Hey Guys,

How does one calculate motor fusing requirements?

I would like to fuse before the starter and at the main disconnect. (10 ft away)



Example: 3 HP Motor: 460 (480) Volts: 4.8 FLA:

Would I take 4.8 * 120%? (Before motor starter)

Would I take 4.8 * 150%? (At the Disconnect)



How does one calculate disconnect fusing with 5 motors. (Using above example)



"You definitely want to add all of the MAX amps for a total full load rating. That becomes the worst case scenario. Then add 10 to 50% for heat de-rating. Your local code should be consulted for the exact amount of de-rating based on length of the cable run"



"For panels with motor loads you typically take the motor with the largest current draw and multiply it by 125%, then add all of the other motor loads to that for a total (of motors only). Then add in other loads such as transformers, etc."

Are the above statements correct?

First go to here and look at table 430.52 for the fuse size range, as well as the whole 430 section.
NEC

Each motor fuse if not Design B looks like 175% x 4.8 = 8.4 (next higher fuse is allowed, probably 9A) max. dual element fuse for each

For the feeder see Example D8 in the NEC under Appendix D and 430.62.
For you thats 9A + (4.8 x 4) = 28.2A (next lower fuse is used, probably 25A)

Statement 1: It depends, I'm not sure exactly what this refers to. Your cable length of 10' isn;t long enough to worry about derating. There is a derating for more than 3 current carrying conductors in a conduit, see Table 310.15(B)(2)(a).
Statement 2: That is for the calculation of a panel main.

How would I properly fuse at the disconnet; for one motor?
8.4+4.8=13.2????

9A max. for a dual element fuse for one.

So let me get this straight.
I would fuse each leg before the motor starter with 9 amp fuses.
And since i using a disconnect bus drop that is fused i should use the same size fuse?

Could you tell me if I am on the right path.


Motor Name plate:

15 hp Motor

208 Volt

37.5 Amps FLA

Panel Fuse: Time Delay fuse

37.5A x 175%= 65.625Amp

Buss box fuse: Time Delay fuse

37.5A x 175%=65.625Amp

Wire size in panel: #6

37.5x120%=45 amps

Wire size going to Buss box:

140 ft x (37.5x120%) amps x 1= 6300

5.5 x 1000000/6300= 873.016

Wire size at 80% power factor= #6

Conduit size

1 inch EMT

Overload device: Thermal/Magnetic

37.5x115%=43.125 Amps

You can use this website to check your calculations. And don't forget to read the NEC, that's where it all comes from.

http://www.electrician2.com/calculators/motor_ver_1.html

The fuse and wire size is based on the table in the NEC not the motor nameplate. Only the OL setting is determined from the nameplate amps. Use 125%, not 120%. I don't follow your voltage drop calculation, but yes #6 should be fine in a 1" conduit. 175% + next standard size is the max. fuse you can use. I usually use 125%x FLA with a nice bussmann low peak fuse with no problems.

Wow, great website! Lots of great information there that I'm looking forward to reading. Have you seen his "gallery" of electrical burn pictures? Those have to be the most horrifying images I have ever seen, and bring it all into perspective as to why we have all these rules and regulations to follow.

Yes, there are lots of great calculators and articles. The burn stuff I've seen too many times now from electrical safety training. Its the best thing to use to convince people to use there PPE for sure. The last slide show I saw was a time elapse of a guys electrocuted leg over the course of a few days. It got really disgusting just before they amputed it. But it worked on me anyway, i'm a PPE believer now.