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12-12-2012, 11:40 PM   #1
Mtn_Brewer
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I have a ~5 amp AC unit I am installing in a fermentation chamber and it has blown fuses up to a time delayed 8 amp fuse. Smaller and fast blow fuses blew immediately when the unit was cycled on but the 8 amp time delay went through 4 cycles or so and then blew. Everything seems wired correctly. I suspect that the surge on start up is strong enough to blow the fuse. I'm worried about increasing the fuse amperage any higher. Has anyone else experienced this problem? Anyone have a solution? Thanks.

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12-13-2012, 01:35 AM   #2
PintoBean
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Use a 15A fuse, a motor draws on average 225% of the running load amps (RLA). So 5x225%=11.25. Round up to next standard fuse size, 15A. You never want to run anything on a circuit where the input power is really close to the fuse rating. I'm assuming this is a fuse in your home fuse box, and the wiring in your house is rated for at least 15A.

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12-13-2012, 03:10 AM   #3
rickyspeak
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I don't want to sound like a dick picking a fight I'm just curious how you came to this answer. Do you have a citation or reference for that 225% number or is it just a standard that you like to use?

Quote:
 Originally Posted by PintoBean Use a 15A fuse, a motor draws on average 225% of the running load amps (RLA). So 5x225%=11.25. Round up to next standard fuse size, 15A. You never want to run anything on a circuit where the input power is really close to the fuse rating. I'm assuming this is a fuse in your home fuse box, and the wiring in your house is rated for at least 15A.
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12-13-2012, 12:58 PM   #4
ChuckO
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Here's a discussion thread of inrush current http://control.com/thread/1026195635 which more or less supports PintoBean's statement when you don't know the exact motor design. You can find quite a bit of information if you google "motor inrush current"

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12-13-2012, 05:11 PM   #5
PintoBean
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Quote:
 Originally Posted by rickyspeak I don't want to sound like a dick picking a fight I'm just curious how you came to this answer. Do you have a citation or reference for that 225% number or is it just a standard that you like to use?
Just a rule of thumb I learned to use at work. You could figure out an axact number if you knew every motor parameter, but for the purpose we're talking about here, this is sufficient.

If you want, take a look at your house AC unit RLA (it should be on the unit on a sticker somwhere), and then look at the breaker size for it in your panel and use that ratio.
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12-13-2012, 06:14 PM   #6
ryan_george
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Also consider that when you plug this A/C unit into the wall, you are working with a 15A/20A breaker in the panel. No other fuses would be installed. 15A or 20A fuse should be fine, assuming all of your power wires can handle it...

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12-14-2012, 12:37 AM   #7
Mtn_Brewer
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Thanks for the info everybody. Very interesting. I did call LG and ask what the inrush current is for my unit and the customer support in Bangalore could not give me an answer. I guess I will try a 15 amp fuse and see how it goes. My PID has a max rating of 10 amps internal relay, so I guess I will have to install an external relay with a high amp rating.

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