resistive loads

[GerryB]

I was reading an old thread about electric baseboard heat. How much on a circuit, continuous load, etc. It was if two 8' strips could be on a 20 amp circuit. At a continuous load that was 20.84 amps so the answer was no.
One poster suggested the voltage was probably over 240v so it might be ok. He was corrected by another poster who said in a strictly resistive load if the voltage goes up the current will go up in proportion.
I am trying to understand that. Wouldn't a1500 watt 6' baseboard heater at 240 volts would draw 6.25 amps, but if wired 120 volts draw 12.5 amps?

 

[don_resqcapt19]

I was reading an old thread about electric baseboard heat. How much on a circuit, continuous load, etc. It was if two 8' strips could be on a 20 amp circuit. At a continuous load that was 20.84 amps so the answer was no.
One poster suggested the voltage was probably over 240v so it might be ok. He was corrected by another poster who said in a strictly resistive load if the voltage goes up the current will go up in proportion.
I am trying to understand that. Wouldn't a1500 watt 6' baseboard heater at 240 volts would draw 6.25 amps, but if wired 120 volts draw 12.5 amps?

The resistance is fixed and does not really change so a heater rated 1500 watts at 240 will only be a 375 watt heater if you connect it to 120 volts.

 

[LarryFine]

Gerry, This is from another thread, but it kind fits here:


A simple explanation of Ohm's Law: One volt is capable of pushing one amp through one ohm.

If you increase the voltage and/or reduce the resistance, the resultant current increases proportionately.

Don't get confused by the difference between applying a different voltage to a given piece of equipment . . .

. . . and . . .

Equipment designed to produce (or use) a given amount of power when supplied by a different voltage.

 

[Tulsa Electrician]

120/240 squared x1500/120

120/240= .5
.5 squared = .25
1500 x .25 = 375

 

[winnie]

One source of confusion in these discussions is clarifying what remains constant and what changes.

If you had some sort of _constant_ 1500W load, then when you run it at 120V it will draw twice the current vs being used at 240V. A simple baseboard heater probably isn't a constant wattage load, but I could imagine one that could optionally be connected for 120V or 240V.

As others have said, a baseboard heater is probably a constant resistance load, and the current drawn is proportional to supply voltage, lower voltage giving lower current.

Jon

 

[petersonra]

120/240 squared x1500/120

120/240= .5
.5 squared = .25
1500 x .25 = 375

because the resistance elements will not be as hot at 120 V, the resistance will be a little less at 120 as opposed to 240 V, so the heat produced will be a tad more. than your calculation came up with, but the idea is close enough for rough calculations.

 

[suemarkp]

I'm also not sure exactly what voltage is used to determine heater wattage. It seems like it may be 125V or 250V. So it will draw less at 120V or 240V. It seems like a 4000W heater rated at 250V just makes it on a 240V 20A circuit, as it draws 16A at 240V. Otherwise, manufacturers would be smart to market heaters with odd sized wattages that just first the circuit limits (e.g. 12A, 16A, 24A...) if they used the voltages the NEC wants for calculations (120, 240,...).

 

[Tulsa Electrician]

At 375 watts for the 120 volt source. 375/120= 3.125 amp using ohms law.
You could go another route as well
And get ohms.
1500/240= 6.25 amp
1500/ 6.25 squared = 38.4 ohms
120/38.4. = 3.125 amps
Don had mentioned 375 watts.
So I thought I would share how he may have arrived at that figure.
A lot of electricians can not or do not understand the relationship.
There is a question on the the PSI test that ask this type of question.
Since to OP was trying to understand how this takes place I thought some math examples might help.
"I am trying to understand that. Wouldn't a1500 watt 6' baseboard heater at 240 volts would draw 6.25 amps, but if wired 120 volts draw 12.5 amps"
He could also use
250/240 squared x 1500/250
This would show an amp draw increase.
He has also ask about that.
New voltage 250 / rated voltage 240 squared
,250/240 squared = 1.085
Squared sum (1.085) times rated watts (1500)
1.085x1500= 1627.5
Take that sum and divide by new voltage.
1627.5 /250= 6.51 amp
1500/240= 6.25 amps
375/120= 3.125 amps
Now the op can use 208 volt and find an answer.
Now he can understand the relationship.
Voltage applied to wattage@ rated voltage.
Once you know ohm (R) you can go the other route as well. R would be constant. Than use E/R just change E.
120/38.4= 3.125
250/38.4. = 6.510
240/38.4 = 6.25
208/38.4= ?

For in field work taking voltage readings, find R for rated equipment.
Then under load take voltage reading do math and compare to amp reading. This will help in trouble shooting. You get a volt reading of 240 and an amp reading of 4 you have a problem for this 1500 watt @240 volt equipment. Then do same at load end 4 amp at load you voltage would reflect a lower reading or damaged equipment.
The low voltage at load would mean you have an issue in the branch circuit/ feeder. Good voltage and low amp reading would indicate an issue with equipment.
This is why we should have an idea how to figure.

Getting late hope I did math correctly. If not some one correct please.

 

[garbo]

I was reading an old thread about electric baseboard heat. How much on a circuit, continuous load, etc. It was if two 8' strips could be on a 20 amp circuit. At a continuous load that was 20.84 amps so the answer was no.
One poster suggested the voltage was probably over 240v so it might be ok. He was corrected by another poster who said in a strictly resistive load if the voltage goes up the current will go up in proportion.
I am trying to understand that. Wouldn't a1500 watt 6' baseboard heater at 240 volts would draw 6.25 amps, but if wired 120 volts draw 12.5 amps?

I was reading an old thread about electric baseboard heat. How much on a circuit, continuous load, etc. It was if two 8' strips could be on a 20 amp circuit. At a continuous load that was 20.84 amps so the answer was no.
One poster suggested the voltage was probably over 240v so it might be ok. He was corrected by another poster who said in a strictly resistive load if the voltage goes up the current will go up in proportion.
I am trying to understand that. Wouldn't a1500 watt 6' baseboard heater at 240 volts would draw 6.25 amps, but if wired 120 volts draw 12.5 amps?

Was taught 50 years ago that for a 100% resistance load if you halve the voltage the current is reduced to only 25%. Still holds true today. Many of my fellow electricians had problems with that. Never went above 80% of the circuit breaker rating even if load was all baseboard heaters. Therefore would never go over 12 amps for a 15 amp breaker & 16 amps for a 20 amp breaker.

 

[LarryFine]

Was taught 50 years ago that for a 100% resistance load if you halve the voltage the current is reduced to only 25%. Still holds true today.

Not quite correct. If you halve the voltage, the current also halves, which results in one fourth the power.