Motor Circuit Sizing

[Abtech Electric]

I have been in discussions with an "Engineer" over the following. He insist that
a 30amp breaker 208volt three phase will be adequate for a 7.5HP motor on an air compressor. Problem one is that he thinks all that needs done is take 125% of the nameplate rating for the circuit, breaker and disconnect size. Problem two is that when he reads the manufacturers recommendation is does not know the difference between a time delay fuse and a breaker.
Recommendation only list fuse size. Problem three is that he insist because of the starting means for reduced voltage starter that it may remain on a 30amp circuit.
I tried to get him to listen to my argument that 430.6 prepares the minimum ampacity as per table 430.150. I tried to explain that the NEC is the minimum requirements allowed despite name plate or manufacturers ratings. No where did I read that the starting means allows this to be lower than the table allows. I have tried to show him these code minimum requirements and my calculations from 430.6, 430.52 and table 430.150 and he holds his decision to a manufacturers recommendation that he his reading wrong.
Has any one ran across this before. Inspectors won't give there time to put together a proper legal response. My only other thing I could do is to hire a real Engineer to do the calculations and help back me up.
Thanks

 

[iwire]

Unless you bought the motor and the compressor and assembled the units yourself I agree with the engineer.

If this is a factory assembled unit with a label you are required to follow the label per 110.3(B), table 430.150 will not apply.

 

[Abtech Electric]

Yes - but he is basing his calculation from a " fuse recommendation" and we are using breakers.

 

[augie47]

as Bob states, 110.3(B) rules ! If the manufacturers instructions are included in the listing and labeling and call for a 30 amp FUSE then you need a 30 amp fuse in the system.
If the mfg calls for OVERCURRENT protection the fuse or breaker is allowed.
If there are no manufacturer instructions, then 430.250 is your guide.

 

[Abtech Electric]

Yes - thank you and I have not denied that a 30amp fuse is recommended and acceptable and will be located at the units service disconnect. I only have a subpanel with branch breakers and NO fuses. The manufacturers recommendation DOES LIST a breaker size to use and ONLY list the fuse size.
Therefore wouldn't I use table 430.150 to size the breaker in the subpanel with NO other recommendation listed? When I showed the Engineer the listing was for a fuse and does not mention anywhere a breaker recommendation he told me "fuse or breaker it doesn't matter".

 

[iwire]

The manufacturers recommendation DOES LIST a breaker size to use and ONLY list the fuse size.

Is the above a typo?

 

[augie47]

I get gun shy when folks say "compressor" because of all the "special" HP ratings, but if the motor is identifeid as a 7-1/2 hp, I would use 430.150, as my guide plus installl the 30 amp manufacturer's fuse.


Are they using the fuse as overload protection ? (vs shirt-circuit/GF protection)

 

[Abtech Electric]

From who? And what?

 

[mivey]

From who? And what?

:-? :grin:
In response to what?

Type slower and more clearly. It appears you are thinking one thing and typing another.

 

[Abtech Electric]

I get gun shy when folks say "compressor" because of all the "special" HP ratings, but if the motor is identifeid as a 7-1/2 hp, I would use 430.150, as my guide plus installl the 30 amp manufacturer's fuse.


Are they using the fuse as overload protection ? (vs shirt-circuit/GF protection)

Two major things help the compressors life expectancy. Proper voltage and ventilation. We all know that compressor rooms are hotter than the 86 degree allowed before temperature corrections therefore I will be using #8awg
even though #10awg squeaks by if you figure the air compressor room will be around 100 degrees.
We will be installing the motor starter at the unit with overload protection set at the nameplate. Side note - When I suggested to put the 7.5HP on a 50amp circuit he flipped out and said that if I did that and something went wrong with the motor the breaker wouldn't trip before serious damage could happen to the motor. I then told him that the overloads in the starter would protect the motor from that he didn't know what I was talking about.

 

[Abtech Electric]

Yes - thank you and I have not denied that a 30amp fuse is recommended and acceptable and will be located at the units service disconnect. I only have a subpanel with branch breakers and NO fuses. The manufacturers recommendation DOES NOT LIST a breaker size to use and ONLY list the fuse size.
Therefore wouldn't I use table 430.150 to size the breaker in the subpanel with NO other recommendation listed? When I showed the Engineer the listing was for a fuse and does not mention anywhere a breaker recommendation he told me "fuse or breaker it doesn't matter".

Oops - sorry. Trying to do to many things at once. I mean who likes working Saturdays?

 

[augie47]

If it's not a "factory assembly" (starter-associated wiring etc) , and just a compressor with a motor, it's odd there is any factory recommendation in my experience.... but so be it. You are obligated to meet the manufacturer's requirements.

 

[iwire]

When I suggested to put the 7.5HP on a 50amp circuit he flipped out and said that if I did that and something went wrong with the motor the breaker wouldn't trip before serious damage could happen to the motor. I then told him that the overloads in the starter would protect the motor from that he didn't know what I was talking about.

Again I agree with the engineer, you can not code wise use a larger breaker then the labeling of the unit allows. It was tested under certain conditions and you must install under those conditions.

If your worried about the 10 AWG then go ahead and run 8 AWG or 6 AWG or even 1 AWG but the breaker or fuse size can not exceed the labeling.

This is not a "motor" it is more of an "appliance" or an HVAC unit. It was designed, tested and listed as an entire unit, the tables in the end of 430 do not apply.

 

[iwire]

The manufacturers recommendation DOES NOT LIST a breaker size to use and ONLY list the fuse size.

OK now that makes sense, in this case you are NEC required to install a fuse of the size the manufacturer recommends somewhere in the circuit.

The following is directly addressing HVAC equipment but the same concepts apply to your compressor.

Sometimes the manufacturer of the combination-load equipment will specify fuses as the overcurrent protective device. This is important information and must be followed. If the nameplate says fuse only, the equipment has been evaluated and tested only with a fuse. The manufacturer has determined that only a fuse provides the proper overcurrent protection for the hermetic refrigerant motor-compressor and the other internal components. Use of a circuit breaker would be in violation of the Code Section 440-4(b) and 440-22(c). This would also be in violation of Section 110-3(b). This is the equivalent of not following the manufacturer?s instructions provided with the equipment. Not following the instructions is the same as not following the Code.

http://www.iaei.org/magazine/?p=3944

 

[nakulak]

why are you so intent on arguing with the engineer. express yourself nicely in writing to CYA, then get paid and go to the next job. if and when it blows up or doesn't work, give them a nice price to fix it right COD and tell the owners they should go to the engineer for reimbursement.

 

[Abtech Electric]

Great. This is kind of what I was looking for. We used to run into this problem with HVAC equipment that was listed only with fuse recommendation. Simply fixed - fused disconnect at the unit which is what I am planning on doing here. Larger breaker needed for the start up.
NEC is our minimum requirements allowed. I do not always wire to the minimum that I am allowed ie: using the #8awg conductor. I have not had to
look at anything at a minimum since I took my test and don't plan on trying to relearn unless I have to be tested again. However, I do not over do it either. Just a little here and there due to past experiences and what makes me sleep better.
I hear other contractors tell customers that they are wiring their building as
"per code". I translate that into "I'm wiring your building as cheap as I can".
I am primarily a design/build contractor but since our economy I taking anything I can and running into problems like this.
Thank you for all your time this Saturday.

 

[iwire]

why are you so intent on arguing with the engineer. express yourself nicely in writing to CYA, then get paid and go to the next job. if and when it blows up or doesn't work, give them a nice price to fix it right COD and tell the owners they should go to the engineer for reimbursement.

I was also thinking along those same lines, an RFI to cover my rear and whatever the response is just run with it.

 

[Abtech Electric]

why are you so intent on arguing with the engineer. express yourself nicely in writing to CYA, then get paid and go to the next job. if and when it blows up or doesn't work, give them a nice price to fix it right COD and tell the owners they should go to the engineer for reimbursement.

I went down this road before with not rectifing a code issue properly. I expressed my concern with the owner/architect verbally and in writing. The engineer said that it wasn't illegal and we went on even though I knew better. During the final inspection the building failed due to the infraction and did not get to open on time. I got 100% of the blame. The owner was ticked to say the least and I got a lashing from the local inspector for knowingly installing something I knew was incorrect. And that letter I wrote was my confession. I asked the inspector what was I supposed to do chain myself to the building in protest and he replied "I don't care it is your problem". Good thing was, I did get paid to fix it.