Motor question with opp. to teach

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andinator

Senior Member
Location
Lilburn Georgia
What my situation is :

A 75 FLA 3 phase 208/120 motor driving a big aircompressor. In the motor control panel is 3 70A fuses (or are they?).

How I am told to wire it:

Run out of 1200A MDP with 3 #6 THHN CU to fused disconnect. Run 3 # 6 THHN CU to motor . Install a 50A breaker in the MDP. Install 45A fuses in the disconnect. Recieved call today saying perhaps we need a 60A breaker in the MDP.

How I figured it....

200A breaker in the MDP. Nonfused disconnect. # 4 wire throughout.




I'm a resi electrician who started doing commercial 1 year ago. How is my thinking wrong? I'm desperate to learn. Will it work how I'm told to wire it?


Thanks
 
I would like to see the HP rating on the nameplate and size feeder circuit per 430.250, don't forget 125% for conductors, I get #3s possibly #1s depending on connection temp ratings.
 
Is this a factory assembled packaged unit?

Tank, compressor, motor and motor starter, or is this something assembled in the field?


If it's a packaged unit it should have a tag with all the info you need on the frame or control enclosure.

If its assembled in the field you need to go right off the motor tag and you need to start with the HP not the FLA.
 
Factory assembled! Perhaps the info given to those above me are from the factory. But the 75 FLA number is correct. So with the numbers and the OCPD that is going to be installed tomorrow (60A) this is going to work? Motors confuse me.
 
Chris, my friend...Remember the level of my idiocy. Motor controls aren't til next month in school. No one I work with inspires confidence in me. Teach me, brother...If I go and find MCA and MOP what do they stand for and how can I interpret? This whole thing may be correct, but SOMETHING doesn't seem right. If I am totally wrong..I'm also totally cool with that.
 
Reverse engineering this, 75FLA @ 208V is probably a 25HP motor, for what that's worth. Doesn't really matter, everything should be done from FLA.

The conductors need to be rated no less than 125% of the FLA, so 75 x 1.25 = 93A. ASS-u-me-ing 75 deg. C copper wire such as THW , 3 conductors in conduit, you would need to use #3 wire (which you may not find, so #2). If you use 90deg C wire such as THHN you can theoretically use #4, but nothing smaller. So you could* use a 90A CB for the #4 THHN or a 100A CB for the #2 THW. None of these numbers are adjusted for voltage drop here, that's also important to keep in mind.

*But... here's the complication. You need to look at the magnetic trip levels of said CBs, because if too low, they will trip when you attempt to start the compressor. If they don't say on the breaker, then you can assume it is low, i.e. 250-400% of the breaker rating. The instantaneous inrush current of the motor can be as high as 1100% FLA, but is usually lower, i.e. 800%. So your CB needs to have instantaneous trips that are adjustable or set higher than 600A. If you get a cheap off-the-shelf CB with no data or adjustability, a 90A breaker may trip at 225A, in other words, all the time. If you up-size the breaker, you need to then up-size the wire too! If you chose to go with a cheaper breaker, make it a 250A (based on 600A / 250%), then use 4/0 wire. A 200A or even a 150A CB might work if it has higher instantaneous trip settings, just keep that 600A as a minimum target. But better yet, do your homework and look for a 90 or 100A breaker that has adjustable instantaneous trip setting that will go to at least 600A, more if possible, just keep in mind that it needs to be listed for use in that MDP though.

If you have this circuit breaker in the MDP, and there is nothing else on the circuit but this one load, you technically do not also need fuses in the disconnect; a non-fused one would be fine. If however, because of limited choices in the MDP you end up with a 150A, 200A or 250A breaker, then by all means put fuses down stream to save yourself on copper costs. Recommended Dual Element fuses for a 75HP 208V motor is 100A, so you are back to the #3 wire size as a minimum (#4 THHN is rated for 95A, so that's a no-go after all unless you put in 90A fuses and risk the occasional blown fuse; but don't do it).

No matter what, don't bother with 50A or 60A breakers and 45A or even 70A fuses, that's all a complete waste of time!
 
First, I agree with I-wire....if it's a package unit, go by the mfg. specifications.
If not then I have to say I don't see it the same way Jaref does.
I would agree on a #3 Cu conductor (25 hp 74.8 x 1.25 = 93.5).
As far a the #4 is concerned, I doubt any of your components are listed for 90 deg so #3 would be the smallest IMHO)
The breaker could be up to a 175 amp (ignoring exceptions) (74.8 x 2.5 = 187). Because of the inrush mentioned, you can still use that breaker on the #3 wire.
That said, I also wouldn't jump to rule out fuses. You would need to take into account the avaiilable fault current of the system and the aic at your compressor. This should be comapred to the withstand rating on your motor starter. Fuses may be the ideal means to protect your starter from excessive fault currents.
My opinion.
 
Jraef said:
(#4 THHN is rated for 95A, so that's a no-go after all unless you put in 90A fuses and risk the occasional blown fuse; but don't do it).
Click to expand...


The 95 amp rating (90 degrees C) of #4 THHN is unusable when sizing motor conductors.
 
1) The conductors are sized to 125 percent
( 74.8 x 1.25 = 93.5) 3 AWG based on 75 degree terminals

2.) Since sizing Short Circuit & Ground Fault in this case is based on a Inverse Time Breaker which is 250 percent.
( 74.8 x 2.50 = 187) Size up = 200A Breaker

Now we will assume you are sizing a fuse at the unit for seperate overload device.....so without any nameplate info lets assume "all other motors" so you size the it based on the nameplate which i dont think you gave....so lets ass-u-me the 75 figure you gave.....

3) Overload ( basically because you refered to fuses at the motor ) now if that is what you are speaking of it would be 115 percent of the FLC on the plate.....so
(75 x 115 = 86.25 ) go ahead and put in 80A fuses....
* If the 115 percent does not hold can use 130 percent

Thats my shot at it with such little info.....other factors could still apply.

Edit- lol.......seemed i left off a few amps..lol...but does not change the end results....
 
Last edited:
andinator,

Your way of doing it was good except for the wire, #3 is needed. The people

telling you to put a 45a or 60a circuit breaker on a 75a load really don't

understand, it's probally a duel voltage motor and they are seeing the wrong

info.

Like others have said, get the info that came with it, follow the manf. instructions.
 
radiopet said:
1) The conductors are sized to 125 percent
( 74.8 x 1.25 = 93.5) 3 AWG based on 75 degree terminals

2.) Since sizing Short Circuit & Ground Fault in this case is based on a Inverse Time Breaker which is 250 percent.
( 74.8 x 2.50 = 196) Size up = 200A Breaker

Now we will assume you are sizing a fuse at the unit for seperate overload device.....so without any nameplate info lets assume "all other motors" so you size the it based on the nameplate which i dont think you gave....so lets ass-u-me the 75 figure you gave.....

3) Overload ( basically because you refered to fuses at the motor ) now if that is what you are speaking of it would be 115 percent of the FLC on the plate.....so
(75 x 115 = 86.25 ) go ahead and put in 80A fuses....
* If the 115 percent does not hold can use 130 percent

Thats my shot at it with such little info.....other factors could still apply.
Click to expand...


these calculations are correct but only if the number you said was the FLA is actually the FLC. FLC = full load current as determined by the tables in article 430. FLA = full load amps the motor draws when running at its designed speed, voltage, current and temperature (not starting or overloaded). (the code says you are supposed to look up the motor in the tables in 430 based on the HP of the motor to determine FLC) the code says that if you only have the running amps, you can use it to determine (the values calculated above), but if the equip is new you surely have all the info. of course, the FLA given you may actually be the FLC, and you are good to go. note: all of the above assumes this is a run of the mill motor (not vsd or something crazy)
 
Atleast it gives him the method to try.....we simply dont know all the factors because his post provides limited information.

Again as othes have stated it should have installation instructions with the equipment that should be followed.
 
stupid fingers

stupid fingers

:grin:


"( 74.8 x 2.50 = 196) Size up = 200A Breaker "

doesn't change your facts, but one of us has a busted calculator
 
augie47 said:
:grin:


"( 74.8 x 2.50 = 196) Size up = 200A Breaker "

doesn't change your facts, but one of us has a busted calculator
Click to expand...
lol....is my math off.......oh yeah....did it with my phone but it still works out.......lol

opps......lol...........sorry 187 ...lol....yeah i was doing it on the road with my motorola Q phone....i am flying to a seminar...lol...sorry
 
The job is done. It is an Ingersoll Rand compressor with two motors. The only info included was the FLA, and when I returned this morning, the motor nameplate said 48.3A running current. I wish I had more info, but the last three days have been intense. I'm new to commercial work, so I couldn't have changed anything.

I watched it cycle twice this morning and BAD wanted to take and amp reading, or hook up the fancy meter to catch the spike, but had no time. We wired it as I described, the tech from IR came to do start up and said we had to upsize the breaker.:roll:

Final job...1200A MDP 208/120 SquareD Iline to a fused disconnect to the compressor. #4 wire with a 60A breaker in the panel, 45A fuses in the disconnect. The whole thing weirded me out.:cool:
 
radiopet said:
lol....is my math off.......oh yeah....did it with my phone but it still works out.......lol

opps......lol...........sorry 187 ...lol....yeah i was doing it on the road with my motorola Q phone....i am flying to a seminar...lol...sorry
Click to expand...

that's dedication to the Forum :smile:
 
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