120V through a 250V Heater

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winnie said:
I suspect that the OP is a spammer. New user, pops in, asks a simple question, lets the discussion build without any input, and ends up with a bit of cred that they use later to post spam or disinformation. With that said, they did start an interesting conversation amongst us real folk :)



Once you get up to the scale of multiple heaters (say a house with a bunch of electric baseboards heaters) you end up having to deal with load diversity maths.

A single resistance heater rated at 250V will have lower wattage when run at 230V. But in a house these heaters are being cycled on and off because of thermostats, and the total power being delivered has to average out to the heat load of the building. Same power at lower voltage means _higher_ current.

How do we reconcile that lower voltage into a resistor means lower current, but lower voltage into the house means higher current (for resistance heating)?

For the house to be at the same temperature, the same total BTU needs to be delivered. For an individual heater at lower voltage, the heater draws lower current, operates at lower power, and needs to run longer to deliver the same total BTU.

When you average across a bunch of heaters, each cycling on and off on its own thermostat, in aggregate delivering the same total BTU, the load diversity gets changed, and the aggregate average current must go up. So instantaneous current to an individual heater goes down as voltage goes down, but average aggregate current to the entire house goes up to deliver the same BTU per hour.

-Jon

-Jon
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Hi, it was a genuine question that I asked, thanks everyone for your detailed response.
 
topgone said:
~260W, 259.2W it is!:
Resistance per heater 250V, 375W) = E^2/P = (250^2)/375 = 166.67 ohms
@ 120V, the current per heater will be = 120/166.67 = 0.72 A
With 3 heaters, you have Amps = 2.16 A
Wattage = 2.16 X 120 = 259.2 W!
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thanks, this is exactly what i needed
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Someone else explained that I can go about doing it this way:
P1 = V1^2 / R.
P2 = V2^2 / R
Thus, P1 / P2 = (V1 / V2)^2
= (120 / 250)^2

I think i would need to multiply this ratio by 375 and then by 3 which also gives me the same answer.
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The main concept I was trying to grasp is calculating power of heaters rated at different voltage rating than what is actually being used to power the heater. i think that concept is clear now. I appreciate everyone's responses.
 
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