What fuses for motors?

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mbrooke

mbrooke

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What class & type of fuse would work for the following:

200HP / 300 amp fuses

150HP / 225 amp fuses

125HP / 200 amp fuses

100HP / 150 amp fuses

60HP / 90 amp fuses

40HP / 60 amp fuses

20HP / 30 amp fuses

15HP / 25 amp fuses

10HP / 20 amp fuses

5HP / 10 amp fuses

3HP / 6 amp fuses

1HP / 3 amp fuses

All motors are 460 volts 3 phase


Do I size the wire to the fuse, or do to the motor's FLC?

1.15 and over service factor means fuses are sized at 125%, but under 1.15 I use 115%, correct?
 
Wire is sized to 125% of full-load current in Table 430.250

I thought we went over using fuses for running overcurrent protection a thread or two back. The 300 amp fuses may not allow a 200 HP to start. In that case a 500 could be allowed. Do you really want to have 500 amp fuses providing running overcurrent protection for a motor that should draw 240 FLA?

430.52(C) (1)(b)
 
ptonsparky said:
Wire is sized to 125% of full-load current in Table 430.250
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Even when the fuse is acting as the overload and larger then the FLC?



I thought we went over using fuses for running overcurrent protection a thread or two back. The 300 amp fuses may not allow a 200 HP to start. In that case a 500 could be allowed. Do you really want to have 500 amp fuses providing running overcurrent protection for a motor that should draw 240 FLA?

430.52(C) (1)(b)
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There has to be fuses out there that can handle inrush and also provide descent over current protection- I'd think.


FWIW, I came across these:


https://m.littelfuse.com/~/media/electrical/application-notes/littelfuse-motor-protection-guide.pdf


http://www1.cooperbussmann.com/pdf/6c202d99-3cc3-4cc3-beaa-a565a95db302.pdf
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My experience usually ended in single phased motors. Under 5 HP are disposable, depending on how critical they are to your process so the cost of a NEMA starter may not be warranted. IEC starters are a different story.

Fifty horsepower motors are in the neighborhood of $3,000 and you will need at least ATL start. Why skip the overload relay?
 
ptonsparky said:
My experience usually ended in single phased motors. Under 5 HP are disposable, depending on how critical they are to your process so the cost of a NEMA starter may not be warranted. IEC starters are a different story.

Fifty horsepower motors are in the neighborhood of $3,000 and you will need at least ATL start. Why skip the overload relay?
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The way I figure it fuses are already required. So why use separate overloads?
 

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ptonsparky said:
I can't read the details, not that it matters.

You are the EE, if you are happy with fuses, go for it.

The Bussman and Littelfuse reps will be glad to take your call.
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But- I am embarrassed to admit that I don't know the inrush properties of 3 phase induction motors for various horse powers when switched on at various points in the sine wave, when there is a momentary "blink" and when there is residual magnetism still in the iron.

I know transformer inrush varies based on those 3 factors, and, I'd imagine its the same for motors.
 
mbrooke said:
But- I am embarrassed to admit that I don't know the inrush properties of 3 phase induction motors for various horse powers when switched on at various points in the sine wave, when there is a momentary "blink" and when there is residual magnetism still in the iron.

I know transformer inrush varies based on those 3 factors, and, I'd imagine its the same for motors.
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But, only worst-case numbers matter, right?
 
Impedance of source along with supply conductors (feeders and branch circuit conductors) can effect how much surge current may flow when first energizing a motor. More impedance will be current limiting, however will also result in less initial motor torque - soft starters intentionally do this.
 
kwired said:
Impedance of source along with supply conductors (feeders and branch circuit conductors) can effect how much surge current may flow when first energizing a motor. More impedance will be current limiting, however will also result in less initial motor torque - soft starters intentionally do this.
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Can we assume infinite bus?
 
Let me get this straight. An EE is advocating NOT using a motor starter on 3 phase 480 motors in excess of X HP. WOW is this "value engineering" gone way wrong?

First time you blow one fuse, single phase that motor and ruin it, your value engineering has cost me a lot of downtime, labor, and money.

Spec motor starters with electronic overload relays. Spec fuse blocks with class J, RK1, or CC fuses ahead of these motor starters. Size fuses in accordance with overload relay setting, motor HP, and maker of starter recommendations to provide type 2 level of protection. In my experience this is generally around 175% of motor FLC. Small fans and pumps can often be closer to 125% as they never run at motor FLC much less SF amps. The branch circuit breaker protects the conductors. The fuses protect the starter and limit current to a level of SCCR the starter can survive. The overload relay protects the motor. Done right all you do is replace a fuse, reset an OL relay, reset a breaker, or maybe replace a motor. Done wrong and it can get real ugly.

Or better yet, spec a factory built MCC and let them do the work.

There are so many good reasons to have a motor starter with current limiting fuses/MCCBs. The only reason to not have them is "penny wise pound foolish" mentality. That type of engineering will get you a very bad name in a hurry. IMHO what you are suggesting is downright dangerous.

Okay rant off and a I humbly apologize if I misunderstood what you are proposing.
 
mbrooke said:
But- I am embarrassed to admit that I don't know the inrush properties of 3 phase induction motors for various horse powers when switched on at various points in the sine wave, when there is a momentary "blink" and when there is residual magnetism still in the iron.

I know transformer inrush varies based on those 3 factors, and, I'd imagine its the same for motors.
Click to expand...

It much the same MBrooke

OL's are sized 430.32, OCPD's 430.52

OL's are slower than OCPD's , so they are a tighter tolerance to FLA

It's all constructed so we don't have to run 'fat' wires :bye:

~RJ~
 
Russs57 said:
Let me get this straight. An EE is advocating NOT using a motor starter on 3 phase 480 motors in excess of X HP. WOW is this "value engineering" gone way wrong?
Click to expand...

Yup, its even been done in old building MCCs.

First time you blow one fuse, single phase that motor and ruin it, your value engineering has cost me a lot of downtime, labor, and money.
Click to expand...


I'd think that once the motor goes single phase, it draws more current that causes the other fuses to blow.

Of course there will be single phase protection, if the feed into the MCC drops a phase the power is cut to the banging contactor.

Spec motor starters with electronic overload relays. Spec fuse blocks with class J, RK1, or CC fuses ahead of these motor starters. Size fuses in accordance with overload relay setting, motor HP, and maker of starter recommendations to provide type 2 level of protection. In my experience this is generally around 175% of motor FLC. Small fans and pumps can often be closer to 125% as they never run at motor FLC much less SF amps. The branch circuit breaker protects the conductors. The fuses protect the starter and limit current to a level of SCCR the starter can survive. The overload relay protects the motor. Done right all you do is replace a fuse, reset an OL relay, reset a breaker, or maybe replace a motor. Done wrong and it can get real ugly.
Click to expand...

The little fuse pdf says RK fuses alone will work.


Or better yet, spec a factory built MCC and let them do the work.

There are so many good reasons to have a motor starter with current limiting fuses/MCCBs. The only reason to not have them is "penny wise pound foolish" mentality. That type of engineering will get you a very bad name in a hurry. IMHO what you are suggesting is downright dangerous.

Okay rant off and a I humbly apologize if I misunderstood what you are proposing.
Click to expand...



Why dangerous? The fuse will protect the wire and the motor from thermal damage.

Doing more with less has many advantages.

Kindly showing my green light :):angel:
 

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romex jockey said:
It much the same MBrooke

OL's are sized 430.32, OCPD's 430.52

OL's are slower than OCPD's , so they are a tighter tolerance to FLA

It's all constructed so we don't have to run 'fat' wires :bye:

~RJ~
Click to expand...

True, but I figure with the same wires the fuse can do the same of the overload.
 
mbrooke said:
I'd think that once the motor goes single phase, it draws more current that causes the other fuses to blow.
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You would typically be mistaken with this thought. For example, if the motor is not loaded substantially, the current in the remaining phases may not increase significantly.
Some 60 years ago the NEC started requiring running Overload protective devices in each phase, for this very reason.
 
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