Fuses Help please

tim_callinan

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Join Date
Apr 2012
Location
Annapolis
Posts
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Im teaching a course in Motor control in about a month. I have a couple of questions about fuses if anyone can help me.
Im trying to show the students some examples of overcurrent protection.
We have a 1/3 HP 3 phase motor with a mechanical brake attached. Its possible to turn on the brake and have the motor current ramp up. The motor steady state current is about 1A per phase without any load but when I turn on the brake it can increase above 6 amps. The fuse holders we have and we can purchase different ones if its a better idea - hold a fuse around 10x38mm.
For whatever reason the midget fuses we got are around $20 per fuse in McMaster Carr. Essentially I want to turn on the brake - let the students see the current running above 5 amps or so and then hopefully the fuse will blow in a second or two. I made a quick video showing what Im trying to do.
Any advice or assistance is defo appreciated. Its not my money but I dont want the college paying twenty bucks per fuse!
https://www.youtube.com/watch?v=ZQ7CJ3z4ewE
Tim
 
I wouldn't use fuses for this. A much better real world example would be to use a motor overload relay. This would allow you to show how motors can and do run at higher current at startup, and how the overload will allow the motor to run above it's FLA rating for a short amount of time. The right overload would also give you the benefit of being able to simply reset it after each demonstration.

In all the plants I have worked in, fuses were used for wire protection, and to clear fault current. Not to protect the motor.


Bubba.
 
Last edited:
Hi Bubba - thank you first of all. We have motor overload relays in the motor starter so thats good. Can I ask what do you mean by wire protection. Our three phase circuits would have a circuit breaker back in the panel that would protect against any short circuits so Im confused what is the point of fuses then?
Tim
 
i'm not sure if i will give you the best answer.

i think that you will agree that all wires have a current limit.
you cannot put a 50 amp load on a 24 gauge wire, it will melt in seconds.

fuses serve not only to protect motors and other devices, but wire as well.
so for a 12 ga. wire, you would put a 20 amp fuse for example.
when wiring the panel, you size the wire based on the load and then size the fuse based on the load rating.
for the motor you are talking about, i would use 12 ga. wire to the motor and size the fuse at 125% of full load (don't have code book in front of me) or up to a max of 250% (going from memory).

if i am incorrect, someone please correct me.
james
 
Hi James,
Thank you for the reply so the 1/3 HP motor - it says for Full load it has 1.6 amps. Ive seen it go up to approx 8 amps when the brake is turned on. Just out of curiosity - could we not use 14GA wire to run to the motor?
Back at the main electrical panel in our lab - the three phase circuits have 20 AMP breakers on them. Are extra fuses required then?

I suppose Im trying to simulate a conveyor system getting stuck or something like that. The motor will continue to run and probably will burn out. If the brake on the motor is engaged - the current is approx 8 amps. This will not trip the main braker bc it is 20amps so im guessing motor people do not use fuses for this but rather setting it at the motor starter or motor overload relay?
Tim
 
Hi Bubba - thank you first of all. We have motor overload relays in the motor starter so thats good. Can I ask what do you mean by wire protection. Our three phase circuits would have a circuit breaker back in the panel that would protect against any short circuits so Im confused what is the point of fuses then?
Tim

You could use either fuses or a circuit breaker to protect the branch circuit wiring feeding the motor starter and motor, but neither of them offers appropriate motor overload protection.

Also, although you're correct in your assumption that #14 wire has adequate ampacity for your application, in the real world, your students should never wire a power circuit with wire smaller than 12 gauge.
 
I would seek out the publication "Bussman SPD 2017" or Bussman Selected Protective Devices 2017.

It has a full writeup on how fuses work, motor circuit components required by code, and full motor charts with recommended fuse, switch, conduit, and wire sizes. It is an invaluable resource for anyone in the industry.
 
Also, although you're correct in your assumption that #14 wire has adequate ampacity for your application, in the real world, your students should never wire a power circuit with wire smaller than 12 gauge.

Many plants and facilities do have a "no under 12 AWG" rule for anything 240 or 480V - but it is very common to do so for motors 5HP & under and not against code.

In fact, some motors in that range will not have the terminals that can handle #12 if you try to use something like VFD cable - you will have to use #14.
 
Fuses are not normally used for overload protection in the real world.
Use a proper dimensioned overload relay instead. Fuses are used as short circuit protection and protects the installation. They can and should be rated higher than the FLA rating of the motor. They need to be able to hold a current at least 6 times higher for a period of time depending on start current and load type, so that needs to be considered when selecting wire gauges.

EDIT:
You can get overload relays that does both act fast in case of a short circuits but also protects the motor in case of overload.
 
Last edited:
In the real world. Brake-resistor is used for rapid motot stop. The break current would not go through the fuse, but the brake-resistor. The brake-resistor heats up very fast. It is normally installed outside of the panel.
 
i'm not sure if i will give you the best answer.

i think that you will agree that all wires have a current limit.
you cannot put a 50 amp load on a 24 gauge wire, it will melt in seconds.

fuses serve not only to protect motors and other devices, but wire as well.
so for a 12 ga. wire, you would put a 20 amp fuse for example.
when wiring the panel, you size the wire based on the load and then size the fuse based on the load rating.
for the motor you are talking about, i would use 12 ga. wire to the motor and size the fuse at 125% of full load (don't have code book in front of me) or up to a max of 250% (going from memory).

if i am incorrect, someone please correct me.
james

This is the standard electrical code, it also depends on the Voltage too, also type of motor. Different Voltage and motor has different rules.
 
Hi James,
Thank you for the reply so the 1/3 HP motor - it says for Full load it has 1.6 amps. Ive seen it go up to approx 8 amps when the brake is turned on. Just out of curiosity - could we not use 14GA wire to run to the motor?
Back at the main electrical panel in our lab - the three phase circuits have 20 AMP breakers on them. Are extra fuses required then?

I suppose Im trying to simulate a conveyor system getting stuck or something like that. The motor will continue to run and probably will burn out. If the brake on the motor is engaged - the current is approx 8 amps. This will not trip the main braker bc it is 20amps so im guessing motor people do not use fuses for this but rather setting it at the motor starter or motor overload relay?
Tim

The fuse is used to protect the wire, not the motor. If the fuse is 20A, you need at least 12AWG, the bigger the safer, so the fuse will pop before the wire starts to burn.
 
Hi James,
Thank you for the reply so the 1/3 HP motor - it says for Full load it has 1.6 amps. Ive seen it go up to approx 8 amps when the brake is turned on. Just out of curiosity - could we not use 14GA wire to run to the motor?
Back at the main electrical panel in our lab - the three phase circuits have 20 AMP breakers on them. Are extra fuses required then?

I suppose Im trying to simulate a conveyor system getting stuck or something like that. The motor will continue to run and probably will burn out. If the brake on the motor is engaged - the current is approx 8 amps. This will not trip the main braker bc it is 20amps so im guessing motor people do not use fuses for this but rather setting it at the motor starter or motor overload relay?
Tim

yes, motor starter with overload to protect motor. Circuit breaker is to have enough current for motor full load current times 1.75(this value depends on the voltage and type of motor. you can lookup the code).
 
Many plants and facilities do have a "no under 12 AWG" rule for anything 240 or 480V - but it is very common to do so for motors 5HP & under and not against code.

In fact, some motors in that range will not have the terminals that can handle #12 if you try to use something like VFD cable - you will have to use #14.

Wire length is also a important factor. And also ambient temperature.
 

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