Table 430.52 Time-Delay Fuse Size.

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Skokian

Member
Location
Skokie, Illinois
A new compressor has a motor with an FLA of 10.5 at 230 Vac single phase. It trips upstream 20 amp time delay plug fuses.

Exception No. 2(b) would allow the time delay fuses to increase from 175% (18.375 amps -> 20 amps) to 225% (23.625 amps).

Exception No. 1 allows up-sizing to the next larger standard fuse. However, Exception No. 2(b) states said fuse "... shall in no case exceed 225 percent ..." of the motor FLA.

Can I up-size to 25 amp time delay fuses or am I still stuck with 20 amp time delay fuses?

This compressor is used with a dry-pipe fire sprinkler system in a church in a large city.

Tx. in advance for any assistance or opinions.
 

Strife

Senior Member
A new compressor has a motor with an FLA of 10.5 at 230 Vac single phase. It trips upstream 20 amp time delay plug fuses.

Exception No. 2(b) would allow the time delay fuses to increase from 175% (18.375 amps -> 20 amps) to 225% (23.625 amps).

Exception No. 1 allows up-sizing to the next larger standard fuse. However, Exception No. 2(b) states said fuse "... shall in no case exceed 225 percent ..." of the motor FLA.

Can I up-size to 25 amp time delay fuses or am I still stuck with 20 amp time delay fuses?

This compressor is used with a dry-pipe fire sprinkler system in a church in a large city.

Tx. in advance for any assistance or opinions.
You say the motor is 10.5 at 230? What voltage is applied to it? Your FLA will be adjusted based on voltage. Applying 240 will reduce the FLA, whereas 208(add 2-3 volts drop) you're looking at 13-14 FLA easily.(so 13*1.75=22.75, 14*1.75=24.5)
Compressors can be a lot more demanding than regular motors(you catch the piston in that position right before maximum compression and it'll take a lot more to start than most other motor applications).
However, are you sure you got time delay fuses? A 20A time delay should easily handle 30A for that half second it takes to start.
I never seen breakers trip at 175% of the FLA(even on compressors), so it seems a little strange, something else might be wrong there
 
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texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
A new compressor has a motor with an FLA of 10.5 at 230 Vac single phase. It trips upstream 20 amp time delay plug fuses.

Exception No. 2(b) would allow the time delay fuses to increase from 175% (18.375 amps -> 20 amps) to 225% (23.625 amps).

Exception No. 1 allows up-sizing to the next larger standard fuse. However, Exception No. 2(b) states said fuse "... shall in no case exceed 225 percent ..." of the motor FLA.

Can I up-size to 25 amp time delay fuses or am I still stuck with 20 amp time delay fuses?

This compressor is used with a dry-pipe fire sprinkler system in a church in a large city.

Tx. in advance for any assistance or opinions.

To be compliant you are stuck with the 20 amp fuse. I wonder if there is another issue, like maybe the compressor is not unloading after stopping and causing to much inrush.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Edit to add (slow poster) - As already said by others:

I think the code is clear - you are stuck with 20A. I suspect this one would not be the first with 25A fuses

But I never use fuses and rarely deal with single phase - so who knows.

Just curious - I suspect you have already checked all of this:
Is this a long run with #12 (or even #14) conductors? You may wish to look at the voltage drop. Excess VD could be causing long accelleration time and that is causing the overload.

Another is the compressor unloader is bad out of the box and is not relieving between starts. The hard starting is causing the overload.

These are two that I have run into on new small compressor installs .

edit to add (Is there an echo in here.......here........here.........

ice
 
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augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Does the motor actually have a HP rating (other than SPL).
430.248-250 is normally used for SC-GF protection rathewr than nameplate FLA.
Might give you a different answer.
 

Skokian

Member
Location
Skokie, Illinois
Motor actual voltage & running current.

Motor actual voltage & running current.

You say the motor is 10.5 at 230? What voltage is applied to it? Your FLA will be adjusted based on voltage. Applying 240 will reduce the FLA, whereas 208(add 2-3 volts drop) you're looking at 13-14 FLA easily.(so 13*1.75=22.75, 14*1.75=24.5)
Compressors can be a lot more demanding than regular motors(you catch the piston in that position right before maximum compression and it'll take a lot more to start than most other motor applications).
However, are you sure you got time delay fuses? A 20A time delay should easily handle 30A for that half second it takes to start.
I never seen breakers trip at 175% of the FLA(even on compressors), so it seems a little strange, something else might be wrong there

Good questions. The actual voltage is 240.0 Vac and the running current is 239.1 Vac. However, the motor has a service factor of 1.15. The measured current draw is 11.23 to 11.78.

Also, I forgot a key parameter in my query. Namely, that the starting code is Code "J",which will draw from 7 to 8 times the rated FLA.

The plug fuses are definitely time delay (slow blow) types for sure. Also note that there is no unloader on these small (2 Hp) compressors. However, it runs less than once per hour and runs for less than one minute. These dry-pipe compressors are used to keep air pressure in a sprinkler system upstream of the "dry pipe valve", 40 to 50 psi in this case. They only run to make up for leakage in the system.
 

Skokian

Member
Location
Skokie, Illinois
Compressor motor wiring.

Compressor motor wiring.

Edit to add (slow poster) - As already said by others:

I think the code is clear - you are stuck with 20A. I suspect this one would not be the first with 25A fuses

But I never use fuses and rarely deal with single phase - so who knows.

Just curious - I suspect you have already checked all of this:
Is this a long run with #12 (or even #14) conductors? You may wish to look at the voltage drop. Excess VD could be causing long accelleration time and that is causing the overload.

Another is the compressor unloader is bad out of the box and is not relieving between starts. The hard starting is causing the overload.

These are two that I have run into on new small compressor installs .

edit to add (Is there an echo in here.......here........here.........

ice

Another good question.

The wiring is #12 AWG. The run from the fuse box to the compressor motor starter is around 5 feet and another three or four feet to the compressor motor. There is no main fuse or breaker in this 8 fuse plug fuse box. This box is supplied by around #4 or so wiring (a guess) from a fusible switch in the service panel switch board with a run of 15 feet or so. This switch board is fed by the utility pole in the alley less than 20 feet away. That's why the running voltage drop is no more than one volt.
 

Skokian

Member
Location
Skokie, Illinois
Motor Hp Rating.

Motor Hp Rating.

Does the motor actually have a HP rating (other than SPL).
430.248-250 is normally used for SC-GF protection rathewr than nameplate FLA.
Might give you a different answer.

The motor nameplate gives 2.0 Hp. Note that the motor is used well into it's 1.15 service factor. I calculate around 111% if prorated down to it's rated 230 Vac instead of the 239 Vac actual running voltage and taking the average running amps as 11.5.

However, you do ask a very good question which raises one that have occurred to me sooner. Namely, am I allowed to use the FLA value of 12.0 amps from Table 430.248 instead of the nameplate value of 10.5 Amps FLA/FLC ?
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
The motor nameplate gives 2.0 Hp. Note that the motor is used well into it's 1.15 service factor. I calculate around 111% if prorated down to it's rated 230 Vac instead of the 239 Vac actual running voltage and taking the average running amps as 11.5.

However, you do ask a very good question which raises one that have occurred to me sooner. Namely, am I allowed to use the FLA value of 12.0 amps from Table 430.248 instead of the nameplate value of 10.5 Amps FLA/FLC ?
Absolutely. Augie raised a good point, rather than just assuming as I did, that you were giving us the table value instead of the nameplate. Looks like you are good to go with a 25 amp TD fuse. Probably will solve the problem.
 

Strife

Senior Member
However, the motor has a service factor of 1.15. The measured current draw is 11.23 to 11.78.

If your measured current running is 11.78 I suspect the FLA it's quite a bit bigger than that.
No one designs a load EXACTLY at 100% of the motor capacity. Most likely 11.78 is about 80% of the FLA. Unless (as I mentioned before), there's something else wrong there. if the FLA nameplate is 10 some and you're drawing 11.78(especially with a higher voltage), either the design was bad (putting more load on the motor) or something else besides what's supposed to creates an additional load(bad bearings, bad valves on the compressor, belt/s too tight? I assume this is a belted compressor??? a belt that's too tight definitely will put a lot more load on the motor, especially at start up)
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
...No one designs a load EXACTLY at 100% of the motor capacity. ...

Small compressors? Yep, they do - that is when they are not outright lying.:jawdrop:

Take a look a your standard Sears Craftsman NEW, 5hp Shop Compressor
Right there in the catalog it says that - and amazingly it has a 15A cord cap:sick:

ice
 

Skokian

Member
Location
Skokie, Illinois
If your measured current running is 11.78 I suspect the FLA it's quite a bit bigger than that.
No one designs a load EXACTLY at 100% of the motor capacity. Most likely 11.78 is about 80% of the FLA. Unless (as I mentioned before), there's something else wrong there. if the FLA nameplate is 10 some and you're drawing 11.78(especially with a higher voltage), either the design was bad (putting more load on the motor) or something else besides what's supposed to creates an additional load(bad bearings, bad valves on the compressor, belt/s too tight? I assume this is a belted compressor??? a belt that's too tight definitely will put a lot more load on the motor, especially at start up)

The motor has a Service Factor of 1.15. This would allow the motor to draw 10.5 x 1.15 or 12.08 amps at 230 Vac or 11.6 amps at 239 Vac. The reason that I quoted the running current as 11.23 to 11.78 is that it varies rapidly and the compressor runs for only a fraction of a minute. Hence, I take the average reading as 11.5 amps. This puts the running current within the rated 1.15 S.F. although it is near the high end of it.
 

Skokian

Member
Location
Skokie, Illinois
Single Pole Overload in 240 Vac Motor Starter.

Single Pole Overload in 240 Vac Motor Starter.

I'm back with another Code question.

The motor starter is a three pole with two poles used to supply the 230 Vac motor. However, only one pole has an overload element. Does this allow (or prohibit) the use of the Exception 2(b) of Table 430.52 fuse size (225% -vs- 175%)?

Tx. in advance.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I'm back with another Code question.

The motor starter is a three pole with two poles used to supply the 230 Vac motor. However, only one pole has an overload element. Does this allow (or prohibit) the use of the Exception 2(b) of Table 430.52 fuse size (225% -vs- 175%)?

Tx. in advance.

IF I read 430.37 correctly, you have acceptable overload protection so you are still good to go.
(Hopefully we will get additional comments)_
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The two cartrige fuses in the disconnect box are FRN-R-20; but, the ones which blow from time to time are 20 amp time-delay plug fuses (edison base) in the upstream eight circuit 120/240 Vac fuse box.

That in itself might be a problem. Plug fuses and associated panels are often guilty culprits due to heat generated by pitted contact points, etc. Removing plug fuses from the supply would be a great improvement IMHO
 

Skokian

Member
Location
Skokie, Illinois
That in itself might be a problem. Plug fuses and associated panels are often guilty culprits due to heat generated by pitted contact points, etc. Removing plug fuses from the supply would be a great improvement IMHO

Good points. I did check the fuses; but, they weren't warm. However, I'll check the wiring connections when I'm back at the site. Since there are no main fuses or breaker in the fuse box in question, I think I would have to coordinate short circuit ratings of the downstream disconnect switch and the upstream service panel board. This would require knowledge of the short circuit current available from the utility. I don't know if I can reasonably get this data. The building in question is an old church building.
 
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