Time Delay Fuse Sizing?

[Mr onetwo]

I am looking for a little advise from industrial/machinery electricians here. I am installing a Nussbaum lift in my garage that is rated 14.1FLA @ 240V 1 ph. I am putting a 30 amp fusible disconnect on it with Bussmann FRN-N-** time delay fuses.I am thinking 20 amp fuses, but is this over or under rating them.17 amp and 22.5 amp are available thru special order....any thoughts on the subject? I am also doing a 3 phase/VFD setup on my drill press.Drive rated at 9.7 amps @ 240v 1 ph. input.....motor is rated at 3.0 FLA @ 230v 3ph. Really not sure what size for this situation?Drive documentation is very confusing.Thanks for your help!!!

 

[sberry]

First,,, its ok but why a fused disconnect? Is this not on a breaker? Ok, second,,, yes you can use fuses to 30 and a 12 wire or better. My Mohawk allowed 14/30 but there is no reason to use the minimum wire and yours may list 12 anyway. You do not need,, or really even want to fuse it that close.

As to the drive, someone will be along that knows more than myself but these setups are often size based that their is thermal protection beyond the input connections,,, so 14 wire and 15 on the input.

 

[alfredeneuman]

By Code, you're allowed a 175% maximum dual element fuse size to protect a single phase motor.

14.1 X 175% = 24.675

If this figure doesn't correspond to a standard fuse size, then you're allowed to use the next size up, which in this case, would be 25Amp.

 

[Mr onetwo]

By Code, you're allowed a 175% maximum dual element fuse size to protect a single phase motor.

14.1 X 175% = 24.675

If this figure doesn't correspond to a standard fuse size, then you're allowed to use the next size up, which in this case, would be 25Amp.

That's a maximum value.Wouldn't it be better to get the fuse sized as accurately as possible so it's not too slow in protecting the motor?What are your thoughts on the low side?

 

[Mr onetwo]

First,,, its ok but why a fused disconnect? Is this not on a breaker? Ok, second,,, yes you can use fuses to 30 and a 12 wire or better. My Mohawk allowed 14/30 but there is no reason to use the minimum wire and yours may list 12 anyway. You do not need,, or really even want to fuse it that close.

As to the drive, someone will be along that knows more than myself but these setups are often size based that their is thermal protection beyond the input connections,,, so 14 wire and 15 on the input.

The breaker is to protect the line.A fusible disconnect is for motor protection.In my experience in the HVAC field motors are always protected by fuses.Just not sure on sizing TD fuses properly.

 

[jeff000]

I am looking for a little advise from industrial/machinery electricians here. I am installing a Nussbaum lift in my garage that is rated 14.1FLA @ 240V 1 ph. I am putting a 30 amp fusible disconnect on it with Bussmann FRN-N-** time delay fuses.I am thinking 20 amp fuses, but is this over or under rating them.17 amp and 22.5 amp are available thru special order....any thoughts on the subject? I am also doing a 3 phase/VFD setup on my drill press.Drive rated at 9.7 amps @ 240v 1 ph. input.....motor is rated at 3.0 FLA @ 230v 3ph. Really not sure what size for this situation?Drive documentation is very confusing.Thanks for your help!!!

I'm with sberry, why go fused over breaker?
And alfred was correct about using a 25a fuse, if you go breaker use a 30a.

As for the VFD, what size what? You seem to have picked the proper drive being double the required size for phase conversion. Use a 15a breaker.

I'm still surprised you americans can use #12 for 30a. I'd personally use #10 for the lift and #12 for the VFD.

 

[jeff000]

That's a maximum value.Wouldn't it be better to get the fuse sized as accurately as possible so it's not too slow in protecting the motor?What are your thoughts on the low side?

In the case of a failure it will pull many multiplies past what the fuse will hold. the time delay is just long enough to cover the inrush spike.

The breaker is to protect the line.A fusible disconnect is for motor protection.In my experience in the HVAC field motors are always protected by fuses.Just not sure on sizing TD fuses properly.

It's been a real long time since I have worked on an MCC with fuses used for the motors, they are all breakers. For better motor protection use a starter.
We use fuses to protect the control side.

 

[Norcal]

If the motor has overload protection the fuses are just for short circuit protection, too low of a rating could result in nuisance fuse blowing/clearing.

 

[rockwithjason]

I'm still surprised you americans can use #12 for 30a. I'd personally use #10 for the lift and #12 for the VFD.

who told you we did? the code specifically excludes #12 from anything over 20a.

 

[alfredeneuman]

who told you we did? the code specifically excludes #12 from anything over 20a.

Exceptions are made for motor loads and AC loads.
It's common to see a breaker bigger than 20A on a #12, although the load can never exceed 16A.

 

[Mr onetwo]

If the motor has overload protection the fuses are just for short circuit protection, too low of a rating could result in nuisance fuse blowing/clearing.

Neither motor has any overload protection.

 

[sberry]

A lift motor really doesnnt need thermal, there is an operator and short duty cycle. The motor wont overload a 12 wire, just needs short protection.

 

[jeff000]

who told you we did? the code specifically excludes #12 from anything over 20a.

Table 310-16
#12, 90°C conductor, 30amps. Unless there is a typo in my code book.

 

[alfredeneuman]

Jeff,

2011 NEC

240.4 Protection of Conductors. Conductors, other than
flexible cords, flexible cables, and fixture wires, shall be
protected against overcurrent in accordance with their ampacities
specified in 310.15, unless otherwise permitted or
required in 240.4(A) through (G).


(D) Small Conductors. Unless specifically permitted in
240.4(E) or (G), the overcurrent protection shall not exceed
that required by (D)(1) through (D)(7) after any correction
factors for ambient temperature and number of conductors
have been applied.

(5) 12 AWG Copper. 20 amperes


The 90° rating is used for derating purposely ONLY in the case of #14, #12, and #10.

Edit: The asterisks after the sizes in table you're on lead straight to this section.

 

[rockwithjason]

Table 310-16
#12, 90°C conductor, 30amps. Unless there is a typo in my code book.

Read the whole section

 

[MTW]

Mr onetwo
I am looking for a little advise from industrial/machinery electricians here. I am installing a Nussbaum lift in my garage that is rated 14.1FLA @ 240V 1 ph. I am putting a 30 amp fusible disconnect on it with Bussmann FRN-N-** time delay fuses. I am thinking 20 amp fuses, but is this over or under rating them.17 amp and 22.5 amp are available thru special order....any thoughts on the subject? I am also doing a 3 phase/VFD setup on my drill press. Drive rated at 9.7 amps @ 240v 1 ph. input.....motor is rated at 3.0 FLA @ 230v 3ph. Really not sure what size for this situation? Drive documentation is very confusing.

The breaker is to protect the line. A fusible disconnect is for motor protection. In my experience in the HVAC field motors are always protected by fuses. Just not sure on sizing TD fuses properly.
Neither motor has any overload protection.

MTW
Fuses can be used for overload protection provided they are sized correctly. See NEC 430.31-32.

Basic intent of the rules when using fuses as overload protection.
It’s somewhat difficult to size the fuses to protect the motor accurately and still provide enough starting capacity for the motor, it’s usually load type dependent.
The basic rule 430.32 says to use up to 125% for 1.15 SF (service factor) motors and 115% for all others. 430.32C states that if the values of the previous section do not allow the motor to start, you may be allowed to go as high as 140% for 1.15SF or 130% for all others.

From the practical aspect, size the circuit on the large size to limit voltage drop and improve starting performance (start time surge). Then use as small of a overload setting, to get the best protection for the motor windings. But it still needs to allow the motor to start and not nuisance trip.

Calculations.
Circuit Short circuit protection.
14.1 x 1.25 = 17.62 min circuit amps. 14.1 x 1.75 = 24.67 max circuit amps. 14.1 x 1.5 = 21.5 preferred circuit amps. So 20A fuses would be good and cheaply available if used for circuit protection.

Overload protection.
Protect at nameplate value if it will start and run (FRN-15).
Otherwise,
SF 1.15 motors, 14.1 x 1.25 = 17.62 A first try setting (FRN-17), 14.1 x 1.4 = 19.74 maximum setting . (FRN-20)
SF 1.0 Motors, 14.1 x 1.15 = 16.21 A first try setting (FRN-17), 14.1 x 1.3 = 18.33 maximum setting. (FRN-20)

This is why a motor starter with separate overload relay is normally used to get accurate protection. I would tell you to check your motor nameplate carefully, if it says TP (thermally protected) all this discussion is a moot point. It has internal protection and doesn’t require external protection. As with a lot of equipment motors, it reduces control cost for the vendor.

For the VFD drive. Size the circuit at the drive input rating x 1.25 as shown above. The VFD should have the required overload limit setting, and current limiting, in it’s programming parameters. Ω

 

[sberry]

The lift motor, mine is 13.9A and allows a 14 with 35A and I suspect its 35A regardless of wire size. The next model up with 17.5A calls for 12 and 45 max. Listed right on the motor,,, wire size and all.
Forget all the mental anguish here and put it on a 12/30 circuit and be done with it. I added a recept for a wire feeder while I was at it.

As for the drive, same deal, 14 wire and 15 breaker ,,, they womnt make you input it with an extension cord and 12. 2534 breaker but the small standard circuit matched to it.

 

[Mr onetwo]

Thanks for all the helpful comments guys! Decisions...decisions I will let you know how I decide to proceed.Lift is pinned and hydraulic parts arrive today.