We’ve had a great response to our blog post, “Ex e Terminal Box Practicalities,” including some good questions that we thought it would be worth addressing in a follow up post. Steve, a senior electrical engineer working in refrigeration dropped us a line to ask about spare terminals. Steve writes:
“I read your article regarding Ex e JB practicalities, and found it very informative.”
Further to the article, I am interested in your views regarding calculated current for:
spare cores of cables terminated in an Ex e JB used for instrumentation (low current consumption)
spare terminals in an Ex e JB used for instrumentation (low current consumption)
The conservative approach is to apply max rated cable current or max terminal current (whichever is less). However, would it be non-compliant to apply a “typical” low load current to the spares to demonstrate max. watts compliance?”
We hadn’t considered the issue of spares in our original post, but in line with that post we think that allowing for max cable or terminal current for spares is extremely conservative. Of course there’s no harm in going that way if the resulting calculated power dissipation is less than the rated MDP of the enclosure, but if the calculation comes out showing a higher MDP than rated (and therefore a “non-compliance”) we think you would be entirely justified in changing the assumed current, especially in an instrument/control application. One approach might be to take the circuit protective device rating of the other circuits in the box (typically 2A – 10A breaker for control circuits). Using the maximum current of any other circuit in the JB would also be justified, we think.
Essentially the argument boils down to this – if there’s no credible scenario where the spares could all be loaded up to their maximum current carrying capacity, then there’s no need to allow for that scenario in the calculation. The choice of “typical” current for spares is essentially arbitrary, so we would be choose some sensible value, consistent with the other circuits in the box, that shows MDP is OK.
Thanks to Steve for the question, and for agreeing to let us turn it into a follow-up post. Please feel free to drop us a line if you’ve got a question on this or any other EEHA related topic – we’re always looking for interesting things to blog on about.