im wiring my shop and have 2 pieces of equipment i am not sure of. a
true 3 hp compressor and a 3 hp unisaw. is a 20 amp circuit enough for
each or these or do i need to go to 25 or 30 amp? the compressor says
it needs 15.4 amps and the tablesaw says 15.5 amps. thanks all ...
NEC limit for permanently attached devices is 80% of breaker rating.
Which is 16A for a 20A breaker. So, you're OK on _that_ basis.
Then, there is the issue of 'start up' load, vs 'running' load.
Virtually any motor draws more power spinning up, than it does
running 'at speed'. However, with the table-saw, the start-up
current draw is _probably_ not going to approach the draw of a
maximum-load cut. The compressor probably has 'smarts' so that
the motor starts up 'unloaded', and then picks up the load.
Me, I'd 'over-engineer' the solution, and see what happens. After all,
wire is _cheap_. Pull 10 ga. wire, and use a 20A breaker, initially.
Preferably a 'motor rated' (or slo-blow) one, if I can lay hands on it.
In the context of _that_ wiring approach, If the 20A breaker trips too
frequently, it _is_ safe to replace the 20A breaker with a 25A or 30A one.
i didnt think about the startup draw. thanks. this is going to be
permited and inspected also. im thinking 30 amp is the best way to go
but i am not sure if i can use a three wire plug or if i have to use a
4 wire plug and change all my equipment over to 4 wire. i know i need
4 wire for such things as dryers and stoves but that seems to be
overkill for ww equipment. any ideas?
Easy answer---call the permit guy. They are paid to give you free
advice on such matters. Then go one size over on whatever they tell
you---ie if they say 12ga wire, use 10, if they say 25 amp use 30.
You could look it up in the code yourself if you want, but I would
rather go to the dentist. :D
I wouldn't assume that 30A is the best way to go. Your startup current is
unlikely to be so great that you need to go that high. 20A should be fine.
What are you using now? Are you tripping breakers? What does the equipment
manufacturer recommend? One idiosyncrasy about posting questions like yours
here is that you can get a lot of not-so-informed opinions from a lot of
very well meaning people. Often, they tend to the recommendation that more
is better, and that is simply not always true. There are places to go and
get the factual information you need and you would be better served seeking
those out. Have you asked an electrician? There are some who post here -
it would be worth your while to wait on their input if you don't do any
If you are running 10ga why would you ever use less than a 30a breaker on a
If this was a test question the correct answer would be <min>14 ga wire and a
(long story but it is on the inspector's test)
hint: you can use 310-16 at the full rating on a motor not 240.4(D), O/C device
can be 250% of FLA and round up to next breaker size.
I guess the real answer is "what do the instructions from the manufacturer
Me? Like I said, I'll 'over-engineer' the physical -- the cost difference
is minimal, and it provides additional flexibility for 'later'.
At the same time -- and particularly for 'dedicated' applications -- I tend
to match the breaker 'as closely as possible' to the _actual_ load. So that
it trips 'as soon as possible' in case of device excursion into "abnormal"
Call it 'defense in depth', against Mr. Murphy. <grin>
You're throw'n a dart with the lights out, dude.
Holy cow - what a thread!!
Top of page 5.
If this isn't your manual, click around and find the right one.
Good thing it doesn't matter much (!!) because either you're throwing
breakers, or relying on your motor's thermal protection
And you, Bubba, are a 10 watt bulb in a 100 watt fixture.
It isn't mine. _I_ don't have a unisaw.
What the manual for a UniSaw says is also *irrelevant* to the last question
asked -- which was why would I use something smaller than a 30A breaker on
10 ga. wire. Note that that is a _generic_ question, and was answered as such.
*ASSUMING* the motor _has_ thermal protection, that is. While on a UniSaw
it likely does, it is -not- guaranteed on all other devices. Heck, some
devices don't even _have_ motors. Or internal overload protections.
It is an indisputable fact that if a device --_any_ device -- will operate
satisfactorily -- under all rated conditions -- on a circuit with a 20A
breaker and 12 ga. wiring, it will also operate _exactly_ as well on a circuit
with a 20A breaker and 10 ga. wiring.
If the device is 240V _only_, then 3-wire is code-compliant Hot1, Hot2,
and _ground_ (usually through the conduit itself; flex conduit or non-metallic
sheathed cable does require a ground *wire*). No neutral. If there's
any 120V 'stuff' in the device, (typically lights, timer, etc.) then a
neutral wire, in addition to the ground is required.
Recommendation: pull the neutral to the box, *even*if* you don't use it _now_.
Again, wire is cheap. and it gives the flexibility "down the road" to
run 120V equipment off that box. Or to slap up a double-gang box next to
the 240 outlet, with a pair of 120V duplex outlets -- one pair on each
side of the 240.
I asked my electrician, who is also a woodworker. We put in a 30 amp
circuit for my new Unisaw a couple of years ago. With a twist lock
We also put in a master cutoff switch, with a place for a padlock, to lock
out everything on the new subpanel. The lights are on the old subpanel, as
are the plugs for the freezer, and non-shop tools. I don't want
unsupervised use of the shop by my grown sons, until I'm certain they have
a little more experience and safety training. They didn't take shop in
A master electrician did it right, and I don't worry about it. I have
enough to be concerned about.
On Wed, 28 Jul 2004 14:44:09 GMT, skeezics
thank you all for the advice. an electrician told me 20 amp was
minimum code and should be fine but not to add any 110 outlets off
that. if i were to do that he said go to 25 or 30 amp. both the
compressor and the saw have newish motors so i guess 12 wire and 20
amp will be ok. if it proves to be a problem i can change it after the
inspection. the shop is unfinnished so changing stuff is no big deal.
Whatever you decide on ampacity, run 10 ga wire, then use the c'brkr size of
your best informed choice.
The Unisaw will run fine on a 20 amp breaker (mine does), and if for some
reason you feel that you need 30 amps later on that circuit, the 10 ga wire
will allow the increase without further ado ... you'll be glad you did.
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