electric question

If uoy wanted to run this heater with 300 feet of extension cord what size cord would I need

Since it's a resistive load, you can run it on #14, but you will loose considerable heating capacity. Here's what I got running the voltage drop calculator.

Results
1 conductors per phase utilizing a #14 Copper conductor will limit the voltage drop to 6.75% or less when supplying 12.5 amps for 300 feet on a 120 volt system.
For Engineering Information Only:
15.0 Amps Rated ampacity of selected conductor
2.9495 Ohms Resistance (Ohms per 1000 feet)
0.058 Ohms Reactance (Ohms per 1000 feet)
24.0 volts maximum allowable voltage drop at 20%
20.099. Actual voltage drop loss at 6.75% for the circuit
0.9 Power Factor

So, with #14 wire,you'll only be getting 1250 watts of heat out of it. It will work, but not as well. If you think you need the full 1500 watts, you'll need to go all the way up to #10!

You can "get by" with a #14 cord. But you'll lose about 19 volts in the cord. Assuming 120 volts where the cord is plugged in, only about 100 volts will be available at the heater. Consequently the heater's output will be reduced by about 15%.

A #12 cord is better - it will deliver about 108 volts at the heater.

Incidentally your and your animals' safety be sure to include a GFCI in the circuit!

Thanks for all the replies .

And when it shorts out, the cattle will all stand around the tank and stare at the water - but they've already been shocked so they know better than to stick their nose in the water.

BTDT when we tried an electric water heater years ago.

(quoted from post at 00:37:39 11/14/14) Since it's a resistive load,
0.9 Power Factor

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What power factor with a 100% resistance load?

Is this a permanent installation? Or are you planning on having three 100ft extension cords run across the pasture ?
Personally I would consider ploughing in conduit and pulling in 12/2 or 10/2 cable to the water trough. A local GFI with a LED yard light wired in 24/7. A bit of illumination in the area and if the light is out that warns that the GFI has tripped.
Being around water etc. I would backhoe down 10ft and install a ground plate at the power receptacle by the tank heater. 99.999% of people would not but it's peace of mind .

Depends on how well it can work with low voltage With AC applicances - 5% voltage loss is usually the most loss desired. Households are wired with no more loss then 2.5%.

At 120 volts and 12.5 amps it goes like this with a 300 foot cord:

#12 copper - 10.3% loss
#10 copper - 6.2% loss
#8 copper - 4% loss
#6 copper - 2.5% loss

Heaters are unity, resistance only.

1) First off this is a pure resistive (NON inductive) heating element meaning it has a UNITY ONE POWER FACTOR.

2) Since its a pure resistive heating element only, not a motor or a particularly voltage sensitive or particularly voltage critical device, voltage drop in the extension cord and resulting voltage at its input isn't as critical as if it were a motor or other voltage sensitive or voltage critical device.

3) Voltage drop on particular sizes of conductors for X amps at X feet is easy to look up and the other fine gents have already provided that.

THAT BEING SAID if it were me personally I still wouldn't use less then a 12 Gauge extension cord, although sure, a smaller gauge may still "work" fine. Its your money and your choice. As far as supplying an outdoor receptacle (into which that heater may be plugged into) I always specified GFCI receptacles. Livestock are very sensitive to even a slight voltage tingle and will get nervous or even refuse to drink if they ever feel a shock, therefore you may have to take a look at ways to prevent such in the event it were to happen, but hey, Ive used a lot of those water heaters AND NEVER HAD A PROBLEM although another person may report he did. NO WARRANTY

Best wishes

John T

We're about the same latitude, I think. How do you keep the water thawed?

Inadequate cord size means the cord heats up more then it should. Go too small and you can melt it. Been there and done that. I've run many extension cords out to water-tank heaters for my animals. Never tried to 300 feet though. 200 feet yes. None of mine were rated 12 amps either.

Last time I tried to run 300 feet of extension cord it was to plug in the block heater in a diesel backhoe sitting out in the field. It has a 1500 watt tank heater. I used three 100 foot 12 gauge cords. The wires got warm and then sank in the snow - that was no big deal. Then and end melted off one of the cords. I suspect with many of these cords the attachment of wires to the molded plugs has less ampacity then the 12 gauge wires themselves.

VS, here's a bit more hard core engineering data and information to help you decide what size wire to use out to that heater. Lets look at the math and Ohms Law instead of just opinions and preferences.


1) The heater appears to have 9.6 ohms of pure resistance (NO appreciable inductance or capacitance). I say that because the info claims it draws 12.5 amps at 120 volts. To solve for R it equals (V/I) = 120/12.5 or 9.6 ohms.

2) Although it depends on temperature and the enclosure or raceway of if the conductors are in free air or enclosed or direct buried etc etc one typical ampacity for 12 Gauge Copper Conductors often quoted is 20 amps, and 12.5 amps is certainly within that current ampacity rating if you used 12 Gauge wire.

3) HOWEVER since wire still has resistance its NOT a perfect conductor, voltage drop will take place in the wires meaning the entire beginning 120 volts will NOT be available out at the heater.

4) So in reality the complete circuit (heater plus wire) consists of MORE THEN 9.6 OHMS as the wires resistance is in series with the heater.

5) THEREFORE, since the circuit resistance due to a long wire run is MORE THEN 9.6 OHMS, and since I = V/R, the current that will flow through the circuit, wires PLUS the heater, WILL BE LESS THEN 12.5 AMPS ACTUALLY. If you applied 120 volts across 9.6 ohms you get the heaters rated 12.5 amps HOWEVER 120 volts applied across say 9.6 + X Ohms IS LESS THEN 12.5 AMPS. More ohms = less current flow DUH

6) This confirms what I advised below, that as the heater is NOT a motor load or NOT a voltage sensitive or voltage critical device or NOT any sensitive "electronic" device ITS JUST A RESISTANCE/HEATER

PLUS as 12 gauge wire has an ampacity rating of 20 amps which doesn't change regardless if its a foot or 300 feet ALTHOUGH VOLTAGE DROP INCREASES AS LENGTH SINCE IT STILL HAS RESISTANCE

12 GAUGE WIRE WILL CERTAINLY SUFFICE TO CARRY 12.5 AND FEWER (see my math above why) AMPS TO THE HEATER

NOWWWWWWWWWWWW before anyone has a calf, the heater will not develop as much heat energy (I Squared R) if you're pumping less then 12.5 amps through it. IT WILL NOT GET AS HOT as it would if the whole 120 volts is there and the longer the wire run the less volts that end up at the heater.

HOWEVER 12 gauge wire will easily carry 12.5 minus X amps and that's less then its rated 20.

DISCLAIMER Im rusty on Ohms Law and long retired so theres a darn good chance my formulas or math may be incorrect,,,,,,,,,Im pretty sure 12 gauge wire has an ampacity of 20 amps and more (depends on enclosures and temp etc) and that ampacity DOES NOT depend on how long the wire is while voltage drop increases as length increases. So if I made an error in my match or formulas will some electrical experts PLEASE CORRECT IT.

FINALLY, (dont have a calf anyone) Im NOT saying its how I would do it,,,,,,,,,,Im not saying its perfect,,,,,,,,,Im ONLY saying 12 gauge wire will suffice to conduct 12.5 and less amps and that heater plus the wires resistance will result in a circuit current of LESS THEN 12.5. CAN ANYONE SHOW THAT INCORRECT????????????? I wouldnt be surprised as Im rusty on Ohms Law grrrrrrrrrr

John T

Thanks everybody for the comments and help . This will not be a permanently installaed cord it will only be for the cold months also it will go to my corral and livestock will not be able to tread on the cord.

(quoted from post at 14:46:16 11/14/14) Inadequate cord size means the cord heats up more then it should. Go too small and you can melt it. Been there and done that. I've run many extension cords out to water-tank heaters for my animals. Never tried to 300 feet though. 200 feet yes. None of mine were rated 12 amps either.

Last time I tried to run 300 feet of extension cord it was to plug in the block heater in a diesel backhoe sitting out in the field. It has a 1500 watt tank heater. I used three 100 foot 12 gauge cords. The wires got warm and then sank in the snow - that was no big deal. Then and end melted off one of the cords. I suspect with many of these cords the attachment of wires to the molded plugs has less ampacity then the 12 gauge wires themselves.

Click to expand...

Exactly . Those molded on plug ends are trouble sooner or later. They get cut off around here and replaced with a premium male plug. And the femal end is often replaced with a weather resistant receptacle box with covers and a HD T-Slot duplex receptacle .