I assume the 18 amp is based upon the motor name plate.
If so is the motor 1 phase 3 HP at 230 volts.
Article 430 NEC... 18 amp x 2.5 = 45 amp, is the maximum size for the motor branch circuit overprotection. That means a 2 pole 45 amp circuit breaker is required. Using a smaller rated circuit breaker may not pass the motor inrush current and will trip while the motor is starting.
The motor branch circuit conductor size is 18 amp x 1.25 = 22.5 amp, reqiures a # 10 awg wire. This wire is connected from the load side to the circuit breaker to the line side of the starter, then from the load side of the starter to the motor leads.
The motor starter overload relay element must be selected according to the starter manufacture overload relay chart.

 

hi stripedbass
 reddog has summed up the motor very well but just to highlight some thing extra.
If your load on each phase is say 10 amps each then you would use a10amp 2 or 3 pole breaker not a 30 amp for each pole. Also note if the load is polyphase then the breaker poles must be "ganged" together. That is, like good union members --1 out all out.

Hope this helps as well
Don

Let's say the motor specification on design drawing says 3HP 230 VAC 3 phase. I look in my Ugly book and it says the motor will pull 9.6A at 230 VAC. It also says to use a 15 amp breaker. Does this amp rating represent amps/phase? I take then that I must gang three 15A breakers and that No.12 wire would be OK to use.

thanks,

striped.bass@erols.com

It appears that some clarifications are needed pertaining to the original posting to avoid ambiguities.
Please, could you explain your load center 240/120V 3phase panel, while noticing that there are:
1. 208V/120V three phase four wire panels
2. 208V three phase three wire panels
3. 240V/120V single phase three wire panels
4. 240V single phase two wire panels
5. 240V three phase three wire panels
6. 240V/139V three phase four wire panels
etc.
 

240V/(139)? three phase 4 wire Delta high.

Extending my etc.
You may possibly have
240V delta (open = two transformers, or closed = three transformers)
Then, either one or both transformers may have a centertap which will provide 120V from centertap (neutral) to each transformer secondary winding end terminal. This way there may be either:
1. 240V/120V three phase, 4-wire (which you probably have since 240V/120V 3phase 4wire panel does not guarantee that you actually have the power supply 240V/120V 3phase 4-wire type. It could also be 240V/120V single phase three-wire power supply connected to the 240V/120V 3phase 4wire panel)
2. 240V/120V three phase, 5-wire
power supply.
Assuming that the motor is 230V single phase, then 2-30A (twin) circuit breaker can be used if the motor has soft start, else 2-40A is maximum if you do not find 2-45A circuit breaker. The 2-15A twin circuit breaker would be undersized.
If the motor is a 230V three-phase type, then you may need 3-30A (three-pole) circuit breaker (if the motor has the soft start) or higher rated up to 3-40A, if you do not find 3-45A.

Responding to the May 6 post -
Let's say the motor specification on design drawing says 3HP 230 VAC 3 phase. I look in my Ugly book and it says the motor will pull 9.6A at 230 VAC. It also says to use a 15 amp breaker. Does this amp rating represent amps/phase? Yes it does - this is the current drawn in each phase lead to the motor.I take then that I must gang three 15A breakers and that No.12 wire would be OK to use. Preferably, you would use a 3-pole 15 A breaker for this situation, not 3x single pole breakers; # 12 wire would be fine; you will also need overload protection of the motor - the breaker provides the branch circuit overcurrent protection against short circuits and will not protect the motor against overloads. The NEC requires each motor over 1 HP to have separate overload protection.

Hi,
Let us calculate.If it is British HP i.e. P=3x745.7=2240W=2.24kW.If the named current In=9.6A i.e. motor ir star-connected(In=P/3xUnxcos phi x eff).I don't know cos and efficiency. Calculate please and You get approximately refered result.For delta-connection motor's current (in windings) would be three times lower. We need
to use double protection for line and motor.Overload protection have to be 1.25-2.5xIn.Usually overload relays or automate sweech breakers used for motor's protection are regulated. Also we must to montage electromagnetic trip device for line's and motor's protection from short circuit current. But this protection have to keep up motor's start current (usually Im=5-7In).
  You can use two devices - sweech breaker(for example GE V/395-003316 C16 -In 16A Im=5-10In) and thermal overload relay (for example for motor's contactor GE CL45 it be GE RT1P). Or You can use one device with double protection produced especially for motors protection.For ex. GE manual motor controller V/326-000160). You can find other devices in sites http://www.abb.com http://www.ge.com etc.
  Thank You and
  Best regards
tabler,lithuania
   

hi
 Back again, sorry i've been away and missed this thread growing. I put a star on you mr peterb. That submission sums it up rather neatly. I take it that the "ugly book"  is your reference manual?
 Just a foot note The over load would be selected using the current rating of the motor. (there is a contention that you can go to 125% of the flc on the name plate. If that is so in the americas so be it but in a lot of places it isn't considered good)
 Most switch gear manufacturers will size the gear for you based on the load details. I prefer to do this and check the values against my references. This means that if it all goes pear shaped then I'm after the reps to fix it. (and it's a handy second opinion)

Regards Don

Suggestion to/Comment on "stripedbass (Industrial) May 6, 2001"
marked <<>> below.
Let's say the motor specification on design drawing says 3HP 230 VAC 3 phase. I look in my Ugly book and it says the motor will pull 9.6A at 230 VAC. It also says to use a 15 amp breaker. Does this amp rating represent amps/phase?
<<Yes, it does.>>

 I take then that I must gang three 15A breakers and that No.12 wire would be OK to use.
<<One must exercise a caution in here. Wire AWG No. 12 made of aluminum is the minimum size. Wire AWG No. 12 made of copper may be somewhat oversized since normally the 15A protective device has aligned the AWG No. 14 copper wire. Then, one must keep in mind a distance of the motor feeder run. If it happens to be long, e.g. over 250 feet, then the voltage drop will pose a problem and the higher AWG must be considered, e.g. AWG No. 10 aluminum or AWG No. 12 copper. Also, the motor locked-rotor current (LRA) must be kept in mind since the higher LRA is, the higher AWG may be needed to start the motor.>>