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Thursday, July 30, 2015
Question:
Hi Charlie!
First of all, thank you for all you do! I start every day with your forum and value the information you pass on. It’s also interesting to see what challenges people from other parts of the country and aspects of the trade are facing on a day-to-day basis.
Most of the work I do is for the oilfield and gas industry. We build “skids” – portable oilfield structures – for use in Wyoming, North Dakota, Colorado, New Mexico, even as far away as Africa. One of the challenges I faced was a box fill calculation on an instrumentation run on a 125 HP Booster Pump. We run a twisted, shielded pair to each of five different instruments in the same conduit run and drop each one out of an explosion-proof tee (GUAT). The horizontal multi-stage centrifugal pump comes with a few instruments already attached. They have a flexible cord that comes from the instrument. We pull this cord into our GUAT and terminate there.
As you know, once you terminate within a conduit fitting, a box fill calculation must be done. The first GUAT in the run contains four twisted, shielded pair that pass through, one #14 AWG equipment grounding conductor that ties all of the instruments and conduit fittings to ground and a fifth twisted, shielded pair that gets terminated to the flexible cord coming from the instrument. All twisted, shielded pairs are #18 AWG. I can perform a box fill calculation using the values given in Table 314.16(B) for the four #18 AWG conductors making up the termination to the instrument and the #14 AWG equipment grounding conductor (single volume allowance per 314.16(B)(5)), but I’m a bit stumped about what volumes to use for the four twisted, shielded pair that pass through. As you can imagine, this calculation gets done twice more, each time with one less twisted, shielded pair passing through as they drop off at the next two instruments. The last twisted, shielded pairs in the run make it back to a junction box by the pump motor and terminate there (no box fill calculation necessary). Any light you could shed on this would be much appreciated!
Wild in Wyoming - Jimmy Alm
A
Answer:
Hey Jimmy thanks for your question and the kind words. Wyoming is great country and a nice wild as compared to some cities. I think you have the concept well covered and have pointed out an issue that should be addressed, possibly by a Public Input to the NEC. The first sentence of 314.16 uses the word "approved size" and I'm sure you know that is up to the AHJ. In your case because the equipment can be installed in multiple locations many different AHJ's will likely be involved.
If one wire allowance is sufficient for all the equipment grounding conductors in a box, as stated in 314.16(B)(5),then one wire allowance should be sufficient for all the shields, I know, I'm out on a limb and don't have claws, but its free advice. Until you raised the question, it was probably overlooked or unasked in many installations.