I want to install three electric baseboard haaters on a 20 amp 240 volt circuit.The heaters are 1500 watt.The 240 volt circuit has 4800 watts.Do I have to use the 80% rule that limits me to 3840 watts?????? They will each have their own thermostat.I will be installing them in a motel.
Thanks bob

Based on 424.3 I would say yes.

If the supply is actually 208v, the heaters will run at 1300 watts, which total only a hair (60 w) over your 80% figure.

Larry:
Not to burst your bubble, but
208 volt, 20 amp equates to 4160 watts
80% equates to 3328 watts
1300 watts x 3 units equates to 3900 watts

Been there, done that

John

Sorry, Hotline...but your math is in error.

The heaters are not "constant amp load" heaters.
Here is how you calculate the expected load:

Assuming the heater nameplate says 240 volts, 1500 Watts. Using Ohm's Law, we find how many Ohms of resistance the heaters offer.
Now we look at the new voltage. Again using Ohm's law, we use the Ohms value, and divide 208 by that number. This will give us the amps at that voltage. Using that new amp number, we find our watts of demand.
Now, I'm just using "ballpark" math, but it looks like the heaters will put out less than 1200 watts at 208, which would allow you to comply with the 80% rule.

I'm really not sure that you can count on any of these calculations and suggest that you just check one at 208 if that's what you're going to use. I wouldn't expect the hot resistance of the element to be the same at 208 and 240VAC.
Joe

Okay, here's the real calculation (I hope):

1500 watts / 240 volts = 6.25 amps (at 240v)
240 volts / 6.25 amps = 38.4 ohms (constant)

208 volts / 38.4 ohms = 5.42 amps (at 208v)
3 x 5.42 amps = 16.26 amps (three heaters)

208 volts x 16.26 amps = 3382 watts (at 208v)

But the wattage is not important, the current is. 16.26 is a CH over the 80% of 20.

By the way, at 208 volts, 16 amps (80% of 20) is 3328 watts, so again, 3382 is slightly over.

I have a 240 volt SINGLE PHASE service.208 is a three phase wye.I have 240 volts 20 amp circuit 20 x 240 gives me 4800 watts.My question is do I have to knock this down to 80% x 4800 to 3840 watts?? I think it works this way.I have a 1500 watt heater 240 volts divide by 240 volts gives me 6.25 amps.times 1.25% would give me 7.8 amps for one 1500 watt heater. I'm going with two 1500 watt heaters that would be 15.6 amps four amps under 16 which is 80% of 20 amps.

Okay. What matters is the 3 x 6.25, which equals 18.75, exceeding 80%.

However, thermostatic heater controls cycle power, so the odds are that no heater will be powered for 3 hours (barring initial turn-on in freezing weather), not to mention all three being on for 3 hours.

So, you might have an argument against having a continuous over-80% load.

Guys George Little pointed out a code section that is right on target.

part of 424.3(B)


It does not matter if it is a continuous load or not, the branch circuit must be 125%