warning system overcurrent protection

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Jps1006

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I have been asked to install a warning system that sets off a strobe and horn if a chemical tank level gets too high. There are currently level indicators with programmable contacts that will close on a preprogrammed level. This is all fed with #12 THHN on a 20-amp breaker. I would like to build this system using #16. The Siemens guy that sold and programmed my LOGO! 230RC (very cool device, I even bought the software) says I don't need to provide supplimental fusing and I can fuse all my #16 at 20-amp. Overload is not an issue and they will be short-circuit protected.

I am having a hard time finding the section that would cover this. Neither 430, 640, 725, or anything in chapter 8 seems to be a good fit. I'm inclined to use 430, but this isn't motors. Any thoughts?
 
I think 240.4(D)(2) {'08 Code} might provide the answer you need
 
Hmmm.....

The contacts of the Logo (and yes, they rock!) are rated at 5A, and thus I personally would be worried about any arrangement that didn't protect these contacts against overloaded. But many people think I am paranoid about issues like this...
 
You must protect the wire. Din rail mounted fuse holders are very cheap, and the breakers have come down in price also. Don't put anything over 1A on the logo's outputs, drive a relay. PLC relay outputs are not rated for inductive loads and will weld quickly.

A horn is an inductive load.

Steve
 
StephenSDH said:
You must protect the wire. Din rail mounted fuse holders are very cheap, and the breakers have come down in price also. Don't put anything over 1A on the logo's outputs, drive a relay.
PLC relay outputs
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are not rated for inductive loads and will weld quickly.

A horn is an inductive load.

Steve
Click to expand...

unless you're using an ancient plc, there are no relays in the output cards. You can add them in your panel if you choose, and I recommend
 
mcclary's electrical said:
unless you're using an ancient plc, there are no relays in the output cards. You can add them in your panel if you choose, and I recommend
Click to expand...

Ancient or New you could always purchase relay cards, and they will always serve their purposes. This is a brick plc, and most of them have some/all relay outputs built in to make them universal to customer voltages. The relays outputs are generally rated for higher current then DC/Triac, but can't compare to the ratings of an ice cube relay. Plus when the relay burns out you lost an output, instead of replacing the ice cube relay. I am sure you are going to love hiring a programmer to move an output when it could have cost you 10 dollars to replace a relay.
 
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