volts500 wrote:
Doug Miller wrote:

In article , indago wrote:


If the heaters are not too far from the panel, say more than 50 feet of
wire, you can use the 12 gauge wire, using just one circuit for both
heaters.

2000/220 = 9.1 Amps 1500/220 = 6.8 Amps 9.1 + 6.8 = 15.9 Amps

Not so fast. If the heaters are rated at 220V as he stated (not 240V) but his
service is actually 240V (as is very likely), the currents will be almost 20%
higher (10.8 and 8.1 amps, respectively) for a total of 18.9 amps, requiring a
30A circuit because...


Think about what you are saying, Doug. A 2000 watt resistance heater
stays a 2000 watt heater no matter what the voltage applied. Same for
the 1500 watt heater. The resistance stays the same. Only the amps
and volts are variables. The higher the volts, the lower the amps.
Indago's calc was correct, as he figured it on the (worst case) lower
voltage. One 20 amp circuit is sufficient for both baseboard heaters.

Wrong! E=IR Voltage = Current * Resistance (or E/R = I) and
P=IE Power = Current * Voltage

Since the resistance effectively stays the same ( not exactly, as the
higher voltage will create a little more heat, raising the resistance
very slightly, we CAN ignore the change for this example ), when the
voltage goes up, the current must also go up for the first equation to
stay in balance. Also, by inserting the first equation into the second,
we get P = E squared / R. So a 2000W heater at 220V has a 0.04132 ohm
resistance. ONLY the resistance can be treated as a constant, as it is
the only physical thing that doesn't (effectively) change. So at 240V,
the POWER becomes 240 * 240 / 0.04132 or approximately 2380W, and the
current will be 9.9A, not 9.1. Since US standard line voltages are
usually closer to 240 than to 220 in real life, using 240 for the
calculations is the safest and most appropriate method. Regardless, as
voltage rises in a circuit, if all physical constructs remain the same,
the current MUST rise, and so will the power.

All of this goes out the window in other countries, power grids, etc.
Take with a grain of salt, as this was based entirely on the laws of
physics, and we all know that science is completely out of favor in the US.

Husky