This thread got me looking at Table 450.3.B, for small transformers. This transformer in question is a good example:
277V/120V, 1KVA
primary 3.6A
Secondary 8.3A
Primary only protection, currents less than 9A,
Maximum rating is 167% x 3.6 = 6A No provision for Next Size Up
Primary and Secondary Protection,
Primary Maximum rating is 250% x 3.6 = 9A.
Secondary, currents less than 9A, 167% x 8.3 = 13.9A.
Again, no provision for NSU.
I am baffled for a couple of reasons;
1. How come I never noticed this before? :?:dunce: I checked back three cycles - no change. I don't recall ever working a small transformer before - unless it was in a control panel. I would have used primary and secondary fuse for that .
2. So what are they telling us - Use fuses? That doesn't seem reasonable.
So why don't I use 240.4.B, NSU for CBs 800A or less. T450.3.B has provision for NSU for currents 9A or more. So, I thinking they would have included the NSU if they thought it was necessary.
Anybody got any clues where I am missing the point?
Color me a confused
worm