Fuse sizing

[kjroller]

Not doing this but just for my knowledge I am wondering how to size a fuses for a disconnect. Lets say 100 amp service needs fusing. How would I calculate this versus a breaker I know how to size a breaker but are fuses different? I also know how to size fuses for a motor but just wondering for a situation like the one above how they are sized

 

[infinity]

For a service the OCPD, whether as a fuse or as a circuit breaker would be the same size.

 

[CoolWill]

Not doing this but just for my knowledge I am wondering how to size a fuses for a disconnect. Lets say 100 amp service needs fusing. How would I calculate this versus a breaker I know how to size a breaker but are fuses different? I also know how to size fuses for a motor but just wondering for a situation like the one above how they are sized

100 amp service would get 100 amp fuses.

 

[kjroller]

So the rule of 80 or in this case 83% load since its a feeder so i could use 83 amps on a 100 amp fuse ?

 

[kjroller]

So instead lets say it draws 100 amps in that case I would need 125 amp fuse

 

[winnie]

If you actually have a continue 100A load you would use 125A OCPD (fuses or breaker) and 125A conductors.

If you have an article 220 calculation for a residence that comes to 100A you use 100A OCPD and 83A conductors.

 

[infinity]

So the rule of 80 or in this case 83% load since its a feeder so i could use 83 amps on a 100 amp fuse ?

There is no 83% load rule but you are allowed to use conductors sized at 83% of the size of the OCPD due to load diversity on the service.

So instead lets say it draws 100 amps in that case I would need 125 amp fuse

You can size the service based on the load calculation if it comes out to 100 amps then you can use a 100 amp OCPD.

 

[kjroller]

Question then isnt it good practice to try to keep your breaker at 80% of the current rating or does service calc take that into account when you calc a 100 amps for load and use 100 amp breaker?

 

[infinity]

Question then isnt it good practice to try to keep your breaker at 80% of the current rating or does service calc take that into account when you calc a 100 amps for load and use 100 amp breaker?

No need to worry about 80% because the load calculation has taken that into account.

 

[kjroller]

So in other words use service calc for exact amperage on main OCPD but on a branch circuit only load to 80%.

 

[infinity]

So in other words use service calc for exact amperage on main OCPD but on a branch circuit only load to 80%.

A branch circuit can be loaded to 100% if it's a non-continuos load, 80% if it's a continuous load.

 

[jim dungar]

So the rule of 80 or in this case 83% load since its a feeder so i could use 83 amps on a 100 amp fuse ?

Enclosed OCPD have the same limitations whether they are fuses or breakers. Continuous loading, at 100%, can only occur for 3 hours.