Motor Protection Question

[Roger]

The branch circuit protection for a single phase 115V 3HP motor would be ___ amps using dual element time delay fuses.

1.) 60 amps
2.) 80 amps
3.) 40 amps
4.) 70 amps

This is a made up question....I calculated 60 amps. Would you guys agree with this or am I doing something in error?

[Homer]

Yes, 60A looks correct for a time-delay fuse. For an inverse-time breaker, the maximum size would be 90A.

A 3HP, 115V motor has an FLA of 34A from tables §430.248 or §430.250.

For a time-delay fuse you take 175% of 34A which is 59.5A, so you can round up to 60A based on §240.4(B) and §240.6 .

So, what's the minimum ampacity of the circuit conductors?

1.) 40.5 amps
2.) 42.5 amps
3.) 55 amps
4.) 60 amps


Homer

[Roger]

HOMER, I calculate this to be 42.5 amps. 34 x 1.25 = 42.5 (Article 430.22 1999 code)

I'm going to try a question for overload relay protection but need to review that some....will post question later today.

[Roger]

Related question. So if I calculate say 23.5 amps then I could use a 25 amp fuse but not a 30? Only can move to next size up? What if the motor start up blows this fuse can you continue to move up or do you most likely have some problem with the motor?

[Homer]

So if I calculate say 23.5 amps then I could use a 25 amp fuse but not a 30? Only can move to next size up? What if the motor start up blows this fuse can you continue to move up or do you most likely have some problem with the motor?If you calculate 23.5A for a time-delay fuse then you can round up to the next size listed in §240.6. Sometimes people are surprised that 25A is a standard size listed in that section.

If you were dealing with breakers instead, many would use a 30A breaker instead of a 25A breaker as it may not be a stock item. However, this would technically be a violation. The rule says the next size in the table, not in stock at your supply house.

Remember, you're already oversizing the fuse to begin with when compared to the conductor ampacity.

Homer

[Mr Fixit eh]

So if this motor were converted to 230V, the FLA would be 17A, and you would use a time-delay fuse of 30A? The branch circuit conductor ampacity would be 17A x 1.25 = 21.25A? Can 12AWG handle 21.25A?

[Homer]

So if this motor were converted to 230V, the FLA would be 17A, and you would use a time-delay fuse of 30A? The branch circuit conductor ampacity would be 17A x 1.25 = 21.25A? Can 12AWG handle 21.25A?You're pretty good Mr. Fixit. It looks like you've caught on to the NEC rules quickly.

1) Yes, the fuse is calculated as 29.8A (175% x 17), so it's rounded up to 30A.

2) Yes, the required conductor ampacity would be 21.25A.

3) Yes, #12 can handle 25A for this application based on §240.4(G).

Homer

[Mr Fixit eh]

Thanks, Homer.

This is all assuming that the motor has built-in thermal over-heating protection.

It is frustrating that the codes are not available online. This forum is truly a great resource, however. I truly appreciate the efforts of all who participate.

[Homer]

This is all assuming that the motor has built-in thermal over-heating protection.Either built-in or external, as long as it is provided for in addition to the OCPD.

Homer