ECN Forum Forums General Topics of the Trade General Discussion Area ampacity question

hmmmm...the utility rates their x-formers, 15,20,25 kva. sometimes, if one x-former is a feed for 3 or 4 residences , i think it's
larger....

so can this thread relate back to it???

Sparky,

You have to take Demand loading into account.
Does anyone know what percentage they may figure for typical residential housing? It's probably pretty low percentage-wise.

ok let me take over the dumb seat for a while the question was a 100 amp panel actually can serve 200 amps am i correct in that. now i never saw a clear cut answer to that question so now I am going to say yes 100amps or really 80 percent of 100 amps down each leg but in real terms you could only bring 80 percent of 50 amps down each leg because of the neutral conductor only being sized for a total of 100 amps now am i right

doc,
does the ol' sine wave apply to ampacity here? A & B side?
i can read 240 voltage, but cannot amprobe from A to B.
Taking the demand into account, there must be a defined amount of x-former va ???

Bill,

As you mentioned, the demand percentage for a typical residential application is rather low. The optional method (220.30) states the first 10 KVA at 100% plus 40% of remainder. The "remainder is listed at 220.30(b)

Frank (cinkerf)

doc,

In a 'typical' residential situation where there is no Harmonics (or other weird stuff) the Currents on the 2 legs would sort of cancel each other out and therefore very little would flow on the neutral. And anything at 240v would not be using the Neutral anyway. Until recent years the Neutral was even allowed to be downsized because of this.

So, in short, if you were using exactly 80 amps on each leg the Neutral would be carrying - 0 - It wouldn't matter what size it was.

So, it could be looked at as being able to use (if perfectly balanced on a 100a 240v service) 100 amps per leg at 120v for a total of 200A (@120v) or 80A continuous per for a total of 160A (@120v).

Like Scott said, this is not the conventional way of looking at things or proper terminology because your House is supplied with 240v Service which happens to be capable of being used at 120v only because of the Neutral tap.

These are my thoughts and may or may not be considered proper or correct. If anyone has anything to add to this or corrections to something feel free to Comment.

Bill

Bill,

You explained in a way that was taught to me early on. Easy to understand. I've always looked at the situation in that manner.

Frank (cinkerf)