I am .83A over load calculation...

[77bbod]

Hi, I am planning on installing 2 8' baseboard heaters running at 240V. I WANT to use 12/2 with a DP breaker at 20A.

So according to my calculations these 2 will be at 4000 watts and figuring for 125% continuous load this will make it 5000 Watts. This brings me to a amperage of 20.83A. Now do I need to be so **** to jump up to 30A circuit and 10 gage wiring just because I am .83A over my load calc??? This seems a waste of money to upgrade to 30amps and more expensive wiring for that small amperage. Anyone know a reason why I should that I don't see?

I also have no idea where to find a double pole line voltage t-stat rated for 30 amps...All of them I seen are good for 22A.

 

[SGKent]

First verify the current draw of each unit with the manufacture's specs. It is not uncommon for a device's wattage to be rounded up (or down). Second, when you figure AC wattage or backing into current from it, you need to use a power factor. The formula for AC is (amps x volts x power factor). In a DC circuit it is only (amps x volts). I could not find a true AC calculator online. AC is not a constant voltage so the factor is usually around .85 but I have not calculated 240 before. If my math serves me, a 2000 watt AC device will draw more amps than a DC device at the same voltage but it is late and I am tired so it could be the other way. Also when a cold resistor comes on it draws more current than when it is hot. So what is the current draw when that first surge hits them cold? Call the manufacturer. Worst case is put each on a seperate circuit. Have one come on at say 68F and one come on at 65F. That way if one can't keep up the other will come on but if one can do it then it will stay on longer which will lower costs as you will have less surges when they come on to pay for. Just my humble opinion.

 

[Norcal]

2 baseboard units will need to be split into 2 circuits, they draw over 16A between the 2 & 16A is the limit for a continuous load on a 20A circuit.

 

[Cryptic1911]

^ yeah, from what I've been told, you should only load to 80% of what the breaker is rated for

 

[sberry]

I have seen this argument on other code forums, Norcal is more qualified than I am so this is just an opinion but I wouldn't lose sleep over it if I had my heart set on 12, to me it wouldn't be a big deal to run 10 but since there is a T stat and if it shuts off at any time within 3 hrs its not continuous.

 

[Charles (in GA)]

Electric heating, by code, is defined as continuous no matter if it runs 3 hrs or just 30 minutes. Lighting is also defined as continuous. Other items may not specifically be defined as continuous and it is up to the installer to determine if they are continuous based on the 3 hr rule.

The OP was mistaken in his calculations by taking the LOAD and multiplying by 125%, rather, he should have taken the 20 amp breaker and multiplied by 80% to determine what the circuit is allowed to handle, and then compare it with his ACTUAL heating load.

20 amps times 80% is 16 amps, that is the max "continuous" load of the circuit.

4000 watts divided by 240 volts is 16.666 amps, still too much for the circuit. If you see any voltage drop at all, the amperage will go up. Run the wire, split it up, you will be happier when it is all over, rather than second guessing yourself.

Charles

 

[JBurgess]

If you see any voltage drop at all, the amperage will go up. Charles

True for a motor, but not so for resistance heating elements, the amperage will fall with voltage drop.

 

[MrMark]

Electric heating, by code, is defined as continuous no matter if it runs 3 hrs or just 30 minutes. Lighting is also defined as continuous. Other items may not specifically be defined as continuous and it is up to the installer to determine if they are continuous based on the 3 hr rule.

The OP was mistaken in his calculations by taking the LOAD and multiplying by 125%, rather, he should have taken the 20 amp breaker and multiplied by 80% to determine what the circuit is allowed to handle, and then compare it with his ACTUAL heating load.

20 amps times 80% is 16 amps, that is the max "continuous" load of the circuit.

4000 watts divided by 240 volts is 16.666 amps, still too much for the circuit. If you see any voltage drop at all, the amperage will go up. Run the wire, split it up, you will be happier when it is all over, rather than second guessing yourself.

Charles

It doesn't matter which way you do it, it's the same calculation. THe math is the same either way.

 

[sberry]

Electric heating, by code, is defined as continuous no matter if it runs 3 hrs or just 30 minutes.

Good to know, will read it. It does look like he could use the 22A tstat and 10 wire though?

 

[JBurgess]

It doesn't matter which way you do it, it's the same calculation. THe math is the same either way.

Not a point worth argueing over the end results are the same. Since it has been brought up.....

422.10 (A) Individual Circuits. The rating of an individual
branch circuit shall not be less than the marked rating of the
appliance or the marked rating of an appliance having combined
loads as provided in 422.62.
The rating of an individual branch circuit for motoroperated
appliances not having a marked rating shall be in
accordance with Part II of Article 430.
The branch-circuit rating for an appliance that is a continuous
load, other than a motor-operated appliance, shall
not be less than 125 percent of the marked rating, or not
less than 100 percent of the marked rating if the branchcircuit
device and its assembly are listed for continuous
loading at 100 percent of its rating.
Branch circuits and branch-circuit conductors for
household ranges and cooking appliances shall be permitted
to be in accordance with Table 220.55 and shall be
sized in accordance with 210.19(A)(3).

 

[JBurgess]

Good to know, will read it. It does look like he could use the 22A tstat and 10 wire though?

With a 25a amp breaker as long as they are not corded.

or 2 15 amps circuits with 14 ga.

 

[sberry]

I outright forgot there was code section dedicated to heat.

 

[Charles (in GA)]

Good to know, will read it. It does look like he could use the 22A tstat and 10 wire though?

Yes, Article 100 Definitions, tells us about the max current for three hours or more being a continuous load.

Article 424.3 (B) tells us "Branch-Circuit Sizing. Fixed electric space-heating equipment shall be considered continuous load."

Charles

 

[Charles (in GA)]

I suppose it is largely a matter of whether you are trying to determine...........

1.... how large a circuit breaker you need for the load you have..... or....

2.... how much load a given size breaker can carry.

Since you are taking amps (breaker size) I tend to want to convert the actual equipment load or draw to amps right away to do any 125%/80% calculations. Kinda odd (to me) to see watts multiplied by 125%

Charles

 

[tcianci]

One of the posters mentioned multiplying by the power factor. IIRC, that only applies to non-resistive loads i.e.:capacitive or inductive.

 

[Falcon67]

FWIW - my shop heater is a 5000W 240V unit. It pulls 21A on each leg according to my hand held meter. I used 10 gauge wire and 30A plugs and breaker on the circuit.

 

[77bbod]

I forgot about this thread...Well after thinking about this some more I went ahead and wired both heaters to one circuit on 12/2 at 20Amps. Originally I used the assumption that a foot of baseboard heat is 250 Watts and using that the circuit would be overloaded under an amp. Then a wild hair inspired me to acutally look at the nameplate of the heater and it shows 1900 watts for the board. So I believe I am good to go using that info off the heater themselves and my load is now 15.83A which is under the 16....charles I see your point and it is more logical to calculate it that way. So the lesson learned here for me is to really know the ratings of what you've got when planning this stuff.