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Monday, July 27, 2015
Question:
Re: CQD answer published Friday, July 17, 2015 - Existing Range Circuits - Insulation
Good morning Charlie,
I have followed daily your Q&A emails for quite a while and find many are very informative. I would like to add to the ”tape insulation” debate as follows:
All cabling used in the field has a continuous sheathing of approved insulation appropriate for the duty required, ranging from 0.5mm to as thick as 25mm or more on big cables and can range from thermoplastics to polymeric rubbers.
When making a splice/joint, requires that the insulation to be cut back to accommodate a component being secured or a pair of wires twisted together for a joint. The code calls for tinning the wire ends or soldering the twisted pair- ensures continuity especially with a twisted pair (which we all know can loosen through movement or vibration and now is not a continuous flow path when left unsoldered).
Now the re-covering of the exposed wire ends should be with a similar single wall thickness of insulation- there are numerous heat shrink insulation sheaths available that meet code.
Using a thermoplastic type adhesive tape does not constitute a single wall thickness- each layer is separated by an adhesive, which over time loses it adhesion due to heat, aging or not being applied to a clean surface. I have come across insulation tapes that can pass current due to the type of plastic/adhesives used, thus causing both short/long term dilemmas’.
Should the code be more specific in this area by emphasizing the requirement for the same thickness of such insulation by sheathing and not using tape due to the adhesive not making a single wall thickness?
Your comment on this point would be appreciated as I feel it adds more to the efficiency/reliability of the code and articles 110 and 310- which still seems a grey-area.
Kind regards, Geoff
A
Answer:
Hey Geoff thanks for your question and comment. We are happy that you enjoy the CQD.
I think there is always room for improvement so often suggest that individuals submit Public Inputs to provide better code language. The wording in 110.14(B) is "insulation equivalent to that of the conductors" and does not specifically state it must be in a single layer, or that it be listed, other than for direct burial. At least one insulating tape manufacturer (of a listed product) provides instructions for a double half lap layer of tape for applications up to 600 volts.
And as Christel pointed out, 310.104 states that insulated conductors must comply with Table 310.104. No mention of insulating tape there and 110.14 is titled "Splices" so does that section apply if there is no splice or joint? I wish I had a drink chip for every conductor that was taped in the field to "fix" an insulation damage problem.
So by all means submit a PI to improve the wording. But a somewhat famous saying that is used by some that frequently submit proposed changes "Be careful what you wish for - you just might get it, and sometimes more than you wanted"!