How to Measure Conduit Trade Size

What it is

Conduit is sold by trade size, such as 1/2, 3/4 or 1. The trade size does not equal the inside or outside diameter. A 1/2 trade size rigid conduit has an outside diameter of 0.840 in., and a 1/2 trade size EMT has an outside diameter of 0.706 in.

The NEC gives each trade size a metric designator. Metric designator 16 is the same conduit as trade size 1/2. It is a name, not a metric measurement.

How to measure it

1. Identify the conduit type first. EMT, rigid, IMC, PVC and flexible conduit each have their own outside diameters. See How to Identify Conduit Types.
2. Measure the outside diameter with a caliper, or wrap a tape around it and divide the circumference by 3.1416.
3. Match the reading to the table for that conduit type.
4. Check the marking. Most conduit is printed or stamped with type and trade size every few feet.
5. For fittings, measure the conduit, not the fitting. Fittings are sold by the trade size of the conduit they fit.

Trade size identification table

Rigid metal conduit and PVC Schedule 40 and 80 share the outside diameters of steel pipe of the same nominal size. IMC has a slightly smaller outside diameter than rigid but uses the same thread. For exact values and wall thickness, see the Conduit Trade Size and Metric Designator Chart and the product standard.

Key points

• Trade size is the same for all conduit types, but the measured diameter is not.
• EMT is thinner and smaller outside than rigid conduit of the same trade size.
• Inside diameter sets conductor fill. Fill is calculated from NEC Chapter 9 tables, not from trade size.

Common mistakes

• Calling 1/2 EMT "5/8 conduit" because of its measured diameter.
• Treating the metric designator as millimeters of diameter.
• Using a pipe size chart for EMT or flexible conduit.
• Ordering fittings by the measured diameter instead of trade size.

Governing standards: NEC 300.1(C) (metric designators and trade sizes); NEC Chapter 9 (conduit dimensions and fill); ANSI C80.1, C80.3 and C80.6 (rigid, EMT and IMC); UL 651 (PVC conduit).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.