I thought the 125% rule was for calculating sizes for dedicated load circuits like hot water heater.
Why would a 1800 watt plug in heater be pulling 2250 watt load?
Also, why would a 20 amp circuit trip from a 18.75 amp load after couple hours? Heat buildup in the panel?
Any resistive load should be figured at 125% weather it is the branch circuit, overcurrent or even a plug and cord appliance.
If a 20 amp circuit has anything else in use with an 1800 watt heater you can count on the circuit opening each and every time, especially after an hour or so.
Any 15 amp receptacle weather used on a multi-outlet 20 circuit or not will fail when more than a 12 amp load is being used for an extended amount of time. This is where a lot of electricians make the statement of not using the stab lock on the back of a 15 amp device comes from.
The tension on the blades of the male plug will start to fail due to the heat of the receptacle.
A small test can be done with this type of portable heater that helps anyone to understand what is happening inside the receptacle. Simply plug in a small electric heater and let it run for about an hour. Unplug the heater and hold the male plug in a closed hand to capture the heat of the male plug inside the closed hand.
If is feels hot then the tension of the receptacle is being lost and at some point in time the prongs of the male plug gets spread apart in order to keep from falling out of the receptacle. At this point the receptacle is failing and needs to be replaced.
I have personally seen where the conductors, yes even the ones wrapped around the screw have started to discolor and melt from the use of portable electric heaters. I have seen many portable electric heaters where the male plug was deformed from the heat produced at that connection point.
Really want to see a problem with portable heaters. Use an extension cord so the heater can be placed closer and then the problems really begin especially from those little brown cords that cost $1.19 at the local Family Dollar Store.
When the receptacle starts to fail and arcing starts between the blades of the male plug and the tension of the inside of the receptacle we now have an added resistance that is in parallel with the load.
Also in a parallel circuit the total resistance will always be less than the lowest resistor thereby causing more of a load on the overcurrent than the heater itself is pulling.
Should there be a resistance of 25 ohms at the receptacle in parallel with the heater then the 20 amp overcurrent device is maxed out.