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Old 03-12-2013, 07:23 PM   #1
RichBenn
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Default Converting from Dual 120V to 240V

After designing a dual circuit 120V system and building most of the panel, I am thinking of converting to 240V before I finish. I was just getting ready to drill the holes (two in each kettle) for the elements. I have two separate 120V, 20A circuits in close proximity, which is why I thought to go this way.

But now a 240V 30A 4 prong circuit is available where I can brew. I'll probably add a contactor, as my DPST switch, although rated for 20A, would seem a bit marginal for regular use, depending upon how big the elements and fused protection (Opinions?)

I now have two separate PIDs mounted to a heat sink, one driving two 25 Amp SSRs for the boil kettle and the other driving two 25 Amp SSRs for the HLT. The PIDs outputs are wired through a switch so that the boil kettle and the HLT cannot be driven simultaneously.

So other than the switch vs. contactor question above, :
1. In converting to 240V, should I just wire the SSRs in parallel? Or put on on each leg of the 240V?
2. I'm only doing 5.5 gallon batches. What size element would you recommend? 3500 watts?
3. Other than wire sizes, cords and connectors(arrgh), what other things should I be on the lookout for?
4. Anybody need some unused 120V heating elements and connectors? (LOL - I'll post in the classifieds if I do this)

-Rich



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Last edited by RichBenn; 03-12-2013 at 07:40 PM. Reason: The DPST switch amperage was wrong
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Old 03-13-2013, 04:18 AM   #2
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But now a 240V 30A 4 prong circuit is available where I can brew. I'll probably add a contactor, as my DPST switch, although rated for 20A, would seem a bit marginal for regular use, depending upon how big the elements and fused protection (Opinions?)
If it's rated for 20A, it's rated for 20A. You should be fine using it all the way up to 20A with no issues. In reality, the switch can probably handle more than 20A, but they engineer in some safety margin and tell you to use it at 20A or less.

With 20A, you can handle a 4500W element.

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So other than the switch vs. contactor question above, :
1. In converting to 240V, should I just wire the SSRs in parallel? Or put on on each leg of the 240V?
You can put an SSR on each leg of the 240v if you want, but it isn't necessary. Personally, I would just eliminate 2 of the SSRs in your original design. Run one of the 120V hot legs directly to the element receptacles and put an SSR on the other hot leg going to each receptacle.


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2. I'm only doing 5.5 gallon batches. What size element would you recommend? 3500 watts?
3500W will work fine, but you have 30A available and a switch that can handle 4500W, so I'd go with the 4500W if for no reason other than reducing the wait time on heating things up.


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Old 03-13-2013, 01:46 PM   #3
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Putting an SSR on both legs is including an additional failure point with no benefit. Put an SSR on one hot leg, and run the output of the SSR to some switched means (contactor, relay, or manual switch) with the other hot leg. There are countless reasons why running straight from the SSR to the element is a bad / unsafe plan. A means of physically disconnecting both legs to the element is the smart way to do it.

Throw in the biggest element your setup will allow - you can always throttle a 4500w element down - you can't throttle a 3500w element up.

-Kevin

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Old 03-13-2013, 01:58 PM   #4
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Put an SSR on one hot leg, and run the output of the SSR to some switched means (contactor, relay, or manual switch) with the other hot leg. There are countless reasons why running straight from the SSR to the element is a bad / unsafe plan. A means of physically disconnecting both legs to the element is the smart way to do it.
I know what you are saying here, but the exact wording used might mislead some people. It is perfectly fine to have the SSR connected to the outlet or power cord that feeds your element, but you do want a switch in there SOMEWHERE between the main power and the element.

main power --> SSR --> switch --> element is fine
main power --> switch --> SSR --> element is also fine
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Old 03-13-2013, 04:07 PM   #5
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Fair - there's no appreciable difference if you put your switch before or after the SSR, so long as the switch breaks both legs of the 240v feed and is a true mechanical switch, not an SSR. Thanks for clarifying.

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Old 03-13-2013, 06:07 PM   #6
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Quote:
Originally Posted by Walker View Post
I know what you are saying here, but the exact wording used might mislead some people. It is perfectly fine to have the SSR connected to the outlet or power cord that feeds your element, but you do want a switch in there SOMEWHERE between the main power and the element.

main power --> SSR --> switch --> element is fine
main power --> switch --> SSR --> element is also fine
In my design, I have a switch on the control side of the SSRs. This is a 3 position switch(HLT/OFF/BOIL). There is also the 20 amp DPST switch that controls the overall power. There are two issues I see with this:

1. Since the circuit breaker is at 30 amps, I'll need to protect the switch (DPST) with 20 amp fuses to protect in the event of a short.
2. If an SSR fails in the "on" state, then it's possible that an element could still be on and I don't know it. (like I switch from HLT to boil PIDs, and the HLT is still on)

It's this second issue that made me wonder if that's why people used contactors in lower wattage applications. But it's not like I'd have my hands in the kettles or HLT without the main power off, and a ground short or both elements on at once would cause the GFCI to fire and/or fuses to blow anyway, so this is probably over obsessing the safety scenarios.

And thanks, Walker, for the recommendation to go to 4500 watts. I'd already gotten to that conclusion by time I'd read your response, but it's nice to have confirmation.
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Old 03-13-2013, 06:30 PM   #7
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Some suggestions:
* Check all your switches and indicators. You will probably want 240v indicators for power input, power to elements, etc.
* Make sure your panel can only power one element at a time. You could use a 3-way switch rated for 30amps or a low power switch and contactor for each element.
* Use an input power cord rated for 30 amps since your ckt breaker is 30 amps. I would recommend 4-wire 240v so you have a neutral for 120v pumps, etc.
* I rely on the breaker to protect the elements and 240v wiring and a small fuse for PID, pumps, and other low power control wiring. All my 240v wiring is rated for 30 amps.

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Old 03-13-2013, 06:54 PM   #8
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Originally Posted by RichBenn View Post
In my design, I have a switch on the control side of the SSRs.

...

2. If an SSR fails in the "on" state, then it's possible that an element could still be on and I don't know it. (like I switch from HLT to boil PIDs, and the HLT is still on)

It's this second issue that made me wonder if that's why people used contactors in lower wattage applications. But it's not like I'd have my hands in the kettles or HLT without the main power off, and a ground short or both elements on at once would cause the GFCI to fire and/or fuses to blow anyway, so this is probably over obsessing the safety scenarios.
Having the switch that controls the low-voltage input to the SSR is great, but I'd still recommend having the contactor in the main power feed to the SSR or from the SSR to the element. Curret that leaks by a functional SSR, or that pours past a broken SSR, can be dangerous to you as the operator, and may cause your element to burn out (dry fire) in some situations. SSR failures of both types are a known occurance, and the peace of mind that comes from knowing that your element is well and truly off is priceless.

Plus, if you put the contactor in the main feed, you can drop the one on the SSR control line as it won't matter.

Of course - just my opinion. You can build it however suits your needs and desires.

-Kevin
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Old 03-13-2013, 11:58 PM   #9
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Having the switch that controls the low-voltage input to the SSR is great, but I'd still recommend having the contactor in the main power feed to the SSR or from the SSR to the element. Curret that leaks by a functional SSR, or that pours past a broken SSR, can be dangerous to you as the operator, and may cause your element to burn out (dry fire) in some situations. SSR failures of both types are a known occurance, and the peace of mind that comes from knowing that your element is well and truly off is priceless.

Plus, if you put the contactor in the main feed, you can drop the one on the SSR control line as it won't matter.

Of course - just my opinion. You can build it however suits your needs and desires.

-Kevin
Just to put into perspective, I cannot eliminate the switch in the SSR control line as it is to control whether the HLT or the Boil pot is active. And I have a low water switch also inline to shut off the SSR.

As to SSR failure, I'm curious about this, as relays (contactors) are far more prone to failure than solid state devices(at least that was the data many years ago). The only advantage the contactor has is the normally open (NO) nature, presumably being unlikely to fail closed. Have there been SSR failures reported here? Did the SSR cause a short, or did a short cause the SSR to open?
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Old 03-14-2013, 11:17 AM   #10
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Just to put into perspective, I cannot eliminate the switch in the SSR control line as it is to control whether the HLT or the Boil pot is active. And I have a low water switch also inline to shut off the SSR.
Got it - I read that and didn't compute it in your original post. You're using one PID to run 2 different SSRs, one for the HLT and one for the BK. In that case, your setup is good to go (unless, of course, you find a switch capable of handling the full load on the 240v side and just use 1 SSR to control either outlet)

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Originally Posted by RichBenn View Post
As to SSR failure, I'm curious about this, as relays (contactors) are far more prone to failure than solid state devices(at least that was the data many years ago). The only advantage the contactor has is the normally open (NO) nature, presumably being unlikely to fail closed. Have there been SSR failures reported here? Did the SSR cause a short, or did a short cause the SSR to open?
There are a lot of conversations on the boards about SSR failure. You are correct that contactors tend to fail open and thus are much safer in their failed state. SSRs have been found to fail in any variety of condition, to include a closed state. The other issue is that even a completely functional SSR will still allow some amount of current to leak by, even if it shows that it is 'off'. This may not be enough current to produce much heat from the element, but it could be plenty of current to stop your heart. Conventional wisdom maintains that safety is paramount, and having a means to actually switch power off when you think it's supposed to be off is even better.

In thinking - if you could get a 20-30a (depending on your element wattage), 240v rated 3 way dual pole switch - if they even exist - you could put that downstream of a single SSR. Then use 1 PID and 1 SSR to control either element, and use the 3 way switch to go between the two, or to make sure that both are off. All the birds - one stone.


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