I am being asked to correct a snow melt installation that never worked. It is used to prevent snow from entering a large HVAC intake vent. There are five units with a namplate stating 208 volts, 14.6 amps. The manufacturer states that 277 volts can be used. I am also being told that each unit has a resistance of 14.97 ohms. I am assuming that by increasing the voltage, that I will reduce the amperage, however, using Ohm's law I= E/R the amperage increases. I have always thought that amperage and voltage are inversely proportional. What am I missing?
Snow Melt
- Thread starter rifi
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DetroitEE
Senior Member
- Location
- Detroit, MI
The snow melt is like a big resistor. So, if you increase the voltage, you will increase the power dissipation.
In the case of a purely resistive load, an increase in voltage will result in a proportional increase in current.
That is, unless the power is delivered to some kind of snow melt controller that contains electronics which regulates the voltage and/or current delivered to the heating cable. But I have a feeling that the snow melt is just a big resistor as I described, with no electronics in between.
If the manufacturer says that 277V is ok, then you should be fine, as long as you're got it on a 20A breaker. Unless the snow melt is considered a continuous load, in which case a 25A breaker would be required. On the upside, the snow will melt faster with that higher heat dissipation!
In the case of a purely resistive load, an increase in voltage will result in a proportional increase in current.
That is, unless the power is delivered to some kind of snow melt controller that contains electronics which regulates the voltage and/or current delivered to the heating cable. But I have a feeling that the snow melt is just a big resistor as I described, with no electronics in between.
If the manufacturer says that 277V is ok, then you should be fine, as long as you're got it on a 20A breaker. Unless the snow melt is considered a continuous load, in which case a 25A breaker would be required. On the upside, the snow will melt faster with that higher heat dissipation!
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DetroitEE
Senior Member
- Location
- Detroit, MI
Looks like the total load is 5125 Watts at 277V. Better divide that by the total length and get a watts per linear foot and triple check with the manufacturer that the snow melt is rated for that value.
When the voltage increases and the amperage decreases the load is a constant power load, like a motor.
The heater you have is a resistor which is not a constant power load. So as the voltage increases the current increases and the power the load absorbs increases.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
The difference between simple math and the real world of equipment. Ohm's Law obeys the rules of math, but a given load won't have the same resistance for all voltage designs.
For a given resistance, an increase in applied voltage will result in an increase in current. But, when designed for a different voltage, the equipment will have a different resistance.
broadgage
Senior Member
- Location
- London, England
The above posts are assuming that the snow melting equipment is a simple resistor of at least approximatly fixed resistance.
If it IS a fixed resistance, then as posted above, an increase in voltage from say 208 up to 277 will increase the current in like proportion, by ohms law.
BUT here in the UK, and presumably elswhere, snow/ice melting tape is available that is not a fixed resistance.
It is made of PTC material (positive temperature coefficient) and is called self regulating.
Such heating tape maintains a roughly constant temperature, and will within limits, draw from the supply as much current as needed to maintain this temperature. The stated current draw would be the maximum under cold conditions.
If worked from a higher voltage, then the running current will be LESS since it is still trying to maintain the same temperature.
The behaviour of such PTC heating units at different temperatures and supply voltages is counter intutitive and can be perplexing.
Two types are available, low temperature for defrosting, and high temperature for counteracting heat loss in hot water supply pipes.
If it IS a fixed resistance, then as posted above, an increase in voltage from say 208 up to 277 will increase the current in like proportion, by ohms law.
BUT here in the UK, and presumably elswhere, snow/ice melting tape is available that is not a fixed resistance.
It is made of PTC material (positive temperature coefficient) and is called self regulating.
Such heating tape maintains a roughly constant temperature, and will within limits, draw from the supply as much current as needed to maintain this temperature. The stated current draw would be the maximum under cold conditions.
If worked from a higher voltage, then the running current will be LESS since it is still trying to maintain the same temperature.
The behaviour of such PTC heating units at different temperatures and supply voltages is counter intutitive and can be perplexing.
Two types are available, low temperature for defrosting, and high temperature for counteracting heat loss in hot water supply pipes.
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