Wire Size Calculator
The smallest gauge that holds the drop — not an ampacity answer.
Work out Wire Size. The smallest gauge that holds the drop — not an ampacity answer. Every rate and local condition is an input, never an assumption.
Not a code compliance tool. Conductor sizing for safety comes from the ampacity tables and their derating rules; this answers only whether the run is long enough to matter for voltage drop.
Minimum size for voltage drop
8 AWG
2.60% drop over 100 ft
This is a voltage-drop answer only. Ampacity — how much current a conductor may carry without overheating — is a separate and stricter question that depends on insulation type, ambient temperature, how many conductors share a raceway and which code edition applies. Take the larger of the two sizes, never this one alone.
How the Wire Size Calculator works
This finds the smallest AWG size that keeps voltage drop within your limit over a given run. That is one of two constraints on conductor size and it is not the safety one: ampacity — how much current a conductor can carry without overheating — depends on insulation type, ambient temperature, how many conductors share a raceway and which code edition applies locally. Take the larger of the two answers, never this one alone.
Also known as: wire gauge calculator · AWG calculator · cable size calculator · wire size for voltage drop
Two constraints, and this is only one
Conductor size is decided by whichever of two requirements demands more copper.
Ampacity is the safety requirement: how much current a conductor can carry without its insulation overheating. It comes from code tables and depends on insulation type, ambient temperature, how many current-carrying conductors share a raceway, and the terminal temperature ratings at both ends.
Voltage drop is the performance requirement, and it is what this calculates. It depends on length, and it is the one that surprises people, because a gauge that is perfectly safe can still be inadequate at distance.
In practice
20 A over 100 feet at 120 V, holding to 3%: the run needs at least 14,333 circular mils, so 8 AWG at 16,510 is the smallest size that works. It comes out at 2.6%.
12 AWG would be the usual choice for a 20 A circuit on ampacity grounds, and over 100 feet it gives 6.6% — safe, and out of specification for performance.
In aluminium the same run needs 6 AWG at 2.7%, because aluminium's resistivity constant is 21.2 against copper's 12.9 — about 64% higher, which is why aluminium runs are consistently sized larger.
Where the figure deceives
This is not a code compliance tool and cannot be used as one. It has no knowledge of insulation type, ambient temperature, conductor bundling, continuous-load factors or which code edition your jurisdiction has adopted.
A conductor that passes this check can be unsafe. Take the larger of this size and the ampacity requirement, always.
It also stops at 4/0. Beyond that, runs use parallel conductors or a higher supply voltage, and that is a design question rather than a table lookup — which is why the tool returns nothing rather than guessing.
Acting on it
Get the ampacity size from the code tables first, then check this, then use the larger.
Consider raising the voltage instead of the conductor size on very long runs. Drop falls with voltage for the same power, and it is often the cheaper fix.
Have anything you are unsure about checked by a qualified electrician. This calculates one property of a circuit; it does not design one.
Frequently asked questions
What size wire do I need?
Two constraints apply. Ampacity comes from the code tables for your insulation type and conditions; voltage drop comes from the run length, and this calculates that one. Use whichever demands the larger conductor.
Why does length change the wire size?
Because drop is proportional to length. A gauge that is fine at twenty feet can be inadequate at two hundred, even though the current has not changed at all.
What is a circular mil?
The area of a circle one thousandth of an inch across — a unit that makes conductor areas come out as convenient whole numbers. 12 AWG is 6,530 circular mils, 10 AWG is 10,380.
Can I use this instead of the code tables?
No, and that is the important point. This answers a voltage-drop question only. Ampacity is a safety requirement with its own tables and derating rules, and a conductor that passes here can still be unsafe.
Should I upsize for future load?
Often worth it on a long run, where the labour of pulling cable dominates the cost of the cable itself. Upsizing later means pulling it again.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/wire-size-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Wire Size Calculator"></iframe>The only condition is that the credit line stays visible. It sits inside the frame, so you do not have to do anything to keep it.
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