ok, I’ll do that!
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So in the garage, on 110 v, I have 2 enders, my pc and an electric heater on 20 amps and never trip a breaker. Maybe i should put the kill o watt on and see what they are running!
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That brings up a really interesting topic (to me, anyway).
Summary: 20A breakers don’t trip until well above 20A and are only there to protect the cabling so care is needed if you use ‘it didn’t trip’ as an evidence of what’s going on!
Breaker ratings are something that confuse a lot of people. The 20A isn’t actually that it will trip at 20A, it’s that it’s sized to match the thermal performance of a cable that is specified to be used long-term at 20A. So it’s guaranteed not to trip at 20A and usually won’t reliably trip until somewhere between 1.2x and 1.5x overload for an hour or more. It’s also a thermal average so this is how you end up with situations like a welder that might draw 40A when actively welding at max power but may have a 20A plug/socket and specify a 20A circuit. It’ll also be specified for something like a maximum of 5 minutes at full power out of every 20 minutes, or something like that.
The other thing is that it’s temperature dependent, to a degree, so if you’re in a cold scenario and running a heater, assuming the circuit breaker is somewhere cold then you’ve actually got a higher rated circuit than when it’s hot. In extreme cases this can cause issues where the circuit protection is in a dramatically different environmental condition to the wiring, such as a hot overloaded breaker cabinet causing unexpected false tripping while wiring in a heated area may overload if the protection is out in a cold external cabinet etc. For that reason I have the 32A protection for both of my EV charging circuits separated with a space on either side so they can adequately cool to avoid false tripping while the usual recommendation is to use 40A breakers to give some headroom, given that a 32A EV charging circuit might draw that for 10 hours or more.
So if you’ve got a steady load like a heater and it’s not tripping a 20A circuit after being used for 2-3 hours, all you can conclude is that it’s drawing less than 24A on that circuit, most likely.
Something like the PC and 3D printers will have dramatically varying current draw while operating as processing loads change, heat loads etc. My gaming PC idles at ~150W but draws around 400W under gaming loads and likely more if I were running a pure stress test. The 3D printer will have a ~20-40W heater on the nozzle and 50-200W into the bed depending on power, but in reality it’ll always be turning those on and off to maintain a set temperature so would likely be half that once the printer is actually up to temperature and running.
This is worth knowing because the only thing that the breaker is actually protecting is the cabling in your walls. It is assuming that whatever is protecting will heat up and cool down like a cable would, which means it’s relatively simple I^2*R power loss and quite a lot of thermal mass. If you then assume that a 20A breaker would protect a 20A plug/socket, for instance, that may not be true anymore because the plug/socket may heat up significantly faster than the cable or have a lower failure temperature. 40A for an hour will be fine for a 20A cable but will almost certainly smoke a 20A plug/socket. This is also true for other stuff like relays (especially solid state relays), contactors, dimmers etc. So a 20A MCB that doesn’t trip isn’t an indication that you could control that circuit with a 20A rated relay/contactor, necessarily.
As a practical example, if you said that each 3D printer is averaging 2A while printing but draws 4A while pre-heating, you could absolutely put 10 printers on a 20A MCB and be fine. Even if you turn them all on at once, they’ll draw 40A for a few minutes or so and then reduce down to 2A, which is well within the thermal trip curve of the breaker. That 40A for a few minutes, however, may well be enough to permanently damage a 20A plug, socket or relay.
Typically fittings like plugs and sockets are protected by needing to be matched to the nameplate ratings of the equipment they’re connected to in order to prevent overload. In the case of the 10 printers that could draw 4A on startup, chances are the nameplate rating says 5-6A or something. In that case, you’d take that 20A socket or relay and hook up 3-4 printers with it and have plenty of safety margin.
Returning to the welder example above, newer welders and EV charging cables actually use temperature sensing in the plugs to avoid overload in this kind of situation. Older ones just put the whole ‘5 minutes out of every 20 minutes’ in the manual and either it didn’t matter for the plug because they’d overheat and break if you tried to go for 20 minutes flat out, or they’d just set the plug on fire which is a hilariously common scenario.
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They explicitly say not to attach the dimmers to fans. I think it is the power factor of universal motors that is the problem.
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oh trust me that breaker is a very cool cucumber right now! only a piece of plywood seperating it from very cold temps
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