Sizing fusable disconnect for compressor

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Weezy

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I'm helping with the installation of a new compressor. I want to check my calculations to ensure the correct disconnect and fuses are installed. The tech from the manufacturer told me to install number 10's and a 40 amp breaker. This seems incorrect to me so I'm already thinking of changing these conductors to number 8's especially since the breaker is over 100 feet from the disconnect. He also told me to install a 30 amp fusable disconnect. So the motor will be on 460 volts, 22 FLA, 15 hp. Does it sound correct to use 30 amp fuses for this motor at the disconnect?

my second question concerns a switch which is being used to alternate between two compressors currently in use. One compressor is being replaced with the new one I mentioned above and the other compressor will remain but won't be needed as often. They want to be able to use some kind of control to alternately switch between the two compressors. They each have different voltage and current ratings. Can anyone recommend where I would start to look for solutions? What kind of control device would I need and is it logical to use one?
thank you
 
Weezy said:
I'm helping with the installation of a new compressor. I want to check my calculations to ensure the correct disconnect and fuses are installed. The tech from the manufacturer told me to install number 10's and a 40 amp breaker. This seems incorrect to me so I'm already thinking of changing these conductors to number 8's especially since the breaker is over 100 feet from the disconnect. He also told me to install a 30 amp fusable disconnect. So the motor will be on 460 volts, 22 FLA, 15 hp. Does it sound correct to use 30 amp fuses for this motor at the disconnect?
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I don't think you need both a 40A CB and a 30A fuse. But either (or both) is acceptable IMO. #10 conductors have adequate ampacity. You can always make them bigger if you want. I don't think you will see more than a few volts drop. Not enough to worry about IMO. I don't know where you got the idea that wires running more than 100 feet should be upsized.

Weezy said:
my second question concerns a switch which is being used to alternate between two compressors currently in use. One compressor is being replaced with the new one I mentioned above and the other compressor will remain but won't be needed as often. They want to be able to use some kind of control to alternately switch between the two compressors. They each have different voltage and current ratings. Can anyone recommend where I would start to look for solutions? What kind of control device would I need and is it logical to use one?
thank you
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How about a 2 position selector switch tied into the existing controls so only one compressor can run at a time.
 
Why do you need a selector switch? Just use the disconnect to turn the one on that you need.

The disconnect must be rated 115% of the fla so 22 amps (1.15) will take a 30 amp disconnect. The problem here is that the motor may be capable of using 40 amp fuses so you would need a 60 amp disconnect

Some motors can be fused at 150% or more of the fla so I would tend to use a 60 amp disco
 
At well > 100ft I would probably use the #8s but an educated answer would require more data on the motor and some engineering. Most likely the 10s would be fine but over the years I have seen Compressors that can be finikey
Most 30 amp heavy duty fusible safety switches are rated 15 HP and the no-fuse Heavy-Duty are rated 20 HP. As Dennis points out, fusing might be a problem so I would tend to go no-fuse unless the mfg. recommends fusible.
I also agree with Dennis** that your selection switch should be installed in the compressor control circuit so a simple selector switch could be used.

** I read too quickly It was Petersona's post that recommend the selector switch & I concur
 
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augie47 said:
As Dennis points out, fusing might be a problem so I would tend to go no-fuse unless the mfg. recommends fusible.
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Beware of the available fault current. The unfused safety switch will likely have a maximum short-circuit withstand rating of 10 kA. Fuses typically bring the switch rating to the interrupt rating of the fuses.
 
mayanees said:
Beware of the available fault current. The unfused safety switch will likely have a maximum short-circuit withstand rating of 10 kA. Fuses typically bring the switch rating to the interrupt rating of the fuses.
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Good point.
I honestly forgot that but I'll fib and say that at #10s and 100 ft I choose to ignore it :)

At 15HP unless the mfg. has a recommended fuse, I'd tend to go with a 60 amp disconnect and 40 amp fuses rather than a 30.
 
Thank you everyone for your input. Since the manufacturer recommended a fusable 30 amp disco, that's what I intend to install. What threw me was the recommendation to use #10's on a 40 amp breaker. I was taught nothing larger than 30 amps for 10's. I was unsure if the 30 amp fuses were a good call but I will go ahead and do it. I am going to take a closer look now at the switch. I was told that it automatically switches between the two compressors when needed. They are also tied into an old time clock wich won't be used with the new compressor. I am used to new construction where everything is sized for me. Every time I am asked to install something at this school I am expected to make it work with limited info and very old equipment. Makes me second guess myself. Thanks again for the suggestions.
 
Weezy said:
Thank you everyone for your input. Since the manufacturer recommended a fusable 30 amp disco, that's what I intend to install. What threw me was the recommendation to use #10's on a 40 amp breaker. I was taught nothing larger than 30 amps for 10's. I was unsure if the 30 amp fuses were a good call but I will go ahead and do it. I am going to take a closer look now at the switch. I was told that it automatically switches between the two compressors when needed. They are also tied into an old time clock wich won't be used with the new compressor. I am used to new construction where everything is sized for me. Every time I am asked to install something at this school I am expected to make it work with limited info and very old equipment. Makes me second guess myself. Thanks again for the suggestions.
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Here are a couple of links to Mike's Free Stuff on motors that will help with your studies

http://www.mikeholt.com/mojonewsarchive/EC-HTML/HTML/Motor_Calculations_-_Part_1~20030303.htm
[FONT=&quot]http://www.mikeholt.com/mojonewsarchive/EC-HTML/HTML/Motor_Calculations_-_Part_2~20030306.htm[/FONT]
 
Weezy said:
Thank you everyone for your input. Since the manufacturer recommended a fusable 30 amp disco, that's what I intend to install. What threw me was the recommendation to use #10's on a 40 amp breaker. I was taught nothing larger than 30 amps for 10's. I was unsure if the 30 amp fuses were a good call but I will go ahead and do it. I am going to take a closer look now at the switch. I was told that it automatically switches between the two compressors when needed. They are also tied into an old time clock wich won't be used with the new compressor. I am used to new construction where everything is sized for me. Every time I am asked to install something at this school I am expected to make it work with limited info and very old equipment. Makes me second guess myself. Thanks again for the suggestions.
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Until you start reading the actual code, this is a common misconception. And even though the code allows it, there are many stuck up electricians who would refuse to put smaller wire to a motor anyway. To me, it is kind of like putting a ground wire in a conduit. I admit, I am not prone to installing conduit without a ground period, even though code allows it. And I have run across many electricians who honestly believe it is required.

Read the references given on motors and you will find that often time you can have a rather large breaker with rather small wires feeding a motor.
 
petersonra said:
I don't know where you got the idea that wires running more than 100 feet should be upsized.
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It is a common rule of thumb for electricians.

It also appears in many job requirements. It is not unusual for the EE to put a note in that tells us to bump up one size for every 100' of 'home run' length.
 
iwire said:
It is a common rule of thumb for electricians.

It also appears in many job requirements. It is not unusual for the EE to put a note in that tells us to bump up one size for every 100' of 'home run' length.
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Not that it would always be true, but every time I see it and I estimate, so that is probably 60-100 times a year it applies to lighting and power branch circuits. It doesn't apply to mechanical equipment loads, or say panel feeders especially when they are already telling you the conductor sizes. It is also usually voltage related as well as in 100' @120, 200' @277
 
That does not appear to be a residential type air compressor. With that said, I would imagine it might have some type of control panel?? If so, all bets are off with sizing the circuit for a "motor". You would actually be serving a control panel. Then you would need to know the power requirements of the control panel and since it isn't a "motor" the conductors would be sized to match the breaker.
 
Strathead said:
Not that it would always be true, but every time I see it and I estimate, so that is probably 60-100 times a year it applies to lighting and power branch circuits. It doesn't apply to mechanical equipment loads, or say panel feeders especially when they are already telling you the conductor sizes. It is also usually voltage related as well as in 100' @120, 200' @277
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I agree with all your points.:)

That's the thing with rules of thumb or any short cuts, they are compromises.
 
mjmike said:
That does not appear to be a residential type air compressor. With that said, I would imagine it might have some type of control panel?? If so, all bets are off with sizing the circuit for a "motor". You would actually be serving a control panel. Then you would need to know the power requirements of the control panel and since it isn't a "motor" the conductors would be sized to match the breaker.
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Since it is a 480V compressor I would say it absolutely is not a residential type air compressor. I can't think of any special control panels on any of the compressors of this nature that I have been around. Branch circuit to a disconnect, motor starter....that's about it.
 
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