IT-07
Cable Conduit Capacity (NEC)
NEC Chapter 9, Table 1 · NEC Chapter 9, Table 4 · NEC 300.22(C) · NEC 300.22(C)(1) · NEC 725 · NEC 770 · NEC 800 · NEC 820
Drawing
The drawing appears when the inputs are complete
- 01Size conduit from cable count and outside diameter using the NEC Chapter 9 Table 1 fill percentages, with plenum, riser and circuit-article flags.
- 02Tip. Fill and jamming are different failures. A conduit can sit well inside its fill limit and still seize a pull, because jamming depends on the ratio of conduit bore to cable diameter rather than on how full it is. Check both before you order.
- 0353 % — Fill, one cable
- 0431 % — Fill, two cables
- 0540 % — Fill, three or more
- 062.8 – 3.2 — Jam danger band
- 07360° — Bend between pulls
NEC Chapter 9 fill and jam risk
Two separate checks decide whether a conduit run will work. Fill is a code limit on how much of the bore the conductors may occupy, set for pulling tension and heat dissipation. Jamming is a geometry problem that can seize a pull in a conduit well inside its fill limit.
Fill percentage
NEC Chapter 9, Table 1 caps the proportion of the conduit cross-section the conductors may occupy. The limit steps down at two conductors, then back up for three or more.
Use cases
- One conductor — 53 %
- Two conductors — 31 %
- Three or more — 40 %
Formula
Fill % = total cable area ÷ conduit internal area × 100
NEC Chapter 9, Table 1
Jam ratio
Three or more cables of similar diameter can wedge into a triangular lock against the conduit wall. The risk depends on the ratio of conduit inside diameter to cable outside diameter, not on how full the conduit is.
Use cases
- Below 2.8 — the triangle cannot form, low risk
- 2.8 to 3.2 — danger band, a pull can seize
- Above 3.2 — cables can pass one another, low risk
Formula
Jam ratio = conduit inside diameter ÷ cable outside diameter
Pulling practice, not an NEC requirement
Conduit material matters. Trade size names a nominal outside dimension, so wall thickness changes the usable bore — the same trade size in PVC Schedule 80 carries appreciably less than in EMT. Select the raceway actually specified before trusting the recommended size.
- 01Conduit fill — NEC Chapter 9, Table 1
- 02Conduit dimensions — NEC Chapter 9, Table 4
- 03Wiring in environmental air spaces (plenum) — NEC 300.22(C)
- 04Raceway exception for plenum ceilings — NEC 300.22(C)(1)
- 05Class 2 and Class 3 circuits — NEC 725
- 06Optical fibre cables and raceways — NEC 770
- 07Communications circuits — NEC 800
- 08Coaxial distribution — NEC 820