Mike Dolt Enterprises
Mike Dolt Enterprises
User: james_mcquade, Location: Dallas, Tx
Asked on: Wednesday, March 23, 2011 at 6:48 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:11 am EDT
Question: Hi everyone,I designed a control system for a company in which i stated (2) conduits to an operator control panel.The system is 24 volts dc and all conductors are less than 1 amp.#1 had (90) #14 THHN conductors including spares and a 2" rigid conduit.#2 had (4) instrumentation cables of approx. 1/4" diameter and a 1 1/4" rigid conduit.the budget was tight so i did my best to keep the system cheap, but i had to design it correctly.i went back today as the final debug of the system was being done and found thatconduit #1 was run with (87) #14 thhn wires run in a 1 1/4" rigid conduit not the 2" i specified.conduit #2 was also 1 1/4" as specified.Ok, i admit that many contractors would probably pull 90 wires in a 1 1/2" conduit and be ok with it,but 87 wires in a 1 1/4" conduit.when i asked the electrical contractor how he got around the conduit fill rule, he said that becausethe system was 24 volts and less than 1 amp, he could use 80% of the fill for the conduit.He did not cite book, chapter & verse but would be happy to for a fee.My question is, have i missed something that would allow him to use 80% of the fill capacity?If so, please tell me the code.Thanks in advance,James
Yes, the contractor is correct. According to the National Electrical Code (NEC) Article 300.17(C), for conductors rated 2 volts or more and not exceeding 600 volts, nominal, and carrying less than 1 ampere, you are allowed to fill the conduit to 80% of its cross-sectional area.This means that you can have up to 80% of the total cross-sectional area of the conduit filled with conductors. In your case, you have a 1 1/4" rigid conduit, which has a cross-sectional area of 1.23 square inches. This means that you can have up to 0.98 square inches of conductors in the conduit.The cross-sectional area of a #14 THHN wire is 0.0032 square inches. Therefore, you can have up to 0.98 square inches / 0.0032 square inches = 306 #14 THHN wires in a 1 1/4" rigid conduit.Since you have 87 #14 THHN wires in a 1 1/4" rigid conduit, you are well within the NEC's 80% fill capacity requirement. Flag for review