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Old 02-02-2009, 04:49 PM   #1
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automatic shutoff rlay


Hello,

I am installing a tankless water heater in my home. It requires a 125 amp breaker.

I have 200 amp service. The 3 largest draws on the panel are the water heater(125A), hot tub(50A) and electric heat(75A). Obviously, if all 3 of these are on at once, I have an issue.

I have a wood stove and do not use the electric heat. The breakers are in the off position all the time. However, because they are there, they must be taken into account in doing the calculations. Just because I don't use it doesn't mean a future owner will want to take out the stove and utilize the electric baseboards.

I have been told that I should consider bumping up to a 400A service. Big bucks, and I want to avoid that if possible.

I am thinking, if I move the 4 electric heat breakers (20,20,20,15A) from the main panel into a sub panel, it would be considered as one 75A draw instead of 4 separate ones.

I am thinking if I could put a relay in between the tankless and heat subpanel that would prevent both from operating at the same time I might solve the issue of possibly tripping the main breaker. I was wondering if there is a relay in existence that works as follows:

When the water heater comes on, the current is sensed, and the power to the heat subpanel will be shut off. Then when the tankless heater shuts off and there is no longer any power to that device, the power will be restored to the electric heat. This way, there is no possibility of a 125A draw for the tankless and a 75A draw for the heat happening simultaneously. This will ensure the main breaker doesn't trip.

Also, if a future owner does utilize the heat and it is drawing current, again, if they have a shower, the heat will turn off for the time they are in, and then come back on when they stop using hot water.

Does anyone know if such a relay exists, and where to get it? I guess it would be a normally closed relay that would be opened by the current running to the tankless.

Any help, or other possible solutions to consider would be appreciated.

Many thanks in advance, Max

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Old 02-02-2009, 05:45 PM   #2
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automatic shutoff rlay


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Originally Posted by madmax55555 View Post
I have 200 amp service. The 3 largest draws on the panel are the water heater(125A), hot tub(50A) and electric heat(75A). Obviously, if all 3 of these are on at once, I have an issue.

I have been told that I should consider bumping up to a 400A service. Big bucks, and I want to avoid that if possible.

>>Yes, if your average current draw does not equal your peak current draw.

I am thinking, if I move the 4 electric heat breakers (20,20,20,15A) from the main panel into a sub panel, it would be considered as one 75A draw instead of 4 separate ones.

I am thinking if I could put a relay in between the tankless and heat subpanel that would prevent both from operating at the same time I might solve the issue of possibly tripping the main breaker. I was wondering if there is a relay in existence that works as follows:

When the water heater comes on, the current is sensed, and the power to the heat subpanel will be shut off.

>A current sense xformer driving a relay possibly through an amplifier, or
a current sense relay, like they used to have in the electromechanical voltage regulators for cars. I couldn't find one of these. It would drop maybe one or two volts across the coil.

I recommend an indicating LED for when the relay pulls in.
You already know where to find the relay/contactor; it's the same as the one inside the tankless heater.

Even better, get a schematic for the heater and we can interface the current sense thingy directly to the heater control, saving the price of an external contactor and staying with low voltage low power circuitry.

Tell the tank manuf. what you want to do; there may already be an internal logic pin that will disable the heater, possibly for this very purpose.

Anyway,
if all the parts you find for this won't play together, a homebrew interface circuit is easy to make. Look on commercial equipment websites. Keywords would be contactor, "current sense", relay, 240vac, logic.

You know that heater will click a lot? And in three weeks I couldn't master taking a shower without getting some cold water. Maybe your heater has a controller with Artificial Intelligence that can correctly anticipate how to keep showers warm.


Last edited by Yoyizit; 02-02-2009 at 06:17 PM.
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Old 02-02-2009, 06:33 PM   #3
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automatic shutoff rlay


I think, depending on your panel, you may have difficulty finding a 125 amp breaker. You may have to go with 2-60's 0r 4-30's, with the latter I've seen the most.

Anyway, I would put the current sensor on the SE conductors and set it for example at 180 amps since this is what you're trying not overload. In some cases it may be possible to have these things running at the same time. (heat and tankless) since the heaters tstats may be satisfied and not actually on

Then just put a contactor in between as you said however, I would have it normally closed so the coil won't have to be energized constantly. If the sensor senses a potential overload it will energize to open the contacts. Also, if the coil burns out, you won't be dead in the water until the coil is replaced. However, this will temporarily put you back where you started as a potential main breaker overload but you'll be cognizant of it and can take precautions untill a repair is made.
Here's some sensing relays
http://web2.automationdirect.com/adc...rated_Switches
Here's some contactors.
http://ecatalog.squared.com/pubs/Mot...8910CT9301.pdf

Last edited by wirenut1110; 02-02-2009 at 06:41 PM. Reason: more info
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Old 02-02-2009, 09:24 PM   #4
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But first run all the prices for all the configurations, parts and your labor, and see how close it comes to the 400A upgrade price.

If there's a 1/3 chance you'll someday need the upgrade, then you should spend less than 1/3 of the upgrade cost for this project.

Most people move within 10 years (in normal economies).

Last edited by Yoyizit; 02-02-2009 at 09:26 PM.
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Old 02-02-2009, 11:44 PM   #5
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Relays such as this are very commonly used to cut out load banks on emergency generators that power pump stations or wells. If properly sized, they'll do exactly what you want.

Just a thought here, the breaker ratings are not the actual amp draw of the load. For example, a hot-tub is on a 50 amp breaker, but usually draws around 35 amps or so. The electric heat will be similar.

As a wild guess, the hot-tub and electric heat will likely be OK on a 100 amp sub-panel. If so, you could easily install a 100 amp 2 pole relay in series with the sub-panel feed, and power it through the current-sensing relay.

Is your electric heat baseboard type? If so, look for a watt rating somewhere on each unit. Add the watts together and divide by 240. The result is amps. If you can't find any rating on the heaters, figure 1.1 amps per foot. The hot-tub should have some sort of nameplate on it that lists actual load.

Rob
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Old 02-03-2009, 03:09 AM   #6
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automatic shutoff rlay


First, thank you everyone for the replies. I will try to address everyone here...

first of all, a link to the product manual

http://www.boschhotwater.com/Portals...25_English.pdf

I have the bigger unit AE125. As you can see, maximum output 26.85 kw. Using the formula, max draw is 112A. BTW I live in Canada, so the wiring is different than US.

to micromind and yoyizit:

Yes, I realize the breaker ratings are not the actual draw. For instance, the plate on my tub says it is 40A whereas it has a 50A breaker. As I said, I think I am actually OK as I stand, I am just trying to cover the bases in case we do sell, however we have discussed this and pretty much plan to stay here awhile (we bought in 1997 so it is already 10 yrs).

Putting both the tub and heat on a 100A subpanel will be excessive I think if the tankless can pull 112 on its own.

Price quoted for 400A upgrade 2-4k, so if I can do it for a third of that I will be happy, hopefully even less than that!!

to wirenut,

I have already found and purchased a 125A breaker. I must have this as you can see in the installation manual I have linked, so several smaller ones not an option.

What does the SE stand for ( SE conductors )?

Also 180 might be a little high, the electrician said that although the panel is 200A, they trip at less than that, I think he said around 180A.

Thanks for the links, I think we are on the right track here....
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Old 02-03-2009, 01:46 PM   #7
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Until you find it somewhere on the Web. . .

If you're going to run 24vac relay current over some distance, the very approx. ampacity for twisted pair, free air, 90℃ insul. temp. is 4A for #24 and 8A for #22.

Voltage drop and resistance to physical damage is another thing. A relay coil can probably stand 10% below rated voltage.

Last edited by Yoyizit; 02-03-2009 at 02:01 PM.
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Old 02-03-2009, 02:20 PM   #8
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Micromind,

you say :

Relays such as this are very commonly used to cut out load banks on emergency generators that power pump stations or wells. If properly sized, they'll do exactly what you want.


Do you know a source for these? Or at very least, what is the proper name for an animal like this so I can search for them on my own?

Yoyizit,

last reply a little too technical for me, haven't had my morning coffee yet, not sure what you are trying to say ?
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Old 02-03-2009, 04:57 PM   #9
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Quote:
Originally Posted by madmax55555 View Post
Micromind,

you say :

Relays such as this are very commonly used to cut out load banks on emergency generators that power pump stations or wells. If properly sized, they'll do exactly what you want.


Do you know a source for these? Or at very least, what is the proper name for an animal like this so I can search for them on my own?

Yoyizit,

last reply a little too technical for me, haven't had my morning coffee yet, not sure what you are trying to say ?
Wire needs to be sized for the current it will carry without overheating its insulation. 90℃ seems to be a safe value for how hot residential wire should get. Wood ignites at 190℃ so you've got some headroom here.
Wire sizes smaller than #14 are rarely used to transmit power, but my posted numbers may help you size the wire to control your contactors.

The resistance of wire causes a voltage drop over distance in wires, and this also needs to be taken into account.

Lastly, wires carrying appreciable power must resist physical damage, though probably no serious consequences would result if your relay control wires were damaged.


Last edited by Yoyizit; 02-03-2009 at 05:00 PM.
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