ampacity of motor conductor

[thomas_b_l]

Please Help anyone.....the question on a preparation test....states What size THW feed is required for a single -phase 3/4, 120 volt and a three phase 2hp,208 volt motor.. Okay i got all of the figures and looked at the tables .. and my answered came up with 24.75 amp...... the problem is int 310.16 the table says that 14awg ,12 awg,and 10 awg for ThW can be used for 2 different ampacity rating ..there is an asterik at the bottom which cross reference to art...240.(d) which states when to you the correct apacity of the 3 different awg wires...... on this motor application which wire do i us ampacity is (24.75amps)sorry for the long post but i needed to explain so i can receive proper help

 

[iwire]

Re: ampacity of motor conductor

As far as 240.4(D) you should look at 240.4(G)

As for the different ampacities listed for one size wire you need to find out the temperature rating of the insulation.

You said "THW" wire that along with 310.13 will get you there.

 

[thomas_b_l]

Re: ampacity of motor conductor

Mr Bob thanks for your reply but the question ask.....What size thw feeder is required for a single phoase 3/4hp ,120 vold and 3phase 3hp208 volt.. the ampacity of the 2 motors comes out to be 24.75amps my delema is according to the table 310.16 the 12awg can be used for 25 amps also it states that in 240..d that in some application the 12 awg is only good for 20 amps the table shows 25ams for 12 awg in this application because the ampacity of the 2 motors equal (24.75am)would i used the 12awg or would i go to the 10 awg based on these to articles .hope am explaining thoroughly to get the answer im seeking thanks

 

[iwire]

Re: ampacity of motor conductor

It is an unwritten rule here that we do not 'give' test answers to people.

We are more than glad to point you in the right direction.

Originally posted by thomas_b_l:
my delema is according to the table 310.16 the 12awg can be used for 25 amps also it states that in 240..d that in some application the 12 awg is only good for 20 amps

240.4(D) does not 'rate' the wire at all.

240.4(D) tells us in certain circumstances we will protect 12 AWG CU with a 20 amp overcurrent device.

That said please read 240.4(G), that section has important info regarding motor circuits.

 

[thomas_b_l]

Re: ampacity of motor conductor

thanks for the insight wasn't asking for answer just at which situation to awg on of the wire per table 310.16 or 240.4d thank very much