I don't think this is true for all space heaters. In the operating manual for one I have, it says that when operating on the Low setting, the power draw is 800W, compared to 1500W at the High setting. ...
True,
some space heaters will have a low watt setting. But at least IME, it's more common to have a thermostat that just switches the main element on/off.
Since the OP provided such limited info, Occam is suggesting that they are the On/Off type, since the OP did say
"I don't even have the heaters set on High." (and the breakers still trip).
... the Low setting, the power draw is 800W, compared to 1500W at the High setting. So, if OP is using heaters like mine, running two on the Low setting should be doable (although perhaps inadvisable) even on the same circuit.
Well, two 800W draws might be do-able (I may explain this in more detail later), but it is against code and not a good practice at all.
Per code, a 15A circuit is de-rated to 80% for "continuous" loads (more than three hours). A space heater can certainly be on "Low" for 3 hours. Two * 800 = 1600, And 1600W/120V =13.333A. But 15A * .8 de-rating = 12A. You are over the code.
That's why it's common to see devices limited to 1440 watts if they could be considered "continuous". 120V ⋅ 15A ⋅ 0.8 = 1440W.
Since circuit breakers have tolerances and and aren't 'smart' devices, they can't protect against this in a precise way. It's up to the user to assure that only one device is on a circuit and there isn't any other considerable draw on that circuit. That's why NEC code is going more and more towards requiring a separate circuit for various appliances (like microwaves, dishwashers, etc), with no other outlets available to plug into to increase the load.
... A breaker trips by two mechanisms. ...
A couple times in recent years, I did a "deep dive" into fuses and circuit breakers for home projects (one line voltage, the others low voltage). There is a *lot* to consider in fuse/breaker choice, if you are trying to provide protection for a circuit that's on the edge of a limit. It often calls for some 'smarts' (timing circuits and comparators, or a uC) and some sort of actual current measurement of reasonable precision. You can spend a lot of time looking through the charts of current vs time vs ambient temperature vs number of cycles of actuation, vs min/max specs and distributions and on and on and on.
Makes great dinner conversation!
It happens a lot more times than you would think! If rooms/areas share a wall then most likely outlets are on the same circuit! It just costs more to cleanly separate the circuits by room. Especially the older houses have a very few circuits that share the load beyond your wildest imagination. I renovated a 2200 sqft old home which had less than 20 circuits for everything in the house. Now I have 45 circuits after I rewired the house according to the latest code and cleanly separated circuits by room/area. I still ended up "cheating" for a few outlets (less than 5) where I could just tap-in from the area next to the wall.
But in some ways, this can be a good thing. You might be less likely to overload a circuit that is spread out across two rooms. Could go the other way too, but I don't think it's "cheating" at all.
In a recent thread, someone complained that the ceiling lights and the outlets in a room were not the same circuit. He thought if he flipped the breaker and the lights were out, the outlets should be off too (no, you *always* check before you touch them).
But it's good this way. If you overload a circuit, you don't want to be sitting in the dark. Best to keep them separate.
-ERD50
