What is conduit fill and how do you calculate it?

Conduit fill refers to the percentage of the inside area of a conduit that is occupied by wires or cables—correct sizing ensures compliance with National Electrical Code rules, safe heat dissipation and easier future pulls. The calculation involves summing the cross-sectional areas of all conductors (including equipment grounding) and ensuring the total does not exceed the prescribed limit (e.g., 40% for three or more conductors) of the conduit’s internal area.

placeholder content

eServicePro Login

Customer Q&A:
CONDUIT FILL

SUBMIT A QUESTION

How Do I Calculate Conduit Fill?

Copper Line Break

OUR SOLUTION FOLLOW-UP QUESTION

WHY CONDUIT FILL MATTERS

Conduit fill directly impacts the safety, reliability, and maintainability of an electrical installation.

Excessive fill increases pulling tension and sidewall pressure, which can compromise conductor insulation and reduce service life. Overcrowded conductors also restrict heat dissipation, leading to elevated operating temperatures and potential thermal damage.

Adhering to NEC conduit fill requirements ensures proper mechanical protection, thermal performance, and capacity for future modifications, resulting in a compliant and durable electrical system.

In general, the NEC limits conduit fill to 40% for three or more conductors, 31% for two conductors, and 53% for one conductor or cable.

Conduit Fill Illustration

When running cable through conduit, you need to know how much can safely fit inside the pipe. That’s why understanding conduit fill and how to calculate it is essential.

Conduit fill is a percentage of how much space inside of conduit is taken up by the conductors running through it. The National Electric Code (NEC) makes recommendations about maximum conduit fill to ensure wires don't overheat and can be safely pulled through the conduit without damage.

HOW TO CALCULATE CONDUIT FILL

To calculate conduit fill, determine the cross-sectional area (including insulation) of each wire you plan to pull and the internal cross-sectional area of the conduit. To calculate the cross-sectional area, divide the outside diameter (OD) of the wire by 2, square it, and then multiply the result by Pi.

FOR EXAMPLE, if a conductor has an OD of 0.25 inches, you would begin by dividing 0.25 by 2, giving you 0.125. Then you can calculate the area using that formula.

The NEC provides tables with wire sizes and conduit dimensions so you don’t need to measure them yourself.

Add up the area of the wires you plan to install (0.049in in example above), then divide that total by the conduit’s internal area (per NEC table). Multiply by 100 to get the fill percentage.

FOR EXAMPLE, if you take the 0.049 square inches from above, and your conduit has an internal area of 1.5 square inches, the fill is about 3.3%—meeting NEC limits for conduit fill.

CONDUIT FILL FAQ

Open AllClose All

If I have a multiconductor cable, like ServicePlex®, do I add up each wire?

No. A cable assembly counts as one conductor or cable. You use the manufacturer’s published overall OD to find its area.

Do I calculate conduit fill by conductor diameter or bare wire size?

You calculate conduit fill based on the overall insulated conductor OD (or published cross-sectional area), not the bare copper or aluminum size.

Why do two conductors get less fill (31%) than 3 (40%)?

Two conductors get less fill due to their geometry and packing efficiency. Two circles (conductors) in a round conduit leave more wasted space than three or more.

Do all types of conduit have the same internal area?

No. Each conduit type has different wall thickness and thus different internal areas. That’s why NEC Chapter 9 Table 4 is specific by material and trade size.

Can I just use 40% conduit fill for everything?

No. You must use 53% for one conductor or cable, 31% for two conductors, or 40% for three or more conductors.

White Quotations I love Service Wire's competitive pricing and quick delivery.

White Quotations Service Wire goes above and beyond to help us meet our customer's needs.

CONDUIT FILL SIMPLIFIED

Copper Line Break

Calculating conduit fill doesn't have to be complicated. Whether you're working with multiconductor cables like ServicePlex®, planning a large installation, or just want to ensure NEC compliance, our team can help you get it right the first time. Contact us today for expert guidance on cable selection and conduit fill calculations tailored to your specific application.