# 7.5 Hp Compressor Fuse



## Karl_T (Sep 8, 2015)

What is the correct fuse size for a 7.5 Hp. 220 3 phase air compressor?

I blew a 15 amp set on first trial. it was running the wrong direction and I found one wire nut inside the motor with terrible connections. It melted the wire nut and then grounded to frame. I guessing that might blow a fuse 

These fuses are spendy, six inch 3/4 diameter cartridge tube type. Need to order more today.


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## CluelessNewB (Sep 8, 2015)

Your motor should list the amps on the name plate, it may say "FLA" (full load amps)  if not try this link:   

http://www.engineeringtoolbox.com/elctrical-motor-full-load-current-d_1499.html

Make sure your wire is large enough (see this table):

http://www.engineeringtoolbox.com/three-phase-circuit-wiring-d_737.html


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## Karl_T (Sep 8, 2015)

I should have included the FLA -  23 amp.

Thought maybe I could/should go a bit bigger on the fuse


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## BobSchu (Sep 9, 2015)

According to the NEC article 430 - Motors- you are allowed to go to 175% of the full load current of the motor with dual element slow blow fuses. The table 430-250 says that motor should pull 24.2 amps at 208V 3 phase 7.5 hp. Close enough for government work. That computes to 42.35 amps for the fuse. So a 40 Amp fuse would work fine, in my experience. Wiring should be rated for the nameplate rating- at 23 amps plus 25% that would be just under 30 amps so it should be fed by #10 wire. Not required to upgrade the wire for the fuse size, the branch circuit is protected by the motors thermal protection.

Bob


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## Karl_T (Sep 9, 2015)

Thank you for the very good information.

I had found a deal on 30 amp on eBay this morning and ordered them. if they don't hold, I'll shop for 40s. I used #10 wire, so that's good to go.


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## BobSchu (Sep 9, 2015)

Hope it works out for you. In my experience, most 7.5 hp motors will start just fine on 30 amp fuses- except for compressors. Compressors are especially hard on fuses due to the starting head pressures. Good luck. 

Bob


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## Wireaddict (Sep 10, 2015)

If the fuses are 6 in. long they're most likely 600-volt type & they do cost more than 250-volt fuses.  It sounds like your fuses are in a 600V, 30A disconnect switch.  If that's the case try FRS or TRS 30 fuses or equivalent which are dual element as Bob said & power it with #10 wire after you get the motor leads straightened out.  175% of 23 amps is about 40A but if you need to go over 30A you'll need a bigger disconnect switch; to save money in that case, get a 60A, 250V fusible switch. And buy FRN/TRN-35 to 40 fuses.


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## Karl_T (Sep 10, 2015)

The biggest fuse my disconnect takes is 30 amp. So, I really hope it works.


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