Friday, July 9, 2010

Part II

Last week we looked at the proliferate use of service cords in the electrical industry. We left off after looking at our ampacity rating tables. Our 30 A load ultimately needed a size 8/4 service cord (SC). According to T400.5(A), in the 2008 NEC we find an 8/4 rated at 35 amperes as its maximum rated ampacity. SC installations must also follow other de-rating factors for; numbers of current carrying conductors, ambient temperature corrections, as well as other installations restrictions.


In a similar method to how we treat other wire types, we must also de-rate anytime an installation of SC's contains either: more than three (3) current carrying conductors (CCC) or where our ambient temperature rises above 86 degrees F. First begin by obtaining your "Core Ampacity" (as in the case above it was 35A for an 8/4, according to table values). The next step (step II) is to de-rate where there are 4 or more CCC's. SC's come in many different conductor counts, but they usually range from 2 to 6 conductors. Our rule for grounded conductors ("the neutral") follows us from 310.15(B)(4), see 400.5(B). If the "neutral" only carries the imbalance load from different phased conductors (ie 'shared neutrals'), we do NOT have to count it as a current carrying conductor. Where it is the "load side" of a 120V circuit (as an example, or in any other single phase voltage branch circuit) you MUST count it as a full current carrying conductor. T400.5 adjusts by a percentage of our core ampacity value. It is the same exact table as you see in T310.15(B)(2)(a). Ambient temperature factors would be used from T310.16 under the temperature column that corresponds to the temperature rating of the Service Cord. (*Note SC temperature ratings are NOT shown in T400.4 and should be taken from the printed or scribes ratings on the outer jacket of the cable). Portable power cables, types G, PPE, and W, and Flexible stage and lighting cables, types SC, SCT, and SCE; follow their own pre-calculated temp rating/de-rating in T400.5(B).



I have also noticed SC's installed in commercial buildings as substitutes for permanent wiring. One contractor used it for branch circuit wiring to feed under-cabinet lights. His installer had fished it into several wall cavities. He explained that he couldn't get the bend radius neccessary to conceal it using MC Cable. Unfortunately for him, he had to remove it. 400.8 specifies 7 specific uses that are NOT permitted when installing SC's. Essentially the rules are: 1) Not used as fixed wiring of a structure; 2) Not run through walls, doors, windows etc; 3) Not concealed by walls, floors, or suspended ceilings; 4) Not installed in raceways, and finally 5) Not where subject to physical damage. If you are ever tempted to run a SC inside a conduit or raceway, DON'T do it! Not only does that suggest a more "fixed type wiring" scenario, but you are not permitted to encase an SC in a raceway (unless specifically permitted to do so under any other section of the NEC or where in an industrial application, under certain restrictions, a maximum of 50' of raceway may be used as a means to protect a SC; see Art. 400.14).


In our earlier scenario where we had a 3 phase, 480V, 50HP motor, our first step is to find the FLA of the motor in T430.250. Under the 460V, 50 HP column we see that to be 65 amperes. Referring back to T400.5(A), our SC Ampacity chart, we find that to achieve this per code, we'd need a size #2 service cable. That is substantially larger than what was currently installed. The #6 being utilized is only rated for 45 amps, a 20 Ampere Deficit!


Finally, to recap, Service Cords are NOT "ordinary wiring." They follow specific rules and MUST be treated in a different manner. The ampacity ratings are much less than typical wiring. The installation restrictions are numerous. It's even interesting to note that you may not install a NEW SC service that has ANY splices or taps. You may repair an existing usage, but you may NOT reutilize it with the splice remaining. Strain relief to keep tension off of joints and terminations must be employed. The voltage rating on many of them is only a maximum of 300V so be cautious that you match the proper insulation voltage rating with the voltage supplied by the service. For example, a Junior Service Cord, type SJO etc... could NOT be used for a 480V, 3 phase motor application. They cannot be encased in raceways, above suspended drop ceilings, or be used inside walls. I urge you to re-read the entire Article 400 to re-familiarize yourself with the SC installation requirements. As always, be safe and remember your responsibility is to keep others safe from your work!

1 comment:

  1. Thanks for sharing such a good post. It is useful information about the electric wiring blog it's truly good and great information well done.
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