Conduit Jam Ratio for Three Same-Size Conductors (NEC Ch. 9)
Three same-size conductors can wedge in a bend when the jam ratio 1.05 x conduit ID / conductor OD...
Example
You enter
- Conduit inside diameter (in, NEC Ch.9 Table 4) 2.067
- Conductor outside diameter (in, Table 5) 0.65
- Number of conductors 3
You get
- Ratio 3.339
Checked against published worked examples from: EC&M (J. DeDad).
Details, formula, and sources
ScopeThree same-size conductors can wedge in a bend when the jam ratio 1.05 x conduit ID / conductor OD (1.05 for the bend ovaling the conduit) lands in the ~2.8-3.2 band. Three 0.65 in conductors in 2 in EMT (ID 2.067) -> 1.05 x 3.18 = 3.34, just clear; 0.70 in conductors give 3.10, jam-prone. Jamming needs exactly three conductors. A pull caution, not a code limit; the NEC and AHJ govern.
Formula or tableratio = 1.05 x conduit_ID / conductor_OD (1.05 for bend ovality); jam-prone if n = 3 AND 2.8 <= ratio <= 3.2.
Source and editionThe conduit-jamming ratio guideline referenced with NEC Chapter 9 (Table 4 conduit ID, Table 5 conductor OD), by name.
Free accessThe jam-ratio guideline is a widely-published cable-pulling rule; the conduit ID and conductor OD are in NEC Chapter 9 Tables 4 and 5 (NFPA 70). The NEC and the AHJ govern.
What governsEstimate. AHJ and licensed professional govern.
API field namesconduit_id_in, conductor_od_in, n_conductors, ratio
- Jam band jam-prone when exactly three conductors and 2.8 <= 1.05 x ID/OD <= 3.2Southwire / IEEE 1185; EC&M (DeDad, 2001)
- Table dimensions conduit ID from NEC Ch. 9 Table 4; conductor OD from Table 5NEC (NFPA 70) Chapter 9
- A caution, not a limit a pull caution; does not check fill area or pulling tensionscope of this tile