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Old 06-22-2011, 09:31 AM   #16
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Just for fun, I looked at the motor nameplate on my 3HP compressor.
Amps 14.0
Code J
SF 1
SFA no number after this.
It is a 3450 RPM continuous duty motor.

Is this really a 3 HP motor

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Old 06-22-2011, 10:13 AM   #17
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Originally Posted by Missouri Bound View Post
You would be best to use a 30 amp breaker with #10 wire for that compressor. How is the compressor controlled? Does it just have a pressure switch? Any information on the compressor would be helpful....a picture would be great. We have run a 5 HP single phase compressor for nearly 9 years now with a 30 amp breaker and 10 ga wire. It's most likely you will need only 2 wires to the compressor, one for each hot leg. You will just need to tape the end of the conductor (white) with black tape to indicate it is a hot wire. Would you just consider running a whip from the panel to the compressor instead of your romex? It makes for a neater installation and you could pull two black or red wires instead of marking the cable.
Well the compressor is control by a pressure switch,with an off and on lever where the wires goes. i got info off the motor if this helps,its a centry motor,HP-5,volts 208-230,time- cont.,rpm-3450,hz-60,ph-1,type- cp,sf-1.00,amps-15.0
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Old 06-22-2011, 10:19 AM   #18
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Originally Posted by rjniles View Post
Do not understand this #8 with 70 amp breaker. 5 HP is 3730 watts. At 240 volts that is a little over 15 amps. 12-2 with a 20 amp 2 pole breaker should be fine.

To the OP, if the compressor is labeled 240 VAC and not 120/240 VAC, you do not need a neutral connection. Only the 2 hots need be connected to the 2 pole breaker.

Well here's the info off the motor it self. Hp-5 Volts-208-230 ,time -cont. ,rpm-3450 ,hz-60,ph-1,amps-15.0,type-cp,sf-1.00. didnt see anything relating to a lable with info on 240vac or 120/240 vac anywhere on motor or compressor.hope this helps out any Thanks Johnx
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Old 06-22-2011, 10:43 AM   #19
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Originally Posted by johnx View Post
Well here's the info off the motor it self. Hp-5 Volts-208-230 ,time -cont. ,rpm-3450 ,hz-60,ph-1,amps-15.0,type-cp,sf-1.00. didnt see anything relating to a lable with info on 240vac or 120/240 vac anywhere on motor or compressor.hope this helps out any Thanks Johnx
My 3 HP
Quote:
Amps 14.0
Code J
SF 1
SFA no number after this.
Here are two pieces of information from compressor motor nameplates.
Both motors have a Service Factor of 1
There is 1 amp difference in motor amps
There is a 2 HP difference in the advertised output.
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Old 06-22-2011, 11:54 AM   #20
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Originally Posted by a7ecorsair View Post
Just for fun, I looked at the motor nameplate on my 3HP compressor.
Amps 14.0
Code J
SF 1
SFA no number after this.
It is a 3450 RPM continuous duty motor.

Is this really a 3 HP motor
Here is the procedure for a 3HP, 240V single phase motor;

430.6(A) 1 states;
(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.

Table 430.248 indicates 17 amps for a 230V 3HP single phase motor.

430.22(A) General states;

(A) General. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motorís full-load current rating as determined by 430.6(A)(1).

17A x 125% = 21.25A

Table 310.16 indicates #12AWG is rated for 25A.

Now before you get your panties in a bunch the asterisk refers us to the bottom of Table 310.16 where it states: See 240.4(D).

240.4(D) states;

(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.

240.4(E) refers to tap conductors which is not applicable here. However, 240.4(G) is a table and tells us that Article 430 applies for motor branch circuit conductors. This means that #12 AWG is specifically permitted to be used for conductors supplying this 25A motor load.

Next we look at sizing the ground fault, short circuit, overcurrent protective device.

430.52 states;

430.52 Rating or Setting for Individual Motor Circuit.
(A) General. The motor branch-circuit short-circuit and ground-fault protective device shall comply with 430.52(B) and either 430.52(C) or (D), as applicable.

(C) Rating or Setting.
(1) In Accordance with Table 430.52. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used. (Also read the exception.)

Table 430.52 states that for an inverse time circuit breaker (ITCB) protecting a single phase motor it is to be sized at 250% of the FLA as stated in Table 430.248.

17A x 250% = 42.5A

Based on the above calculation a 50A ITCB is correct.

If the motor fails to start and run then 430.52(C) (1) Exception No. 2(c) can be applied which states;

(c) The rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes.

12A x 400% = 78A

Since we canít exceed 400% of the motor FLA we round down to a70A ITCB on a piece of #12 AWG conductor.
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Old 06-22-2011, 11:58 AM   #21
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Well here's the info off the motor it self. Hp-5 Volts-208-230 ,time -cont. ,rpm-3450 ,hz-60,ph-1,amps-15.0,type-cp,sf-1.00. didnt see anything relating to a lable with info on 240vac or 120/240 vac anywhere on motor or compressor.hope this helps out any Thanks Johnx
Then the procedure I posted in #5 is code compliant. Anything less is a violation.
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Old 06-22-2011, 02:42 PM   #22
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johnx. On your switch there should be 4 terminals, two which go to the motor and two which will be your 240 volt supply. That's all you need to worry about, no neutral or ground is used. Your 12 ga. wire should be fine, and use a 20 amp breaker for it.
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Old 06-22-2011, 02:59 PM   #23
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johnx. On your switch there should be 4 terminals, two which go to the motor and two which will be your 240 volt supply. That's all you need to worry about, no neutral or ground is used. Your 12 ga. wire should be fine, and use a 20 amp breaker for it.
Only if you want to violate the NEC. Exactly how are you arriving at this? I have stated the NEC requirements. I used the NEC to size the conductors and the OCPD device. Are you using some special code that I don't know about? Do you have any experience sizing motor conductors and OCPD's for motors? Please explain.
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Last edited by electures; 06-22-2011 at 03:10 PM.
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Old 06-22-2011, 03:35 PM   #24
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Here is a link to an online calculator for sizing conductors and OCPD's.
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Old 06-22-2011, 04:55 PM   #25
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hello, I need to now how to wire my compressor? Its a 5hp 230v 15amps 60 1 phaze compressor, I have 12/3 wire for it and its located about 5ft from panel, I have a 220 outlet in my laundry rm we dont use, so i have located the breaker for it and was planning on using those breakers for my compressor,which is a 30amp pole breakers, i just need to know which wires goes to the breaker and to my switch on compressor, the 12/3 has a red,blk,white, bare copper wire for ground,I just need to know where these wires go on the switch of compressor,and inthe panel??? the 220 outlet is wired in panel to the breaker as follows, red,blk to 30amp pole breaker,and white wire to the ground,with no bare copper wire.? i would appericate the help with this ..
Thanks Johnx
There is NO way that this is actually a 5 HP motor.
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Old 06-22-2011, 05:17 PM   #26
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C) Rating or Setting.
(1) In Accordance with Table 430.52. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used. (Also read the exception.)

Look at the end of the sentence. "shall be used".
Right electures, look at the end of the sentence. Then look at the whole sentence. Then put it together. not exceeding...shall be used.


Not trying to start an argument, just stating that I do believe it means you can go up to 250%...not that you have to use a 250%
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Old 06-22-2011, 05:30 PM   #27
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Right electures, look at the end of the sentence. Then look at the whole sentence. Then put it together. not exceeding...shall be used.


Not trying to start an argument, just stating that I do believe it means you can go up to 250%...not that you have to use a 250%

If you don't believe me follow the link to the online calculator in my signature. You will get the same answer. Does that mean the website is wrong also.
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Old 06-22-2011, 05:45 PM   #28
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That website doesn’t say whether it must be nor that it is not to exceed. All it did was calculate at 250%. All I can say is what the code book says…”not exceeding”.

So do I believe you? No. Do I believe that website is completely correct? No. I can say that we agree to dis-agree. That’s cool.
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Old 06-22-2011, 05:53 PM   #29
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That website doesn’t say whether it must be nor that it is not to exceed. All it did was calculate at 250%. All I can say is what the code book says…”not exceeding”.

So do I believe you? No. Do I believe that website is completely correct? No. I can say that we agree to dis-agree. That’s cool.
True, but it comes out to be exactly what I have stated all along. The reason for the high rating is for the starting current of the motor (6x FLA). If a time delay fuse is used instead it is 175% of the motor FLA. So it would be 28 x 175%=49 amps. A 50A TD fuse would be used. TD fuses are designed for motors. A smaller device can always be used, but you run the risk of the OCDP not holding during startup.
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Old 06-22-2011, 06:52 PM   #30
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Going through the Canadian electrical code i get something similar to Electures.

#8 wire with a 70A breaker OR #8 with a 45A TD fuse. For a 5hp single phase 230v.

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