guide; a worked copper-vs-aluminum feeder comparison appears in the case study.
Termination: Where Aluminum Installations Succeed or Fail
Aluminum’s troubled reputation comes almost entirely from terminations, not from the wire itself. Aluminum oxidizes on contact with air, and aluminum oxide is a poor conductor — a connection that starts tight can develop resistance over time as oxidation creeps in, especially if the joint loosens through thermal cycling (aluminum expands and contracts more than copper with temperature swings). The practices that prevent this are non-negotiable:
- Use devices and lugs rated for aluminum (marked AL, CU-AL, or CO/ALR for devices). Never land aluminum on a copper-only terminal.
- Apply anti-oxidant compound at every aluminum termination. It’s inexpensive, and it excludes air and moisture from the joint.
- Torque to specification. Manufacturer torque values exist for a reason — under-torqued aluminum connections are the classic failure mode. Use a torque screwdriver or wrench, not feel.
- Abrade before terminating. Wire-brush the conductor to remove the existing oxide layer immediately before applying compound and landing it.
- Retorque on schedule where the manufacturer or local practice calls for it — some aluminum connections benefit from a re-torque after initial thermal cycling.
None of this is exotic — it’s standard electrician practice for feeders. But it is a reason aluminum belongs in the hands of someone who does it routinely, which is one of the boundary lines drawn in the DIY vs. pro guide.
Insulation and Conductor Types
The material choice (copper vs. aluminum) is independent of the insulation and cable type, but a few notes help when specifying:
- THHN/THWN-2 is the common single-conductor building wire for conduit — available in both copper and aluminum at larger sizes.
- NM-B (“Romex”) for interior branch circuits is copper in the small gauges homeowners encounter.
- USE-2 / URD and similar are typical for underground aluminum feeders to outbuildings.
- Stranded vs. solid: larger gauges are stranded for flexibility; stranding doesn’t change the voltage-drop math (CM is CM), but it affects termination technique slightly.
The calculator only needs the metal and the gauge — insulation type doesn’t enter the voltage-drop formula. It matters for ampacity, wet locations, and burial, which are code questions covered in the code compliance guide.
Sizing Strategy by Material
A practical workflow when both materials are on the table:
- Calculate in copper first to find the gauge that meets the 3% guideline at your load and length (see the complete guide worked example).
- Go up ~two AWG sizes for the aluminum equivalent and verify it in the calculator with K = 21.2.
- Price both — wire, plus any termination upgrades (AL-rated lugs, compound) the aluminum option needs.
- Check device compatibility: confirm the breaker lugs and any subpanel lugs accept the chosen material and gauge.
- Factor in who installs it: if you’re hiring an electrician anyway, their material preference and warranty terms may decide it.
One caution: upsizing aluminum for voltage drop can push you into gauges whose lugs, conduit fill, and bending space need checking — the wire is only one part of the system. When in doubt, the pre-calculation checklist keeps the whole input set consistent.
Related Guides
- Complete Guide — the full overview: concepts, formulas, and worked examples.
- Cost Guide — wire pricing and the economics of upsizing.
- Case Study — a hypothetical feeder costed in copper and aluminum.
- Formula Explained — what the K constant really means.
- DIY vs. Pro — when material choice affects who should install it.
- FAQ — quick answers to the most asked questions.
Get Your Number
Copper is the default for good reasons — predictable, forgiving, welcome everywhere. Aluminum is the value play for long, heavy feeders, provided its termination rules are respected. Neither is “better” in the abstract; the right metal is the one that meets the 3% guideline at the lowest honest total cost, installed by someone qualified to terminate it. Compare both in the Voltage Drop Calculator, price both with real supplier numbers, and let the math — not habit — decide.
Disclaimer: Calculator results are planning estimates based on the values you enter. Actual voltage drop varies with installation conditions, temperature, conductor properties, and load behavior. This guide is planning math only — electrical work must comply with local codes and be performed or verified by a licensed electrician where required. Always de-energize circuits before working on them. This is not a substitute for the NEC or your local electrical code.
