Sunday, June 27, 2010

The Ampacity and Proper Application of Service Cords (SO, SJO, etc...) (Flexible Cords and Cables)

I have seen dozens and dozens of uses for service cords (SC) in the field that are very improper. The use and installations of SC's appear to be a weak point in many technician's training. It seems like the industry has simply adopted their use as a catchall "extension cord" to be used as a multipurpose, fits anything type of cable. However, there are some specific restrictions to SC's and their ampacity ranges are MUCH different than other types of cables and conductors.

Recently I ran across the following scenario. Take a quick minute to test yourself and see if YOU know what the proper size cord would be needed for the following installation:

a 3 phase, 480V, 15 HP, Squirrel Cage motor, operating at approximately 300' away from its supply. I found a 4 conductor SOOW cord, sized at #6 being used. We'll look at the answer next week in Part II of this Blog.

We find that Article 400 "Flexible Cords and Cables" to be our main ruling statute for SC cords. Table 400.4 lists their types, sizes, and usage, among other important data. Three critical tables, but the most often overlooked ones, deal with SC's. These tables, 400.5(A), 400.5(B), and 400.5 are our ampacity and adjustment tables that must be utilized when installing SC's. Lets look closely at table 400.5(A). We find two important columns, A & B. column A is for cords that will have (3) current carrying conductors. So in essence, ALL three phase installations would fall under that first column. Column B is for (2) current carrying conductors. Thus MOST single phase (such as 120V or 240V type installations) would fall under this column. Take a quick moment to write "3 Phase" under Column A and "1 Phase" over Column B in your 2008 NEC Code Book for your future ease of reference.

So a three phase, 208V, 30 Amp load would require an 8/4 service cord! That is a pretty substantial difference from what a normal #10 THWN conductor, encased in a flexible conduit installation would be. The 6/4 SOOW I mentions earlier would have a MAXIMUM ampacity of only 45 Amps.

In Part II we will cover adjustment factors, voltage drops, and installation restrictions. We will also determine the answer to our question posed earlier. Because of the nature of SC's vs other cables, I have created a custom table for technician's easy field reference. Get a FREE copy, by simply emailing a request "Free Ampacity Table" to our web site. Stay tuned for Part II.!

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