In article . net,
Bob wrote:

"Robert Bonomi" wrote in message
servers.com...

Next, what all might be running at one time, _worst_case_.
obviously the lights, plus the air conditioning. The compressor
can be 'trusted' to kick in at 'the worst possible time'.
Add in the dust collector, and the the biggest hog power tool.
Wups! what if there are _two_ people in the shop. might have
two tools running. That puts the theoretical total draw into
the 100A range. which looks like 3 ga wire.

Robert, I think your technical description is accurate. However, there is
one very important key point to remember. The current ratings you use are
full load amps. A table saw cutting 3/4" redwood doesn't draw anywhere near
the current when cutting 2" hard maple. A 2hp and 5 hp motor will draw
approximately the same current when subjected to the same loads. An air
compressor does not draw full load when it first kicks in (excluding initial
startup inrush). The current climbs as the pressure in the tank approaches
maximum. A drill press is rarely used anywhere near full load, unless
swinging a large forstner in hard wood.

The rather dim scenario you painted is highly unlikely to occur, in my
opinion. But it does point out the need to consider useage patterns and
simultaneous loads. Actually this works in favor of most users. Otherwise,
they would be tripping circuit breakers constantly.

If it is a 'dim' scenario, you didn't budget enough for lighting! grin

Note, I was expressly talking about 'worst case' situations -- any rational
engineering *does* _design_ to the worst-case scenario specifications. Yes,
you can 'get away' with 'less' in many situations, but, eventually, it *WILL*
'bite' you, probably "at the worst possible time, in the worst possible way".
There is a reason, borne of painful experience, that O'Brien's Law ("Murphy
was an optimist!") exists.

The _last_ thing you want is the *feeder* to a building with sharp, rapidly
spinning metal thingies to shut down. You lose all the lights -- and thus
can't see _where_ those sharp thingies are -- *and* they're still spinning,
at least until they coast to a stop. This is a significant argument in favor
of a _separately_ protected, separate feeder for the lighting circuits. And
for splitting the lighting across (at least) two circuits, as well. ("Yes,
I _do_ have far more experience in dealing with Messrs Murphy and O'Brien
than I really care to; why do you ask?"

As for the rest, lighting is a 'static' load; an air-conditioner draws quite
close to 'rated' amps while the compressor is running; the air-compressor --
kicking in to 'top off' the tank -- is going to draw around 80% of rated load;
a dust collector runs 'normally' at close to rated load, as well.

your comment about a 2hp motor, and a 5hp one drawing similar current under
similar loading; that claim _is_ true, but only as far as it goes -- static
load, under static conditions. The 5hp motor will draw *considerably* more at
start-up; also at other times when the motor undergoes a significant increase
in load, over the short term.

One final comment: For power feeder/distribution, the 'cost' associated with
"over-engineering" the implementation to 'worst case' specifications is minor.
Materials cost is usually swamped by labor expense. Also, the 'down side
risk' of finding out _after_the_fact_ that the newly done work is 'inadequate'
and requires replacement, puts the minor additional cost of the 'do it right
the first time' approach in the realm of "*really* cheap insurance".