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Situation:

My apartment has a main 50A circuit breaker, and several small appliances with the normal total current never exceeding 40A. There are also 3 240V appliances with higher nominal power: one is 3350W, and two are 5500W. Each is connected to its own circuit breaker.

In normal everyday use these 3 are very rarely used on their highest Wattage, and hardly ever I have more than 2 powered on at any given time. So normally the circuit operates easily under 40A.

If all three are powered on their high-ends, however, that would generate a ~59A current and activate the main circuit breaker, shutting off all power.

What I want:

The main circuit breaker is 4 floors below the apartment and it is managed by the building and the power company so I just can't replace it. But there's no damage that can occur from shutting down any of those 3 appliances, particularly the 3350W one.

I want to establish a first line of defense against total shutdown by first disconnecting only the 3350W appliance if the total current surpasses 40A. If the appliance is already off, or if the current remains over 50A after it is disconnected, then the main breaker would be activated shutting down the entire circuit.

Is it possible to design the circuit to make this happen? how or why?

Edit with answers to the comments:

  • This is a biphasic installation and I have both 120V and 240V.
  • I do have a distribution panel inside the apartment, feeding from the 50A master. This panel has 16 slots I can fill with single or double breakers.
  • I don't have a master breaker in the panel. I thought of adding one but didn't see much value in limiting the overall capacity below 50A until I started thinking of this. It is possible to add one, though.
  • The appliances are one Oven (1000W to 3350W) and two Showers (500W to 5500W each). The reason I wanted to cut the oven is actually practical: you don't want to risk your shower breaker tripping and having to finish in cold water.
  • These are the only appliances on 240V lines, by the way and each has their own double-breakers. 120V holds the refrigerator, freezer, light bulbs and all other outlets and they are split in shared breakers by room iirc.
  • I know I could just set the house rules and instruct everyone to not use everything at once - I was just wondering if there was a way I could be a little more foolproof.
  • The first question is how you're able to modify apartment wiring and overload protection. Do you have free reign to make changes? What limitations apply? – isherwood Dec 7 '16 at 17:33
  • I have free reign over the wiring inside the apartment, the distribution panel is actually not entirely wired yet (we're renovating), but nothing outside of my walls. – moraleida Dec 7 '16 at 17:37
  • Where on the planet are you? (country) – Tyson Dec 7 '16 at 17:43
  • @Tyson I'm in Brazil – moraleida Dec 7 '16 at 17:44
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    There are no devices known to me to automatically shut off power to only one circuit if the total current demand of your entire circuits exceeds a certain value, in this case 40 A . Unless you find that you are actually tripping your 50-A breaker in the basement don't worry about it. You may never ever blow the 50-A breaker and so any effort designed to deal with this would be a waste. In my house the main breaker for the 120-V, 15 A and 20 A branch circuits is a 60-A breaker. If all my branch circuits were drawing 80 % of their limit, that would total 180 A! It has never tripped in 40 years. – Jim Stewart Dec 7 '16 at 19:27
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You can achieve this with 'home automation'.

What you're trying to do is called 'load shedding'. It's not a common task in a residential setting, but it can be accomplished with home automation technologies.

There are z-wave devices to measure current with an amp clamp, and to control appliances in a split-phase (biphase) set.

You would get one amp-clamp and instal it in the panel, measuring current accross your feeder wires.

Then you install one of those relays at each appliance that you want to be able to shed.

You also will need to purchase or build a controller box that talks to both the amp clamp and the relays. MiCasaVerde is one choice I've worked with personally and I bet it could do what you want.

Then the fun part. You get to write a program that runs on the head-end to constantly measure amperage draw and controlls the relays accordingly.

Be careful that you don't create some loop that rapidly toggles the relays though. At high amperages, I could see that being a nuisance to adjacent units (power blinks) or worse case you could overheat and set one of the relays on fire from the arcs that switching causes.

Good Luck! And remember to share your code and parts list here once you've got this working.

  • This would make for a very interesting side project, but it is a little too much work and I can't try it right now :( Now, the one question that is bumping me here is this: the controller box uses wifi to talk to the relay/clamp, right? Even if it was a wired LAN connection, wouldn't that data travel be too slow for the kind of action needed to shut off an appliance before a proper breaker steps in? – moraleida Dec 13 '16 at 4:17
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    the controller box does NOT use wifi or wired ethernet LAN to talk to the relay or clamp. Z-Wave is its own wireless protocol seperate from wifi. Zwave will introduce at most a second of latency on the sensing and on the relay side. But this should be OK depending on how your code works. Circuit breakers are FAR from instantaneous for minor overloading like you're concerned about. As far as current overloads go, they only work instantly for dead shorts. a mere 200-300% overload will take several seconds to cause a trip because a bimetallic strip in the breaker has to heat up. – Billy C. Dec 13 '16 at 4:47
  • Here's a chart for reference. i214.photobucket.com/albums/cc172/480sparky/… Every make and model of breaker might have a different curve, but they all follow the same general idea. You could pull TWICE the rated current for MANY seconds (30-ish), or FOUR TIMES the allowed current for 6 seconds. That's plenty long enough for the zwave system to detect it and shut off the heavy loads. To react fastest you probably want current sensors on the main in, as well as each appliance circuit, so your code doesn't waste time shutting off appliances not in use. – Billy C. Dec 13 '16 at 4:56
  • You could also use zwave to sound audible alarms, or punish the offenders by shutting off or blinking their lights. – Billy C. Dec 13 '16 at 4:57
  • ah, thanks for the graph! I didn't know it would take so long for the breaker to trip on a not-so-overloaded setting: this makes a lot more sense now. – moraleida Dec 13 '16 at 11:37

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