Disconnect, and breaker sizing for a vacuum pump

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jobuTupaki

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I have to size the breaker and a fused disconnect for a vacuum pump at 480V - nameplate rating: 20 HP (which gives 27 full load current, using NEC Table 430.250 (2017 version))

The manufacturer recommends 50A fused disconnect and I am scrambling trying to figure out how that was calculated. See below for my thoughts.

Using 27 as FLC, this gives FLA = 27/1.25 = 21.6
For the disconnect sizes I am calculating doing the following: FLA*225% --> 21.6A*225% = 48.6A, giving us the 50A fused disconnect, and I am going up to the standard size of 60A.
From here, I would use a 50A breaker since FLA is 48.6A?

I feel like I am going crazy/making up calculations trying to sort this out. Motors have always been very confusing to me. If anyone has any insight at all it would be greatly appreciated.
 
LarryFine said:
Welcome to the forum.


Shouldn't that be 27 X 1.25 = 33.75? :unsure:
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Thank you - I have been following for some time now, and finally decided to make an account and ask questions of my own as I see how frequently people respond and how useful this would have been had I posted my concerns in the past.

And to continue, I thought FLC is the "code rating", assuming the worst case of the motor, which is why you multiply nameplate rating (FLA) by 125% to account for any inefficiencies.

So to calculate FLA, which I need to calculate disconnect size and breaker size, I just divided the known 27A FLC by 125%

FLC gives us wiring sizes and FLA gives us overload protection. Source: Mike Holt Lesson
 
infinity said:
Aren't disconnect switches rated for a maximum horsepower?
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Wouldn't that give 27A (using 20HP and using table mentioned above), and the next disconnect size is 30A.... manufacturer recommends 50A.

I think I have been looking at this problem for too long and starting to mix things up...
 
It appears the manufacturer is using the fused disconnect as the branch short circuit and ground fault protection device. Since it is a fused disconnect, you would use 175% of the FLC given in Table 430.250 as the maximum size per Table 430.52 for a dual element (time-delay) fuse.

27A*1.75=47.25A, so the manufacturer is recommending a 50A fuse in accordance with 430.52(C)(1) and Exception 1 of that section.

Since you are using the fused disconnect as the short circuit protection, your upstream breaker can be any size you want (50A or larger) as long as the feeder conductors are properly protected. You could even use a size smaller than 50A if you wanted, but you probably wouldn't be able to start your motor without the overcurrent protection opening.

Use the NEC tables to size the conductors, disconnect switches, and short circuit protection. Use the FLA on the motor nameplate to set the overloads.
 
Welcome to the confused about motors club. Article 430 is complicated, and more so when it allows us to use a 14 awg wire on a 30 amp circuit breaker.
The breaker protects the wire, the overloads protect the motor.
 
jobuTupaki said:
I have to size the breaker and a fused disconnect for a vacuum pump at 480V - nameplate rating: 20 HP (which gives 27 full load current, using NEC Table 430.250 (2017 version))

The manufacturer recommends 50A fused disconnect and I am scrambling trying to figure out how that was calculated. See below for my thoughts.

Using 27 as FLC, this gives FLA = 27/1.25 = 21.6
For the disconnect sizes I am calculating doing the following: FLA*225% --> 21.6A*225% = 48.6A, giving us the 50A fused disconnect, and I am going up to the standard size of 60A.
From here, I would use a 50A breaker since FLA is 48.6A?

I feel like I am going crazy/making up calculations trying to sort this out. Motors have always been very confusing to me. If anyone has any insight at all it would be greatly appreciated.
Click to expand...

The book number is basically HP x 0.746 = kW then divide by 0.85 to take into account efficiency and power factor worst case. This gives us kVA. From there multiply by 1000 and divide by Voltage and square root of number of phases to get to current.

As far as “FLC” and “FLA” there are somewhat made up terms. PRACTICE is to size short circuit protection using NEC value based on HP and overload based on name plate. NEC does not distinguish between the two but some wording says full load current and some says full load amperes so thus the controversy.

A fuse or circuit breaker or supplemental device (UL 489 term) are different devices with different thermal curves. Regardless the thermal curve is 100% useless. Motor thermal properties are very different from wiring. The frame is a huge heat sink and most have cooling fans. The motor is protected by its own thermal devices. But motors have high starting currents and we still need to protect the starter and other parts from short circuits. So NEC gives maximums for how high you can size the device. That is what you follow. So a dual element fuse is 175%. That’s 47 A. You can go to next larger size which would be 60 A. In a breaker it’s 300% so that’s 81 A and you end up at 100 A.

Now I strongly caution you about doing this for three reasons. The first is that standard non-‘motor specific breakers will NOT protect the wiring adequately at this size. But if you use 60 A you will have nuisance tripping issues which is why it is so “oversized”. The second issue is that using a breaker makes it a type 1 protection where using a fast fuse (CC, J, or RK1) makes it type 2 or “no damage” protection. In type 1 on a hard fault the starter will be destroyed in the tripping process. I have seen contacts literally melted off. Third issue is that with modern highly efficient motors a stock UL 489 type B curve is subject to nuisance tripping depending on phase angle…it can’t handle the inrush (NOT locked rotor). A type C (20xLTPU) does better but better is to use a motor specific “breaker” aka supplemental protector. There are several types these days.
 
paulengr said:
...................................... But motors have high starting currents and we still need to protect the starter and other parts from short circuits. So NEC gives maximums for how high you can size the device. That is what you follow. So a dual element fuse is 175%. That’s 47 A. You can go to next larger size which would be 60 A. In a breaker it’s 300% so that’s 81 A and you end up at 100 A.
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Where are you getting this ?
(1) Per 240.6, the "next size up" would be 50 amps not 60
(2) According to 430.52 the max breaker (other than allowed by exceptions) would be 250%
 
Wire is sized at 125% of the CODE amps for the motor. Overloads are sized off the motor name plate. Time delay fuses are 175% of full load current, and breakers are 225% unless the motor will not start then there are exceptions to upsize.
 
minimum conductor ampacity is 125% of NEC current table so yes 27 x 1.25 = 33.75 for the conductor.

dual element time delay fuse can be up to 175% of NEC current table so max fuse is 27 x 1.75 = 47.25, can be next standard size higher which would be 50.

Can't speak for all manufacturers but Square D 30 amp non fused disconnect is rated 20 HP @ 480 volts, I use them on 20 HP motors somewhat frequently. If fused switch is needed will need to go to a 60 amp fused switch mostly because you can't put a 50 amp fuse in a 30 amp fuse holder, not because of rating of the switch itself. That same 60 amp fused switch can be used up to 30 HP motor @ 480 volts.

If you do have a circuit breaker feeding all of this, it would need to be maybe 60 or 70 amps (can be up to 250%, so 70 is max.

All those overcurrent devices are NEC maximums, not a code violation to use lesser values if the motor will start, and many manufacturer charts will recommend something less than what NEC maximums are.

Square D charts recommend 60 amp breakers for this motor and 40 amp time delay fuses, I have never had starting issues with those selections on a 20 HP motor. I even know of one 20 HP motor that is not loaded very heavily that is connected to a circuit originally intended for 10 HP motor, still has 25 HP breaker, and overloads sized for the original 10 HP motor - Kind of is a violation on minimum conductor size, yet never trips anything set for half the HP rating. I don't lose any sleep over it. They still do swap that machine with the original one that had the 10 HP motor when running certain products.
 
jobuTupaki said:
I have to size the breaker and a fused disconnect for a vacuum pump at 480V - nameplate rating: 20 HP (which gives 27 full load current, using NEC Table 430.250 (2017 version))

The manufacturer recommends 50A fused disconnect and I am scrambling trying to figure out how that was calculated. See below for my thoughts.

Using 27 as FLC, this gives FLA = 27/1.25 = 21.6
For the disconnect sizes I am calculating doing the following: FLA*225% --> 21.6A*225% = 48.6A, giving us the 50A fused disconnect, and I am going up to the standard size of 60A.
From here, I would use a 50A breaker since FLA is 48.6A?

I feel like I am going crazy/making up calculations trying to sort this out. Motors have always been very confusing to me. If anyone has any insight at all it would be greatly appreciated.
Click to expand...
I always used quality time delay fuses and for most motors only had to go 125% over motor amps. Only exceptions is for motors that took longer to get up to full
Speed like a loaded air compressor they restarted at 130#'s of air pressure then shut off at 150 #'s. and soms air conditioning/refrigeration units and crushers. Back in the early 1970's best in class Chase Shaumut ( had what looked like a tin playing ) 30 amp 250 volt fuses were the only brand that did not burn out on large refrigeration truck units and some motors. I have seen fuses open up on motors due to excessive start & stopping under heavy loads too many times a hour.. We had a 200 HP rotary screw air compressor that had a sign stating no more then three starts a hour. When machnists changed oil in it he had a habit of bumping if causing one or two fuses to open.
 
jobuTupaki said:
I have to size the breaker and a fused disconnect for a vacuum pump at 480V - nameplate rating: 20 HP (which gives 27 full load current, using NEC Table 430.250 (2017 version))

The manufacturer recommends 50A fused disconnect and I am scrambling trying to figure out how that was calculated. See below for my thoughts.

Using 27 as FLC, this gives FLA = 27/1.25 = 21.6
For the disconnect sizes I am calculating doing the following: FLA*225% --> 21.6A*225% = 48.6A, giving us the 50A fused disconnect, and I am going up to the standard size of 60A.
From here, I would use a 50A breaker since FLA is 48.6A?

I feel like I am going crazy/making up calculations trying to sort this out. Motors have always been very confusing to me. If anyone has any insight at all it would be greatly appreciated.
Click to expand...
Welcome to Forum..
 
Actually, I do believe first time of original membership w/ Mike Holt's Code Forum, was somewhere in between years of 2002 & 2005. Sometimes we get busy, and traveling happens, and you step-away.. Enjoy the forum.
 
RWC/NC. said:
Actually, I do believe first time of original membership w/ Mike Holt's Code Forum, was somewhere in between years of 2002 & 2005. Sometimes we get busy, and traveling happens, and you step-away.. Enjoy the forum.
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Nice, I have been utilizing for about 5 years now. Finally decided to make an account on my own as I see how often it is utilized.
 
For me, kinda like owning you're very first H.D. (Harley Davidson).. and then there's next one, you never really get it completely, "out of your system".. Have safe day @ the office..
 
tom baker said:
Welcome to the confused about motors club. Article 430 is complicated, and more so when it allows us to use a 14 awg wire on a 30 amp circuit breaker.
The breaker protects the wire, the overloads protect the motor.
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I wish more people understood this. You are indeed allowed to have a mismatch between breaker/fuse handle rating and wire size for a motor branch circuit or feeder. Don't stop reading 240.4 til you get to the end, then see 430.21 and 430.24 for wire sizing, and on to 430.52 and 62 for the breaker/fuse.
 
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