I have a commercial kitchen in a school for a class room setting. The exhaust & makeup air roof top unit was custom made for this application.There are two branch circuits for this equipment. One 208v 3ph for motors & one 120v 1ph for control & gas heat operation. Blue print diagrams don`t match what was installed.The motor circuit is what`s the problem, two 3hp motors on same branch circuit, both have starters & heaters and start at the same time. So do I do a motor feeder calculation or branch circuit or go with equipment name plate at 31 amps. That seems low for start up for two 3hp motors at same time. Please help!
I need help with this motor circuit!!!
- Thread starter indymark
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highkvoltage
Senior Member
Re: I need help with this motor circuit!!!
Base it on the nameplate rating. Protection should be based on table 430.52. Coductors based on several factors such as:
1. Lenth of conductor (How far from the panel)?
2. Will this load be concidered continuous?
Sure hope you installed at least a 1" conduit.
Based on the limited information I would fuse feeders at 80 amps. The conductor size #4 copper THHN.
Base it on the nameplate rating. Protection should be based on table 430.52. Coductors based on several factors such as:
1. Lenth of conductor (How far from the panel)?
2. Will this load be concidered continuous?
Sure hope you installed at least a 1" conduit.
Based on the limited information I would fuse feeders at 80 amps. The conductor size #4 copper THHN.
friebel
Senior Member
- Location
- Pennsville, N.J.
Re: I need help with this motor circuit!!!
To: indymark,
I am addressing the matter with the two 3-HP motors, 208-volts, 3-phase.
STEP #1: Table 430.150, the FLA for a 3-Hp, 208-V,is 10.6 amps. 10.6 times 2 motors = 21.2 A. rounding to 21-A.
STEP #2: 430.24, 21amps times 125% = 26.25 A, rounding to 26 Amps.
STEP #3: Table 310.16, 60 degree column , #10 AWG is 30 amps.
Note: As you are aware, you can go to the 90 degree column for derating purposes of temperature and Table 310.15(B)(2)(a) Adustment Factors for More Than Three Curren-Carrying Conductors in a Raceway or Cable.
The 90 degree column #10 AWG copper wire has a 40-Amp rating for derating purposes.
Step #4. Table 430.52 Dual-Element (Time-Delay Fuse) 175 percent times 26 amps = 45.50 amps.
Section 450.52 Exception No.1 will allow you to go the the fuse size that is listed in Section 240.6, which would be a 50 amp fuse.
This should get you started on your problem.
To: indymark,
I am addressing the matter with the two 3-HP motors, 208-volts, 3-phase.
STEP #1: Table 430.150, the FLA for a 3-Hp, 208-V,is 10.6 amps. 10.6 times 2 motors = 21.2 A. rounding to 21-A.
STEP #2: 430.24, 21amps times 125% = 26.25 A, rounding to 26 Amps.
STEP #3: Table 310.16, 60 degree column , #10 AWG is 30 amps.
Note: As you are aware, you can go to the 90 degree column for derating purposes of temperature and Table 310.15(B)(2)(a) Adustment Factors for More Than Three Curren-Carrying Conductors in a Raceway or Cable.
The 90 degree column #10 AWG copper wire has a 40-Amp rating for derating purposes.
Step #4. Table 430.52 Dual-Element (Time-Delay Fuse) 175 percent times 26 amps = 45.50 amps.
Section 450.52 Exception No.1 will allow you to go the the fuse size that is listed in Section 240.6, which would be a 50 amp fuse.
This should get you started on your problem.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: I need help with this motor circuit!!!
friebel
I looked at your calculation, and came up with different answers.
First 430.24 requires 125% of the largest motor and 100% of others. 10.6x1.25 = 13.25
13.25+10.6 = 23.85 amps
Secondly, 430.52 applies to an individual motor. I believe 430.53 applies.
The approach in 430.53(C)(4), for group installations is to use 175% of the largest motor and 100% of the others.
10.6x175% = 18.55
18.55+10.6 = 29,15 amps or a 30 amp fuse.
Although 430.53(C) probably doesn't apply, (B) can, and you can use the same answer.
If the fuse you choose doesn't provide BC-SC-GF protection as required for a single motor, you will need to add additional properly sized fuses for each motor and call your circuit a feeder.
Jim T
friebel
I looked at your calculation, and came up with different answers.
First 430.24 requires 125% of the largest motor and 100% of others. 10.6x1.25 = 13.25
13.25+10.6 = 23.85 amps
Secondly, 430.52 applies to an individual motor. I believe 430.53 applies.
The approach in 430.53(C)(4), for group installations is to use 175% of the largest motor and 100% of the others.
10.6x175% = 18.55
18.55+10.6 = 29,15 amps or a 30 amp fuse.
Although 430.53(C) probably doesn't apply, (B) can, and you can use the same answer.
If the fuse you choose doesn't provide BC-SC-GF protection as required for a single motor, you will need to add additional properly sized fuses for each motor and call your circuit a feeder.
Jim T
highkvoltage
Senior Member
Re: I need help with this motor circuit!!!
friebel. Why wouldn't he use the nameplate rating? It is apparent that the manufacture concidered that both motors at some time, if not always, would be running at the same time. Thus the nameplate amperage is 31 amp times 150% is 46.5 amps times 125% equals 58.125 Amps. Granted I was slightly high on my previous caulc. but #10 wire I doubt will be large enough. #8 is rated for 55 amps #6 is rated for 75 amps. I would pull no less than #6.
friebel. Why wouldn't he use the nameplate rating? It is apparent that the manufacture concidered that both motors at some time, if not always, would be running at the same time. Thus the nameplate amperage is 31 amp times 150% is 46.5 amps times 125% equals 58.125 Amps. Granted I was slightly high on my previous caulc. but #10 wire I doubt will be large enough. #8 is rated for 55 amps #6 is rated for 75 amps. I would pull no less than #6.
- Location
- Massachusetts
Re: I need help with this motor circuit!!!
Jim why would we bother with 430.53 at all?
I did not run the figures but I bet friebels 50 amp fuse and 10 AWG is on the money.
Jim why would we bother with 430.53 at all?
I would figure it as a motor feeder and be done with it.
I did not run the figures but I bet friebels 50 amp fuse and 10 AWG is on the money.
- Location
- Massachusetts
Re: I need help with this motor circuit!!!
Why are you adding 150% and 125% to a nameplate that already includes the 125%?
- Location
- Massachusetts
Re: I need help with this motor circuit!!!
If it says Minimum Circuit Ampacity 31 amps, I would most definitely use that as my conductor size. 10 AWG or larger depending on distance
The name plate should also say maximum overcurrent protection which I would bet to be 45 to 50 amps.
10 AWG with a 50 amp breaker will be fine if the nameplate asks for that.
Always go with the name plate on the equipment if it is provided.
Bob
What exactly does this name plate say?
If it says Minimum Circuit Ampacity 31 amps, I would most definitely use that as my conductor size. 10 AWG or larger depending on distance
The name plate should also say maximum overcurrent protection which I would bet to be 45 to 50 amps.
10 AWG with a 50 amp breaker will be fine if the nameplate asks for that.
Always go with the name plate on the equipment if it is provided.
Bob
Re: I need help with this motor circuit!!!
That`s part of the problem the name plate does not give min. & max., and the control cabinet I don`t believe was built a true control cabinet builder in my opinion -or maybe by the guy that wasn't busy that day.As for length appox. 70ft and I think in this application it be considered continuous.It will also be Inverse Time Breaker
That`s part of the problem the name plate does not give min. & max., and the control cabinet I don`t believe was built a true control cabinet builder in my opinion -or maybe by the guy that wasn't busy that day.As for length appox. 70ft and I think in this application it be considered continuous.It will also be Inverse Time Breaker
highkvoltage
Senior Member
Re: I need help with this motor circuit!!!
Then I stand with my answer. With an Inverse Time Breaker you may have a polyphase motor. The manufacture would not list the name plate amperage at 31 amps if they did not feel this is important. I believe you would use table 430.150 to reference what type of motors you have. If they were polyphase as in table 430.151(B) your nameplate amperage would be much higher. Since we can assume you have wound rotor type motors we now can use 430.52 for your maximum OCPD would be based at 150%. Since table 430.150 list your 3 hp motors at 10.6 amps per motor we really don't know if the other load is internal or if they have already accounted the continuous value. I believe the addition load is internal and thus you would have to multiply the nameplate rating by 125%. I would much rather assume high than low. Fuse at 50 amps and pull #8 THHN. I know this is smaller than my first post but I now have a little more information. You can always downsize the OCPD later without to much expense but if your conductors are to smaller you have a much larger problem.
Then I stand with my answer. With an Inverse Time Breaker you may have a polyphase motor. The manufacture would not list the name plate amperage at 31 amps if they did not feel this is important. I believe you would use table 430.150 to reference what type of motors you have. If they were polyphase as in table 430.151(B) your nameplate amperage would be much higher. Since we can assume you have wound rotor type motors we now can use 430.52 for your maximum OCPD would be based at 150%. Since table 430.150 list your 3 hp motors at 10.6 amps per motor we really don't know if the other load is internal or if they have already accounted the continuous value. I believe the addition load is internal and thus you would have to multiply the nameplate rating by 125%. I would much rather assume high than low. Fuse at 50 amps and pull #8 THHN. I know this is smaller than my first post but I now have a little more information. You can always downsize the OCPD later without to much expense but if your conductors are to smaller you have a much larger problem.
- Location
- Massachusetts
Re: I need help with this motor circuit!!!
If it is not marked I would figure as friebel
has and add in any additional loads imposed by the control circuit.
Or fire off an RFI (request for information) to the engineer, let them deal with it.
If it is not marked I would figure as friebel
has and add in any additional loads imposed by the control circuit.
Or fire off an RFI (request for information) to the engineer, let them deal with it.
Re: I need help with this motor circuit!!!
indymark - Many things still unclear here. Do you actually have (2) 3HP 3-phase motors on the same branch ckt ? This would be unusual. Or do they both have their own branch ckt. O/C protection ? If so, then it is a feeder (more likely).
You say that they both have their own starter & their own branch ckt. running O/L protection.
I assume it is a "kitchen hood" ventilation system with natural gas heat for the makeup portion.
I also assume that under normal conditions the supply portion & the makeup portion would operate at the same time.
A qualified person can figure this out at the job site if need be.
Yes, send a "Request for Information"
indymark - Many things still unclear here. Do you actually have (2) 3HP 3-phase motors on the same branch ckt ? This would be unusual. Or do they both have their own branch ckt. O/C protection ? If so, then it is a feeder (more likely).
You say that they both have their own starter & their own branch ckt. running O/L protection.
I assume it is a "kitchen hood" ventilation system with natural gas heat for the makeup portion.
I also assume that under normal conditions the supply portion & the makeup portion would operate at the same time.
A qualified person can figure this out at the job site if need be.
Yes, send a "Request for Information"
Re: I need help with this motor circuit!!!
Yes it is very unusual! I do have two 3hp motors on one branch circuit. They separate out in the control cabinet to their own starter heaters. Unfortunately this is a fairly good sized school project and the push is on! I have been wanting answers to questions going on 3 months. I had to wait until equipment was installed to get part of the answers. Yes, this is "kitchen hood ventilation system" with gas heat. The other branch circuit is 120v and is only for hood lights, controls for motors, gas heat & duct detector shut down. I am not in charge of the project, however I'm doing things on my own because I want it right. With input I have been getting from here, I think my wire size is correct. Now I need to be sure on the breaker. Thanks again!
Yes it is very unusual! I do have two 3hp motors on one branch circuit. They separate out in the control cabinet to their own starter heaters. Unfortunately this is a fairly good sized school project and the push is on! I have been wanting answers to questions going on 3 months. I had to wait until equipment was installed to get part of the answers. Yes, this is "kitchen hood ventilation system" with gas heat. The other branch circuit is 120v and is only for hood lights, controls for motors, gas heat & duct detector shut down. I am not in charge of the project, however I'm doing things on my own because I want it right. With input I have been getting from here, I think my wire size is correct. Now I need to be sure on the breaker. Thanks again!
Re: I need help with this motor circuit!!!
to indymarh friebel has the way to go 430-150.
but read table if your motors are multi speed full load current will vary in which case nameplate rating is used. Assume load is cont. or check code letter
\430 150 is used for finding amperage
125%of largest motor
than add sum of full load currents of other motors to find min feeder size
to indymarh friebel has the way to go 430-150.
but read table if your motors are multi speed full load current will vary in which case nameplate rating is used. Assume load is cont. or check code letter
\430 150 is used for finding amperage
125%of largest motor
than add sum of full load currents of other motors to find min feeder size
Re: I need help with this motor circuit!!!
Just considering your situation. The NEC requires motor feeder size to be sized at 125% FLC of largest motor plus FLC of any additional motors.
These ratings are taken from the NEC tables, not name plate. Nameplate ampacity is used only for motor overload protection. Feeder protection device sized according to 430.62 Hope this helps
Just considering your situation. The NEC requires motor feeder size to be sized at 125% FLC of largest motor plus FLC of any additional motors.
These ratings are taken from the NEC tables, not name plate. Nameplate ampacity is used only for motor overload protection. Feeder protection device sized according to 430.62 Hope this helps
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