Conduit Fill Calculator
Two cables have the tightest limit. Not a typo.
Two cables have the tightest limit. Not a typo. Conduit fill against the allowed percentage — 53% for one cable, 31% for two, 40% for three or more.
Conduit fill
60.7%
Over the 40% limit for 3 cables — go up a conduit size.
The limits are 53% for a single cable, 31% for exactly two, and 40% for three or more. That ordering surprises people — two cables have the tightest limit of the three rather than sitting between the others. The reason is that the limits are not about physical space. They exist so cables can be pulled without stripping their insulation, and so heat can get out. Two cables jam against each other in a way that one or many do not, which is why the two-cable case is the strictest. Beyond the fill percentage, long runs and multiple bends both make pulling harder regardless of the arithmetic. Most codes also cap the number of bends between pull points, and derate the cable's current capacity once several circuits share a conduit.
How the Conduit Fill Calculator works
Conduit fill against the allowed percentage — 53% for one cable, 31% for two, 40% for three or more. That ordering surprises people, and the reason is that the limits are about pulling and heat, not space.
Also known as: how many cables in 25mm conduit · conduit fill percentage · 40 percent fill rule · is my conduit overfilled
Where to go next
The Conduit Fill question rarely arrives on its own. These are the ones that usually come with it:
- Circuit Breaker Size Calculator — The breaker protects the wire, not the appliance.
- Motor Amperage Calculator — Sized to the surge, not the running current.
- Three-Phase Power Calculator — The √3 is geometry, not a safety margin.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
What is the maximum conduit fill?
53% for a single cable, 31% for exactly two, and 40% for three or more. The limits are the same across most codes because they derive from the same physical constraints.
Why is the two-cable limit the tightest?
Because two cables jam against each other in a way that one or many do not, making them far harder to pull without damaging insulation. The limits are about pulling and heat dissipation, not about physical space.
What happens if I overfill a conduit?
Insulation gets stripped during the pull, and the cables run hotter because heat cannot escape. Both are failures that show up months or years later rather than immediately.
Do bends count?
Yes, though not in the fill percentage. Most codes cap the number of bends between pull points, typically 360° total, because each one multiplies the pulling tension.
Does a full conduit affect current capacity?
It does. Multiple circuits sharing a conduit derate each other, because they cannot shed heat independently. The derating factors are separate from the fill limits and both apply.
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/conduit-fill-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Conduit Fill Calculator"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/conduit-fill-calculator" style="color:#64748b">Conduit Fill Calculator</a> from The Calc Library</p>The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.
Related calculators
Circuit Breaker Size Calculator
The breaker protects the wire, not the appliance.
OpenMotor Amperage Calculator
Sized to the surge, not the running current.
OpenThree-Phase Power Calculator
The √3 is geometry, not a safety margin.
OpenSpeed Distance Time Calculator
Any one of the three from the other two, in four units.
Open