In article , "Eigenvector" wrote:

But back to my question, which was more out of interest than contesting a
ruling. The 5 extra amps is more than just the wire, what about the device
at the other end of that wire - it'll be taking on an additional 5 amps too.

No, actually, they won't. A device draws whatever it draws. A 1200W heater,
for example, pulls 10A at 120V whether the circuit it's on has a 15A breaker
or a 100A breaker.

Maybe not important, if it has its own fuses, they'll blow too. I was just
curious is all.

Nope; see above.

Now this next part is just my mind asking questions, not making personal
attacks.

A lot of people are firing back claiming the wire can handle 65 A, 60 A,
2000A, whatever, those current carrying capacities aren't advertised on the
wire bundle, so how would an electrician know that? I'm presuming an
electrician isn't schooled at the same level as an Electrical Engineer. So
looking at a wire and being able to tell the ampacity of it seems liberal to
me.

You don't tell the ampacity just from looking at the wire. Well, you might if
you have NEC Table 310.16 memorized, but it's the table that tells you, not
the wire.

When they allow higher breaker sizes it also tells me that the NEC
conventions are largely anecdotal or arbitrary as opposed to calculated or
theoretical values - which is even more worrisome to me.

It should be worrisome if your conclusion were correct; fortunately, it's not.

It's important to remember that the Code allows going up *one*, and only one,
breaker size, and then only if the listed ampacity does not correspond to a
standard breaker size. For instance, a wire with a listed ampacity of 50A may
*not* be breakered at 60A, because 50A is a standard breaker.

I would expect
them to state restrictions and rules more along the lines of "This is the
theoretical limit of this particular wire, plus a safety margin of 1.5 - you
may not use something higher than this value" Rather than, "Just use the
next highest one, they don't make the correct one for it."

There isn't really any difference between the two situations, and the latter
is much more convenient to implement in practice.

If they were to
state something like that, I would also expect them to qualify it by stating
the reason why they make that allowance. Like I said, just me asking
questions.

Perhaps the most important thing that you're missing here is the part of
240.4(B) that says "provided all of the following conditions are met" --
conditions which I only summarized in my earlier posts in this thread. It
seems to me that it's time to quote the first one in full, and comment on it:

"(1) The conductors being protected are NOT part of a multioutlet branch
circuit supplying receptacles for cord-and-plug-connected portable loads."
[emphasis added]

Let's examine the possibilities under which the bump is permitted by this
condition:

1) Circuit has no receptacles at all. That means it's feeding either a
subpanel, or direct-wired stationary equipment. In the former case, the load
will depend on which circuits in the subpanel are in use, and is unlikely to
ever be at maximum. In the latter case, the load is precisely known, and the
safety (or lack thereof) is readily determined.

2) Circuit has a single receptacle, or multiple receptacles, for
cord-and-plug-connected NON-portable equipment. Again, the actual load can be
readily determined: you know what's going to be used on the circuit, because
it's sitting right next to the outlets, and it isn't going anywhere. A circuit
supplying outlets for a table saw and a 5HP air compressor would be a good
example of this category.

3) Circuit has a single receptacle for cord-and-plug-connected portable
equipment. It is unlikely that any Code-compliant circuits can exist in this
category: the *lowest* listed ampacity that would be permitted to be bumped is
a 55A conductor breakered at 60A; it is a Code violation to install a
receptacle with a rating lower than that of the overcurrent device on any
circuit over 20A; and any load which requires a 60A (or higher) rated
receptacle is highly unlikely to meet anyone's definition of "portable".

In short, this means that the bump up to the next higher breaker size is
limited to circumstances in which the load is either limited, or more or less
fixed, and readily predictable.


--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.