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Old 10-25-2012, 07:58 PM   #1
tflenker
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Hi all!

Before getting into this post, just wanted to say a quick thanks to such a great community. With the ideas, recipes, and how-to's I've found on this forum I've gone from brewing my first extract batch this previous February to having a full-blown (albeit ghetto) electric AG setup in my basement.

Which brings me to my question. I'm using 5500W elements on a 30A circuit, obviously only 1 element powered at a time. I realized that the switches I've been using are only rated to 25A, and that's where the concern is.

A 5500W element pulls about 23A - is it safe to use 25A switches on this circuit? I ask because everything I've seen has indicated that you should never exceed 80% of a component's rating.

I've searched on the forums and I see that people have successfully used 25A switches and 25A SSR's on a 5500W system, but I'd like to hear someone's opinion that has good understanding of electrical standards/codes. An EE friend of mine seems to think it's a bad idea, but he works solely on power transmission and this seems more in the range of an electrician with more practical experience.

Thanks in advance; hopefully I can contribute something here soon!

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Old 10-26-2012, 07:35 PM   #2
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the 80% rule is more of a safeguard for household wiring. it ensures there is some extra leeway just in case. since we are talking about a switch not a circuit breaker, and also a known constant load, you can *probably* put the full rated current thru it without any consequences. i say probably because that is assuming the rating is accurate. that switch might even handle 30 amps without issue (but theres no way to tell unless you try, which can be dangerous... so just assume the rating is the "max").

if it were a 25 amp circuit breaker, and you wanted to pull 24 amps thru it, you would probably often trip the breaker due the fact that the exact tripping point is not exactly 25.0000 amps- only "pretty close to 25", and the fact that your load might pull a small amount more current under some circumstances. this would cause headaches and make you reset the breaker all the time. if you instead budgeted for 80% of 25 amps, you would likely have no issues. so thats where 80% comes from.

what i would be more concerned about is pulling 22 amps thru a 25amp-rated SSR. the reason is because the closer you get to the max power rating, the exponentialy more heat is generated. your SSR would generate less than half the amount of heat if you switched from a 25A to a 40A SSR. less heat = less cooling required (smaller heatsink) and longer SSR lifespan.

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Old 10-27-2012, 02:49 AM   #3
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Awesome, putting it like that makes sense.

I should have clarified, I'm using a 40 amp SSR, but I saw in other threads people were getting away with 25. The only things I have rated to 25 amps are a couple of switches, everything else is a minimum of 30 amps.

Appreciate the help!

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Old 10-27-2012, 02:55 AM   #4
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I think you will be OK. The 80% rule is somewhat of an enigma in the NEC, it would apply specifically to 'continuous loads' which would be three hours of more. Will you trip a typical 30A breaker at 30A? Not likely, even with hours of draw. Even at 40A or some other slight overload it will tolerate that for quite a while. The breaker's purpose primarily is to protect the insulation of the wiring, and of course for short circuits.

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Old 10-27-2012, 06:26 AM   #5
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When first getting into this, my assumption was that electrical stuff would have a safety factor figured into the maximum rating just like most mechanical things and it would therefore be completely safe to take it up to the rated limit. A bit of research led me to the 80% guideline.

From what porcupine73 is saying there is likely at least fair safety factor, but with all the nuances of electricity it's almost impossible to say what that means in each specific circumstance. Realistically though, you can usually slightly exceed the rating for a good amount of time and be fine.

My final question on this is what happens when a switch fails? Is the main mode to just become welded in one position and really not be of danger since it's not causing a short? or does it explode and kill everyone in the room?

Thanks again.

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Old 10-27-2012, 01:20 PM   #6
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Failure of an electrical component normally results in a few things. Almost always, that component let's the smoke out and is irreparable. If you're lucky, that's it. If you're unlucky the process of failing shorts nearby components due to voltage now having an escape path from the component. The failure could also produce enough heat to melt or ignite the component and nearby devices.

On a good day - breaker trips and you replace a switch. On a bad day, the panel and possibly your house are destroyed by a spreading electrical fire. Not a good outcome - use switches that you know are rated for sustained use at the loads you're using. Not worth saving a few bucks now for the risk down the road.
-Kevin

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Old 10-27-2012, 02:19 PM   #7
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You can keep the setup you have, just add a two pole contactor between the switch and the element. I've bought several from a ebay seller recently for $10 with free shipping. Here is the item #300800311571 its rated 25a for compressors and 35a for resistive loads (heating elements).

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Old 10-29-2012, 03:04 PM   #8
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Several times in my life I've tried to put the magic black smoke back into the box. Never fixes the problem. I suppose there's no sense in being reluctant to increase the cost 1-2% to bump up safety significantly.

Thanks grandqueso, after doing some research on contactors it looks like this is a great way to handle it rather than paying $40 for a switch.

Couple extra items to charge to the game and I'll hopefully be back to brewing this weekend.

Thanks for everyone's input!

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