061210 2113 - Doug Miller posted:

In article .com, "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.

That is *not* correct.

The *resistance* stays the same. Increase the voltage, and you increase the
current. Increase the current and voltage, and you increase the power even
more.

Suppose you have a 10ohm resistance heating element. Operate it at 220V.
V = IR
I = V / R = 220V / 10 ohm = 22 amps.
E = VI = 220V * 22A = 4840 watts.

Now operate the same element at 240V:
I = V / R = 240V / 10 ohm = 24 amps
E = VI = 240V * 24A = 5760 watts.

Same for
the 1500 watt heater. The resistance stays the same.

Right. The resistance stays the same. But the power output changes with
voltage.

Only the amps
and volts are variables. The higher the volts, the lower the amps.

False. The higher the volts, the *higher* the amps.

V = IR

Increase V while holding R constant, and I goes *down*??? Hardly.

Indago's calc was correct, as he figured it on the (worst case) lower
voltage.

Wrong. The worst case would be figuring it at the higher voltage.

One 20 amp circuit is sufficient for both baseboard heaters.

Not if they were rated at 220V but operated at 240V.

If they were rated at 240V, yes -- but that's the whole question here, isn't
it? The installation instructions cited by the OP apparently referred to
installing a 220V circuit, which raises the possibility that they heaters were
rated at 220V and not at 240V. Operating them at a 9% higher voltage than that
at which they were rated will cause them to draw 9% more current, and deliver
nineTEEN percent more power, than rated.

This is correct, and at this point I would connect a temporary line to the
panel and place the heaters on a bench and turn on the power and check the
voltage and current with a clamp-on ammeter at first surge, and then when
the heater heats up, check the current again to get an accurate measure of
just what would be demanded of the circuit. I am assuming the baseboard
heaters are of the calrod class of heater, and the hotter the element gets,
the more the resistance, and the lower the current.