Motor calculations

[Brafs4411]

2014 electrical exam prep book,unit 8 page 258 question 11-12.

Ouestion 11.
If a dual element fuse is used for overload protection,what size fuse is required for 5 hp ,208 three phase motor with a service factor of 1.15 and motor name plate rating of 16A?

Question 12
If a dual element fuse is used for overload protection,what size fuse is required for a 30hp,460v three phase synchronous motor,with a nameplate current rating of 32A, and a temperature rise of 39c

I keep missing this because I have the dual element on my mind and keep going to table 430.52 for my answer. Why does it state dual element fuse but we use 1.25 standard for overload protection, in what instances would we use table 430.52 for calculations?
Thank you
BRAD

 

[Russs57]

I’m not a code guy so I can’t tell what table.

I believe the correct answer for both examples would be not more than 125% of nameplate amps. So 20 and 40.

 

[Dennis Alwon]

Overload is different than over current. Look at 430.32 after getting the fla from 430.250

 

[don_resqcapt19]

This is an application where the fuse is being used to provide both the short circuit and ground fault protection, as well as the running overload protection. It must be sized small enough to comply with the rules that cover overload protection. A dual element fuse should be able to handle the motor starting current even where it has been sized to provide overload protection. Remember the rules in 430.52 are maximum permitted ratings, lower rated devices are permitted.