To avoid these ads, REGISTER NOW!

wire gauge for 240v compressor install?

TwoInch

Well-known member
Joined
Mar 29, 2012
Messages
2,828
Location
NW INDIANA
i have a 240v hardwire compressor i am about to install in the shop. the motor does not list a HP rating, but lists 15amps 240v.

i am adding the 240v circuit, but not sure on what wire gauge to use, and what amp breaker to install. 30amp? 40amp?

possibly run a welder in the future, if that changes things at all.

any help would be appreciated.
 
Last edited:
To avoid these ads, REGISTER NOW!

sloppy

Well-known member
Joined
Mar 3, 2013
Messages
481
Location
Ohio
240v*15a= 3600 watts. 3600watts*.00134102209 = 4.8 HP

would need EF. and PF of motor but 4.8HP should be pretty close.. Unless I did some math wrong with is a good possibility :D

A 20 amp breaker should be plenty for that compressor, cant say much for adding a welder to a circuit since welders power needs can have a pretty wide range.
 

wyliesdiesels

Well-known member
Joined
Aug 14, 2012
Messages
20,256
Location
Modesto, CA
240v*15a= 3600 watts. 3600watts*.00134102209 = 4.8 HP

would need EF. and PF of motor but 4.8HP should be pretty close.. Unless I did some math wrong with is a good possibility :D

A 20 amp breaker should be plenty for that compressor, cant say much for adding a welder to a circuit since welders power needs can have a pretty wide range.

2011 NEC T430.248 lists 17a FLC for a 3HP motor. If the motor says 15a, then i would say its closer to 3HP.

BTW- wire feeding motors is sized at 125%. So 15a x 125% = 18.75a so #12 wire!
 

MrMark

Well-known member
Joined
Jan 25, 2010
Messages
4,626
Location
Southern Cal.
You could run 12 gauge with a 20 amp breaker, but 10 gauge with a 30 breaker will give you room for a bit of growth for not much more money.
 
OP
T

TwoInch

Well-known member
Joined
Mar 29, 2012
Messages
2,828
Location
NW INDIANA
thats what i was looking for guys thanks.

i just needed the minimum for the compressor, and i can go heavier with a higher amp breaker if i feel inclined. im thinking the 10g-30a would do fine.

not sure on possible welders at the moment, been running a 120 unit. havent upgraded yet.

thanks fellas.
 

sands35

Well-known member
Joined
May 29, 2012
Messages
936
Location
St. Joseph, MI
You are using a plug as the disconect?

BTW, Square D has a motor electrical hookup calculator in excel. Google "square d NEC motor calculator". (Or something like that).
 
OP
T

TwoInch

Well-known member
Joined
Mar 29, 2012
Messages
2,828
Location
NW INDIANA
i was just going to hard wire into a regular switch on the wall. add a outlet later if need be.
 

pattenp

Well-known member
Joined
Jun 4, 2008
Messages
10,175
Location
Virginia - USA
What is the so called regular switch you are planning to use? The switch needs to be rated for motor start loads. You can't use a standard wall type light switch.
 
OP
T

TwoInch

Well-known member
Joined
Mar 29, 2012
Messages
2,828
Location
NW INDIANA
What is the so called regular switch you are planning to use? The switch needs to be rated for motor start loads. You can't use a standard wall type light switch.

havent even looked for one yet. i do know that the compressor was installed this way at the previous location. its a 15a motor, i cant imagine its that much of a draw.

FWIW, i was not planning on getting a standard light switch/lite duty type switch.
 
To avoid these ads, REGISTER NOW!
OP
T

TwoInch

Well-known member
Joined
Mar 29, 2012
Messages
2,828
Location
NW INDIANA
is it possible that this compressor could have been running on 120v at the previous location?

i can post pics of the motor in question if that helps. the id tag states only 240v, so i assume no. but if there is something i need to check before firing it up to make sure that it is wired properly, that would be good info.

its just a AO Smith motor, cheap stuff.
 

sands35

Well-known member
Joined
May 29, 2012
Messages
936
Location
St. Joseph, MI
3HP motors can typically be wired for 120 OR 240. There should be a wire diagram on the motor or under a small cover.

Square D calculator:

Square D Calculator

You need to have some sort of disconnect. Either a plain jane outlet wired for 240, an AC pullout disconect or a throw switch disconnect. Leviton (?) makes 30 amp 2 pole switches. Big box home improvement stores also sell switch disconnects for 3HP motors.

FYI - wiring up motors is one of the few things that isn't "logical" at 1st blush. Really need to follow NEC guidelines for a direct wire motor.
 
Last edited:
OP
T

TwoInch

Well-known member
Joined
Mar 29, 2012
Messages
2,828
Location
NW INDIANA
3HP motors can typically be wired for 120 OR 240. There should be a wire diagram on the motor or under a small cover.

Square D calculator:

Square D Calculator

You need to have some sort of disconnect. Either a plain jane outlet wired for 240, an AC pullout disconect or a throw switch disconnect. Leviton (?) makes 30 amp 2 pole switches. Big box home improvement stores also sell switch disconnects for 3HP motors.

FYI - wiring up motors is one of the few things that isn't "logical" at 1st blush. Really need to follow NEC guidelines for a direct wire motor.

it is stated on the outside compressor shroud/cover that is 6.5hp, which is obviously craftsman HP rating :lol_hitti the motor itself states 15a, but leaves the h.p. spot blank for some reason.

i assumed a 3 or 4hp motor could be wired for either 120v or 240v, but the reason i am asking, is that the motor only states 240v, no 120v option, and has no diagrams for wiring. all i have is the 3 wires coming off the compressor in a corrugated metal tube flex conduit.

if i post pics, would it be possible to tell if its wired for 120v by the previous owners? i have now ay of contacting them, so i cant simply call and ask. i did not see the breaker at the former location to see if it was a double or not.

from what it looks like without tearing into the motor, it looks like someone may have been in their before. but i cant say for sure. the windings look like a 2 year old wound them though, i will say that

the compressor and motor are both functioning. i ran it though it paces at before i removed it from the former location. didnt think to check the voltage or breaker and see what was up until i got back home with it.
 

sands35

Well-known member
Joined
May 29, 2012
Messages
936
Location
St. Joseph, MI
A 3HP motor should be 17 FLA amps. The "real" rating is more like 2.5 hp. Yeah, 6.5 is likely an advertising number.

If it's 240, then it's 240. Two hots and a ground.

12 awg THHN (in a conduit) or 10 awg NM wire on a 35 amp double pole breaker. A 30 amp disconnect is appropriate.
 
OP
T

TwoInch

Well-known member
Joined
Mar 29, 2012
Messages
2,828
Location
NW INDIANA
ok so looking around the shop, i have a couple breakers laying in a box. a couple 30a square Ds, and a 40a square D.

the 30a are these http://www.grainger.com/Grainger/SQUARE-D-QO-Circuit-Breaker-1D307

are these appropriate, or do i need to head to the supply shop and buy something different? i did notice that the 30a has three clips that clip into the box, and the 40a has 4 clips. both 30a are the same with 3 clips... normal?
 

MrMark

Well-known member
Joined
Jan 25, 2010
Messages
4,626
Location
Southern Cal.
the 30 amp is a QO series breaker, considered by many to be the finest circuit breaker available. Is your panel a QO? Square D also makes Homeline panels and breakers. They are not interchangeable with the QO.
 
OP
T

TwoInch

Well-known member
Joined
Mar 29, 2012
Messages
2,828
Location
NW INDIANA
not sure. have to go out to the shop and check. the panel has zero 240v circuits right now, but a lot of space for additional add ons.

i hope its the proper type, that would save some cash, so i can get heavier NM

is the QO style common in residential stuff? or no
 

MrMark

Well-known member
Joined
Jan 25, 2010
Messages
4,626
Location
Southern Cal.
From what I have seen, no. Only high end stuff or someone that knows electric. Most builder grade stuff is Homeline/Murray Siemens. Siemens does make some nice panels, but also cheap ones.
 
OP
T

TwoInch

Well-known member
Joined
Mar 29, 2012
Messages
2,828
Location
NW INDIANA
damn, i thought i was in luck. im sure what i have is a low end type panel. but ill check tomorrow.
 
To avoid these ads, REGISTER NOW!
Top Bottom