Fill 'Er Up

Aug 14, 2008 4:10 PM, By Mike Holt, NEC Consultant

The correct raceway fill depends on the conductor type and raceway type

Raceways must be large enough to hold conductors without overheating and avoid insulation damage during conductor pulls. Therefore, we must determine the maximum number and size of conductors to put in a given raceway. That limit is what we mean by raceway fill.

Chapter 3 of the NEC contains Articles for a dozen different types of raceway, including conduit, EMT, and PVC. Chapter 9 and Annex C are the primary references for determining the correct raceway fill for a given application. Annex C contains a different raceway fill table for each raceway type, because each differs in its ability to dissipate heat.

Because different conductor types (THW, TW, THHN, etc.) have different thicknesses of insulation, raceway fill also varies with the type of conductor you use. In fact, each Annex C Table for a given raceway type contains sections for specific conductor types. For example, Table C.8 (Rigid Metal Conduit) contains three sections, each of which lists specific conductor types.

Fig. 1. When different size conductors are installed in a raceway, conductor fill is limited to the above percentages.

Table 1 of Chapter 9 lists the maximum percentage of allowable conductor fill. This number is based on common conditions where the length of the conductor and number of raceway bends are within reasonable limits [Chapter 9, Table 1, FPN No. 1] and how many conductors or cables are to be installed in the raceway.

Number of conductors

Sometimes, you have a specific raceway being run, and you must know how many conductors you can pull into it.

Where multiple conductors of the same size are installed together in a raceway, you can determine the maximum number of conductors from the Annex C tables. Where conductors of different sizes are in a raceway, Chapter 9 contains the information necessary to calculate the required raceway size.