Fuse sizing in a 3 phase circuit

[snagit88]

I had an interview that had a test. One of the questions I supposedly got wrong was sizing each line (1L1, 1L2, 1L3) in a motor branch circuit. The nameplate on the motor was a given 3A FLA. I was asked to size the fuses by 300% using a class CC fuse. I took the 3A and divided by the square root of 3 and then took 300% of that. The EE manager told me I was wrong and that the 3A is across each line. I had thought the 3A on the nameplate was across all 3 phases and to find the particular current on each line you had to take into account the square root of 3. I went along with what he said. But, am I wrong and have I been undersizing my fuses or have they been oversizing them. I haven't had any problems in the past.

Any thoughts?

Thanks.

 

[waross]

If you are deriving current from total watts or total KVA or horsepower, then you may need to use the root 3 factor.
The nameplate current of a motor is the actual current in each phase. You have been very lucky with your fusing.
Using the root of three resulted in your fuse sizing being 173% instead of 300%

 

[mkinnes]

By code you are allowed to size class CC fuses at 300 percent of FLC. 9 amps

175% percent if using Dual Element(Time Delay)

 

[snagit88]

Thanks. At least now I know.

 

[rbulsara]

sangit:

I undestand your confusion, being just out of the school. The term phase and lines are often loosely used in field, but are well understood by experienced persons.

When someone says I measured current in each phase of a panel or a motor, he/she usually means he/she measured the line current.

All equipment nameplate current invariably refer to line currents. For fuse sizing, only line current that matters.

For some insight refer to the following threrad:
thread238-118444

 

[angryman]

Generally speaking, isn't 300% used for fast blow and 173% used for slow blow as far as fusing goes?