(Another) Wiring Question

[Doug Miller]

Why? Is there some reason you think it would not be acceptable to attach a 16A load to a 30A circuit?

Residential electrical supply in the US and Canada hasn't been 110/220 for many years -- it's 120/240 and has been for a long, long time. Doesn't stop people from referring to it as 110 and 220, obviously, but that's not what it is.

My point was this: the OP referred to installation instructions that said (according to his post) to connect the equipment to a 220V circuit. Maybe that was just a mistake on the part of the OP and the instructions really said

240V, or maybe they really said 220V like he said -- and if *that* is the case, that the manufacturer really said 220 and not 240, it's quite possible that the rated output of the heaters is based on their output at 220, and not at the 240 that the OP certainly has in his house. And the distinction is important because if these heaters were rated at 240V, then they can both go on the same 20A circuit -- but if they were rated at 220V they cannot.

Wrong.

Take the case of a resistance heater that emits 2000 watts at 220V. It draws

2000 watts / 220 volts = 9.09 amps. Now calculate the resistance: 220 volts / 9.09 amps = 24.2 ohms.

Note that the resistance is a physical property of the heating element, that does not change no matter what voltage is applied to it.

Now push 240V across that same 24.2 ohm resistance.

240 volts / 24.2 ohms = 9.92 amps.

A similar calculation applies to the 1500 watt heater.

Isn't that exactly the opposite of what you said in the previous paragraph?

Maybe you're right -- but assuming that, without proof, could be dangerous. Which is why I cautioned the OP to check the rating plate to see whether the heaters were rated at 220V or 240V.

Simply knowing what voltage they're rated at is sufficient.

[Doug Miller]

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.

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

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

V = IR

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

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

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.

[indago]

061210 2113 - Doug Miller posted:

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.

[Terry]

Like I have said, I don't know for sure, but I have it in my head that if you can limit the load to 15A or 20A then you should.

My assumption for using 240V was that the OP had made the same mistake we all do in referring to 110-220 and 120-240 interchangeably. If this was the case, then the load would be 14.6A. Maybe I didn't say that too clearly, but that would be my observation. I am sure he got the wattage right, and I would think that he couldn't buy a heater for the home if it was truly rated for 220V.

[Doug Miller]

You seem to be confusing "load" with "capacity". In any event, there is *no* reason why you could not connect a 16A load to a 30A circuit. [snip]

I would think that, too -- but when talking about electricity, assumptions can be deadly dangerous if they prove to be incorrect. The only reason I raised the issue is that the mention of installing the heater on a "220V circuit" raises the possibility that the devices may have been rated at 220V instead of

240V, and hence the possibility that they may draw more current than anticipated when operated at the higher voltage.

[Mark Lloyd]

What actually causes this to happen? The current through a constant resistance will increase as voltage increases.

[volts500]

My mistake, Doug, you are absolutely correct.

[Husky]

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