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#88957 - 08/17/04 10:19 AM Motor Fuses
drillman Offline

Registered: 03/24/03
Posts: 97
Loc: Somewhere in Texas
Thought I would run this by you guys since its been a while since I did this sort of calculation.

I work for a government agency and the warehouse guy picked up a 4 foot tall fan from another dept. The fan has a twistlock plug and a 3 hp, 230/480 volt, 3phase motor. With fla 9/4.4 Amps. SF 1.15.

No starter, no overload protection. He states that he saw it work at the other place when plugged in. I already told the boss we are not going to be hot plugging this thing in and out.

We have fused discos in stock so I came up with the idea of using a fused disco with RK5 fuses at 12 amps. Using 230 volts FLA 9 * 1.25 = 11.25 then round up.

Is this a stupid idea? If so then let me know and I will get a starter.

If I get a starter the heater tables for a starter list the following heaters: 10A - 11A and 11A - 12A. Which one do I get? I figure to round up.

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#88958 - 08/17/04 02:44 PM Re: Motor Fuses
cpal Offline

Registered: 05/17/04
Posts: 165
Loc: Cohasset MA
speaking to the motor running protection only here are a few general thoughts

The motor running protection is established in section 430.32 A, motors more than 1 Hp. Your installation (1.15 SF) allows the MRP set at 125% of Name Plate current 9 A. I agree with your calculation of 11.25A
430.32 (A) (1), States; “Separate Overload Device A separate overload device that is responsive to motor current. This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating:

Note: "not more than" (125%)

I would not apply the 12A fuse. The Buss Electrical Protection Handbook suggests a 10 Amp dual element fuse for this motors overload protection.

This web site, and indicates that the RK 1, and 5 fuses will hold for a minimum of ten seconds at 500% of its rated current. That indicates to me that on hard start or actual overload the 10A fuse will hold to 50A for 10 seconds(or longer). The 12 Amp fuse will hold 60A for ten seconds (or longer).

Generally thermal devices are rated as class 10, 20, and 30 with these numbers indicating the approximate time in seconds they will endure there rated current (generally selected at 125% of motors name plate current).

I would not exceed the 10 Amp device if you intend to connect the motor in the manner you describe.

If you get a motor controller usually the overload devices are selected off the manufactures lable, but the Code will still limit the size to 11.25A.

[This message has been edited by cpal (edited 08-17-2004).]

#88959 - 08/17/04 03:32 PM Re: Motor Fuses
Bjarney Offline

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
I’ll side with cpal. Time-delay/dual-element fuses are fine for motor-running overload, short-circuit and ground-fault protection.

Probably blatantly obvious, but I would caution the user not to use cord and receptacle as a routine means of disconnect and, if the motor doesn’t start on switch closing, don’t leave it on… a fuse may have opened and the motor is single-phased.

#88960 - 08/17/04 07:25 PM Re: Motor Fuses
drillman Offline

Registered: 03/24/03
Posts: 97
Loc: Somewhere in Texas
Thanks for the info. I guess one cannot round up. Not a problem. Like I said its been a while since I did this sort of thing.

To Cpal: thank you for your advice, its been a while since I had to do this sort of thing and I will round down.

To Bjarney: I fully intend to cut the cord cap off and hard wire this thing. Reason number one is that a twistlock recept is not cheap. About the same as a real disco.
Reason number two is that there is no way I would allow a cord cap to be a disco for a 3 hp motor.


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