![]() In this process, you don't need to count raceway and cable fittings (including locknuts and bushings), wire connectors, or cable connectors with their clamping mechanism outside of the box. In no case can this exceed the box volume. The number you get is the total conductor volume. After you calculate all five volumes using 314.16(B)(1) through (5), add them up using the equivalent volumes found on Table 314.16(B). Conductor volumeĬalculating conductor volume is a matter of adding up individual conductor fill volumes and conductor equivalent volume fills - and there are five such volumes. Include only those parts that are marked with their volumes in cubic inches or included in NEC Table 314.16(A). ![]() Then, add to it the sum of the individual volumes of the assembled parts, such as plaster rings, extension rings, etc. ![]() If the box is not a standard size, the volume will be marked on the box by the manufacturer. If Table 314.16(A) is inapplicable for your installation, which it typically will be, how do you calculate box volume? It's a matter of adding up individual volumes of assembled parts. You can use Table 314.16(A) only if the box contains no switches, receptacles, luminaire studs, luminaire hickeys, internal cable clamps, or equipment-grounding conductors. Table 314.16(A) seems to make it simple to determine the volume of a box, but don't get too excited. You calculate box volume per 314.16(A) and box fill per 314.16(B), but make sure your 314.16(A) box volume is greater than or equal to your 314.16(B) box fill. Note: This article is based on the 2005 NEC.īox fill isn’t just the number of wires in the box - it's the total volume of the conductors, devices, and fittings in a box.Įvery outlet box has a specific amount of space for conductors, devices, and fittings. If you’d like to see updated information on this topic, please check out this recently published article, Brushing Up on Your Box Fill Calculations. Contrary to popular electrical opinion, designated rules limit the number of conductors allowed within outlet, device, and junction boxes.Thank you for visiting one of our most popular classic articles. Unfortunately, all too often, electricians remove junction box covers only to find the box crammed full of conductors. Some creative methods have been devised to overpack the conductors into a junction box so that the cover can still be installed. Such methods include, but are not limited to: carefully wrapping the conductors around the inside of the box so that no air space remains mashing the conductors down with the thumb, the heel of the hand or even by applying force with the handle of a hammer. An optional creative method is to back out the junction box screws, insert a blank plate, then tighten the screws, thereby compressing the conductors. Of course, not one of these methods is correct. Article 370 contains detailed requirements for box fill and junction box sizing.Īrticle 370 of the National Electrical Code (NEC) covers a variety of provisions pertaining to boxes (outlet, device, pull junction), conduit bodies, and fittings. It covers the installation and use of all boxes (and conduit bodies) used as outlet, junction, or pull boxes, depending on their use. It also covers manholes (including other electric enclosures intended for personnel entry). Manholes and Other Electric Enclosures Intended for Personnel Entry (new to the 1999 edition), and E. Pull and Junction Boxes for Use on Systems Over 600 Volts, Nominal. This month's In Focus discusses Installation (Part B).ģ70-15 Damp, Wet, or Hazardous (Classified) Locationsīoxes, conduit bodies, and fittings in damp or wet locations must be placed or equipped so as to prevent the entrance of, or internal accumulation of, moisture. Damp locations, as defined in Article 100, are locations subject to moderate degrees of moisture. Partially protected external locations under canopies, marquees, roofed (open) porches, and similar sites are considered damp locations as are some interior locations such as certain basements, barns, and cold-storage warehouses.
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