NYY-J sizing

Rockingit · Jul 20, 2021

Just speccing a job for tomorrow - simple 3ph immersion supply at 12kW. Supply to be constantly on, using the immersion temp stat for control (by request....) via a local isolator. (Have suggested future improvements with timers, stats, contactors etc but for now it's just this)

So that's basically 17A per phase, table 4D4A says that 4mm NYY-J will be absolutely fine at 34A max on some tray, it's only about a 15m run max from the board. C20 in the board, job done. It just all sounds a bit lightweight though for a 'constantly' running 3ph heating supply??

Lucien Nunes · Jul 20, 2021

17A is 17A. If it were bare MICC ref method E/F/G it would be well in on 4L1.5 and just in on 4H1 (although disallowed as a 1mm² power circuit). 12kW is only six pairs of CP62s. Remember those?

I don't like MCB ratings close to long-hour loads, some manufacturers recommend 80% or so to prevent premature MCB ageing. Could you use a B25? If not, can you put blanks either side?

Rockingit · Jul 20, 2021

Lucien Nunes said:
17A is 17A. If it were bare MICC ref method E/F/G it would be well in on 1.5mm² and almost in on 1.0 (although disallowed for being a power circuit). 12kW is only six pairs of CP62s. Remember those?

I don't like MCB ratings close to long-hour loads, some manufacturers recommend 80% or so to prevent premature MCB ageing. Could you use a B25? If not, can you put blanks either side?

lol - I do indeed not only remember with fondness (see my profile pic!) but also can't wait to get back out using them again (in the generic sense) - and yes, I take your point. As for the MCB, it'll be a case of if I can source a C25 off the shelf today (I have a C20 in stock already) but it's going into an almost empty Hager board anyway so loads of ventilation.

Rockingit · Jul 20, 2021

Groan.... can't find 4mm 5c NYY locally anyway so it's a choice of 4mm SY (which in fairness would be fine, I just don't like it) or 6mm H07 which just seems a waste of copper.