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[SOLVED] What size AC breaker, 7A or 10A?

cdevidal

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I want to push no more than 6.9A continuous through one of my circuits; Do I need an exactly-sized 7A breaker, or a slightly oversized 10A breaker to accommodate some startup surge current? I've got a Growatt SPF 5000 ES (outputs 220V euro) and a step-down transformer. The transformer's continuous rating is 1500W. If the transformer is 95% efficient, and the inverter outputs at 230V, that's 1,500W / 0.95 = 1,579W in / 230V = 6.9A. (Did I do that right?)

The load is a 34' class A RV minus the following items: Refrigerator (runs on propane), water heater (propane), 12V anything including lights (separate solar system), air con (heat pump on separate 220V circuit), furnace (using the heat pump), laptops (on the 12V system). No coffee pot.

So the load includes TVs, video games, DVD players, microwave, internet modem, router, cordless phone, medium-sized blender, small kitchen appliances such as InstaPot or dehydrator, small electronics/shaver/things like that. SOMETIMES the 55A 12V converter (about 850W with inefficiency) will be turned on if the 12V batteries run flat but usually it will be disconnected with a switch and the batteries charged from a separate solar charger.

If I go with the 7A will it trip any time an inductive load starts? I think no; It's slow-blow with inertia delay. Time-delay curve shows that a 2x surge causes a break in 10 seconds. That should cover most surges such as from the microwave.

If I go with the 10A will a large unexpected continuous load overheat the transformer? I think yes, since I cannot constantly police the family's energy usage. They might plug in a small electric space heater.

What do you think?
 
At least for the thermal/magnetic breakers we use in the U.S., they should be sized 1.25x max continuous current draw.
Some other types such as magnetic/hydraulic may be recommended for continuous use at 100% of rating.

Over-current protection is generally to protect wires from overheating. They can also trip before stalled motors overheat too badly.

If your concern is overheating a transformer, then a thermostat might be good protection. You can get them in a range of temperatures, and automatic or manual reset. I've seen them with 10A ratings, some 20A.
 
I want to push no more than 6.9A continuous through one of my circuits;
Technically the only way to accomplish that is with a currently limiting device like a buck or boost converter. The load typically determines how much current to pull. A circuit breaker is a protection device not a device that can control control current other than break the connection.
 
Technically the only way to accomplish that is with a currently limiting device like a buck or boost converter. The load typically determines how much current to pull. A circuit breaker is a protection device not a device that can control control current other than break the connection.
And breaking the connection is what I want. Want the family to learn not to push it.
 
At least for the thermal/magnetic breakers we use in the U.S., they should be sized 1.25x max continuous current draw.
Some other types such as magnetic/hydraulic may be recommended for continuous use at 100% of rating.

Over-current protection is generally to protect wires from overheating. They can also trip before stalled motors overheat too badly.

If your concern is overheating a transformer, then a thermostat might be good protection. You can get them in a range of temperatures, and automatic or manual reset. I've seen them with 10A ratings, some 20A.
I don’t know for 100% certain that the problem is overheating, I only know the manufacturer says don’t exceed 1500W. But a thermostat is a great idea; Already reading the air temperature and controlling a fan with an Arduino; Adding a second thermometer and relay in the mix in addition to a breaker would be cheap easy insurance.
 
Good news. I misread the manufacturer's instructions; They want the max to be 50% higher than continuous. Since it's a 3,000W max transformer, 3,000 / 150 x 100 = 2,000W continuous. So I have some overhead. If I use the 10A breaker with a 1.25x thermal overhead that's 8A continuous from the inverter. 8A x 0.95 for inefficiency x 230V = 1,748W to the 110V outlet, which comes under the 2,000W continuous rating. If the family goofs and plugs in something heavy, ? pop! goes the breaker ;) For additional protection I'll monitor the transformer temp and cut it with a relay.
 
For additional protection I'll monitor the transformer temp and cut it with a relay.
I think you may have a problem finding a circuit breaker with exactly that value. Besides constant tripping of a circuit breaker may shorten the life of the breaker. As long as you are considering a relay, have you considered a relay controlled by an adjustable load sensor? There are several load sensing devices that have auxiliary contacts which can control a relay.
 
Rather than using a microprocessor for safety function, you can have an electromechanical thermostat. Clothes dryers etc. use them.


However, these are probably rated for resistive loads; contacts might not last so long on other loads.
 
I think you may have a problem finding a circuit breaker with exactly that value. Besides constant tripping of a circuit breaker may shorten the life of the breaker.
I not only found a breaker with exactly that value, I linked to it in the opening post. Don't anticipate constant tripping; Once the family understands they can't just plug in anything they should figure it out. The total current allowed would be about the same as a standard wall outlet in a house.

As long as you are considering a relay, have you considered a relay controlled by an adjustable load sensor? There are several load sensing devices that have auxiliary contacts which can control a relay.
I like the idea, do you have a link to one such?
 
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Rather than using a microprocessor for safety function, you can have an electromechanical thermostat. Clothes dryers etc. use them.


However, these are probably rated for resistive loads; contacts might not last so long on other loads.
I hate reinventing the wheel particularly for as you said, safety. But in this case it's not for safety, it's to prevent equipment failure, which would not be catastrophic in terms of fire. I'm using normally-open relays so it fails safe, and I already have the microcontroller in place for other purposes. I suppose there's a rare risk of both a software glitch that holds a relay closed AND an overcurrent situation, but the breaker handles that, plus the transformer also has its own breaker and a fuse. Mainly, I'm trying to avoid popping fuses here. My loads are sometimes inductive so yeah, I am concerned about the contacts on that. Might be a good belt-and-suspenders option though, I'll think about it. I am unsure about the temperature setting of the one you linked, since it's not adjustable. Will measure the running temp of the transformer at a comfortably full load is, and see what the temp rises to.
 
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I hate reinventing the wheel particularly for as you said, safety. But in this case it's not for safety, it's to prevent equipment failure, which would not be catastrophic in terms of fire. I'm using normally-open relays so it fails safe, and I already have the microcontroller in place for other purposes. I suppose there's a rare risk of both a software glitch that holds a relay closed AND an overcurrent situation, but the breaker handles that, plus the transformer also has its own breaker and a fuse. Mainly, I'm trying to avoid popping fuses here. My loads are sometimes inductive so yeah, I am concerned about the contacts on that. Might be a good belt-and-suspenders option though, I'll think about it. I am unsure about the temperature setting of the one you linked, since it's not adjustable. Will measure the running temp of the transformer at a comfortably full load is, and see what the temp rises to.
Can you program your BMS? I have a JBD and can set the over current protection value manually. I would rather have the FETS do most of the work over a relay, they cause fewer issues when "hot" switching loads.
 
Can you program your BMS? I have a JBD and can set the over current protection value manually. I would rather have the FETS do most of the work over a relay, they cause fewer issues when "hot" switching loads.
Not really. The BMS does do overcurrent protection but I've got two step-down transformers on two separate branch circuits and the BMS wouldn't know when only one transformer is overloaded. Also, sometimes I'll be on grid power, not battery.
 
Google is you friend. I just bought this one but have not tested the relay function.
Charge-Discharge Meter, DROK 0-90V 100A DC Voltage, Amperage, Power, Battery Capacity Amp-Hour Watt-Hour Time Multimeter, LCD Digital Volt Current AH https://www.amazon.com/dp/B08JB5NQ4B/ref=cm_sw_r_apanp_hRfBtBdzrQ1cQ
For years I used a JLD 404 which you can find with a Google search.
I have one of those too. I bought it because I thought the display looked really slick, and it most definitely does.

The only problem with mine is that it's incredibly inaccurate at lower amperages and is consistently about 5-8% off at higher ones, but that can be corrected for in my head. Under 1 amp and it would be stretching it to call it an educated guess.

I tried to use it on a backup battery that's not used very often, and it wouldn't register any sort of parasitic draw, so when I started my system up it would say 100%, when that definitely wasn't the case, it also wouldn't register my 1 watt LED night lights. Now I use it on my son's lithium Power Wheels to give him a cool display gauge, it works fine for that.

I tried to contact customer support for calibration instructions (there's a hidden setting mode somewhere on the unit), but they didn't seem to have any idea what I was talking about.
 
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