Q: Limiting current draw on a 1000W 120v heating element

[Jack Driscoll]

I've got a 1000W, 120V heating element I'm trying to run off a 1000W
inverter. When I plug it in, the inverter starts beeping and shuts
down, so I think that the inrush current is too high for it. I'm OK
with having less heat output from the element, but I can't replace it
easily. I was thinking I could add some resistance in series with the
heating element to limit the amount of current it's drawing, something
like a light bulb or a length of wire or another heating element. I
also found out during reading about this that since it's a PTC heating
element, it is likely starting out with a large current that will
decrease as it heats up, so I think I might be able to get by running
it on the inverter if I can just limit the inrush current when it's
warming up.

I'm a little cloudy on the math/equations I should use for this and I
don't have much practical experience, so if anyone has any suggestions
or can point me in the direction of what I might need to figure this
out it would be much appreciated.

Thanks,
Jack

[Tom Gralewicz]

The first thing I would do is put a ohm meter on the heater when its cold.  That will tell you what the initial current is going to be:    Volts (120) = Amps * Ohms.   -->  Volts (120) / Ohms = Amps

Possible solutions:

Buy a bigger inverter - running an inverter at its maximum rated output is likely to cause it to fail - at the very least it will run hot (look Ma, 2 heaters for the price of one!)

Manual solutions:  You can try a light dimmer - just make sure its rated for over 1000 watts.  You can turn it down before you turn on the heater and slowly bring it up.

Add a very large resistor in series with or without a bypass switch.  As close as you are to the capacity of the inverter this is still likely to shut it down if the heater doesn't heat up enough or you switch too soon.  And make sure your resistor can handle the power - it could be as large as 500W!  (look Ma, 2 heaters for the price of 4)

Automatic solutions:  There is a device called an "inrush limited" it is designed to have a positive thermal coefficient.  It has a high resistance then goes lower as it heats up.  I don't know if you can get one that will handle the power you are trying to put in.  Also, it may not hold off long enough to warm up the heater before it lets too much current through.

I think the best solution is to figure out the maximum power your heater draws cold then get an inverter at least 50% larger than that.

Tom

--
Tom Gralewicz
Miller Electronics Recycling
(414) 380-1716
www.deadcomputers.com

[Ross Oldenburg]

PTC means "Positive Thermal Coefficient", meaning that as the temperature of the material increases, so does its electrical (and thermal) resistance.  So when the element is cold, you can assume it will have a very low resistance.  The inrush current (in terms of watts) is probably actually way higher than 1kW.  This will only last for a few cycles, and then resistance will climb linearly as temperature does.  So, Tom has a very good idea.  Measure the resistance when the heater is cold, calculate how much current this will pull, and then buy a new inverter that can handle at least 50% more than that.  If you have a multimeter that can handle that much current and can measure Max-Min RMS values (since this is AC), you could actually see how much the inrush current is.

--Ross

[Ross Oldenburg]

Working in data centers I was always told that at the absolute maximum, a load on your power supply should be 80% of the supply's rating.  if that helps.

[Ben Nelson]

I'd also take a look at the bigger context.

If you are running a heating element from an inverter, WHY are you
doing it?

Are you trying to heat a room with an electric heater during a
blackout and running it on batteries through an inverter?

Could you run the element off DC power instead of AC? Could you use a
500 watt element instead of 1000? Why type of heating element is it?
Water? Air? Infrared bulb?
Could you find a 1500 or 2000 watt inverter. Could you just run it
from wall power?

Knowing a bit more about exactly what you want to do might help other
people think a bit outside the box.

-Ben Nelson

[Jack Driscoll]

On Dec 20, 4:19 pm, Ben Nelson <b...@hdvideoguy.com> wrote:

> If you are running a heating element from an inverter, WHY are you
> doing it?

It's the block heater for my ambulance and I'm pretty sure I don't
need 1000W most days. I haven't seen a DC replacement for the type I
have, but I have seen stick-on pads that run on DC. In the future,
I'm planning on switching to LP or diesel block heater. The reason I
want to run it off an inverter is that: A. The outlet box on the
outside of the storage unit where I park got turned off recently,
although I'm not sure yet if it was because I was using it to warm up
the ambulance and B. I'm otherwise tied to the AC if I want to use the
truck in winter: no outlet, no turning off truck.

I'm not planning on doing this frequently, but I would like to have it
as an emergency backup. I have 4 Group 27 batteries in two banks.
Based on my calculations, I should be able to pull 1000W out of one
set and still be >50% charge, allowing me to start on the other set.
For now, I think I'm going to go with the dimmer idea and monitor the
power draw and batteries to make sure I'm not pulling too much.
Thanks for the tips so far.
-Jack

[Mrspeed700]

Jack, i picked up lp tankfor your furness. It is behind my truck. 40$
Rich