Stock tank heaters: wattage, safety, and why insulation beats power

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Research. We have not put a heater in a tank here yet. What follows is how the three types differ, how wattage relates to tank size, and the electrical problem that decides whether the thing works at all. When one has been through a winter here, this page gets updated with the date.

A stock tank heater is an electrical appliance you are putting into water that animals drink from. That is the whole subject.

The three shapes

Floating. Sits on the surface, cord over the rim. Cheapest, easiest, moves between tanks. It is also the one animals can reach, and the cord is the part they reach.

Submersible. Sits on the bottom in a guard. Out of the way, but the cord still comes over the rim, and the element has to stay under water. One that runs dry after the tank is drunk down burns out quickly.

Drain-plug. Threads into the drain hole and heats from below. No cord in the water, nothing loose to pull on. The trade-off is that it is married to that tank — right thread, right material — and the tank now has a hole with a heater in it.

Read the tank-material line on any of them. Plenty are metal-tank only, and a bare element resting against a poly wall will melt through it.

Wattage against tank size

Rough working figures:

• 250 W — buckets and tubs, up to about 25 gallons.
• 500 to 750 W — 30 to 70 gallons.
• 1,000 to 1,500 W — 100 gallons and up, or a wide shallow tank standing in wind.

Confirm it is thermostatically controlled. A typical one starts heating as the water approaches freezing and stops once it is somewhere around 40 F. The ones without a thermostat run whenever they are plugged in, which is the difference between the power bill above and a considerably worse one.

What 1,500 watts actually means

1,500 W at 120 V is 12.5 amps. A 15-amp circuit is rated for 12 amps continuous. One large heater is already past that, and two tanks on one circuit is a tripped breaker you find at dawn.

Most labels also say not to use an extension cord. That is not fine print, it is the hidden cost. If there is no outlet at the tank, the heater is the cheap part, and what you are really buying is a buried circuit and an electrician.

Run the numbers before you buy. 1,500 W cycling at roughly 40 per cent duty is about 14 kWh a day; ninety cold days is on the order of 1,300 kWh. Multiply by your own rate.

Stray voltage, and animals that will not drink

A nicked cord, a corroded connection or a poor ground can put a small AC potential on the water — a fraction of a volt. That is not enough to injure anything, and it is enough for an animal with a wet muzzle standing on wet ground to feel.

A failed element is a different thing entirely. That can energise the water at full line voltage, and full line voltage in a tank kills stock and kills people. The two faults look identical from the outside, which is the argument for the GFCI below and for never treating a tank as safe because the animals seem fine.

Cattle are commonly reported to detect well under a volt. What you get is not a shock, it is refusal — stock walk up and back off, drink in snatches, or quit that tank. The heater looks fine. The tank looks fine. Intake drops and nothing obvious is wrong.

Tank material matters less than bonding. A metal tank that is properly bonded gives fault current a path that is not the animal. A metal tank that is not bonded is arguably the worst case of all, because the whole rim goes live exactly where a muzzle rests. Poly has no path either way.

Note that the insulation advice further down — setting the tank on foam board — deliberately breaks any incidental contact with the earth. That is correct for heat and it is the reason the ground path has to be a real bonded conductor rather than something you hope the dirt is providing.

So put the circuit on GFCI, and take the trade-off honestly: GFCI trips, and one that tripped at nine at night means solid ice by morning. It has to be somewhere you look at daily. If stock go off water and the tank itself is fine, get a meter on it. Anything to do with the animals is a question for a vet, not a product page.

A chewed cord is a two-dollar saving with a long tail. Same argument as the gate latch.

Insulation beats wattage

Heat leaves through the open surface and the walls, and a tank sitting flat on frozen ground wicks straight into the soil.

• Unplug it before you reach into the tank or handle the unit. Every label says so and it is the one habit worth building on day one.
• Set it on foam board or a pallet, not on dirt.
• Insulate the sides — rigid foam behind plywood, or a fitted jacket.
• Cut the open surface down. A lid with a drinking hole in it does more than a step up in wattage.
• Keep it full, and out of the wind. A half-full tank is mostly cold wall.

Same heater, far less runtime — and insulation is the only part of this that still works in an outage.

What we are buying

Outlet first, then the lid and the insulation, then the element. That is the order on this page and we are at the front of it, with none of it bought. Wattage is arithmetic once there is a tank to size it against; the circuit is not. A proper run out to where a tank will stand, on GFCI and with no extension cord anywhere in it, is the figure this page cannot supply and the one that decides whether the rest happens at all.

Buying a heater changes this section. It does not change the tag. The type and the wattage go here, the tank it went into and what that tank is made of, and the electrical work costed apart from the unit, with the date. The Research line at the top of the page holds until one has been through a winter here, because the winter answers the rest of it — how often the GFCI went off and whether it was caught before morning, what ninety cold days added to the bill at our own rate, and whether the insulation did the work claimed for it above.

Before you buy

• Sort the outlet before the heater. No extension cords, GFCI on the circuit.
• Match the heater to the tank material in writing before it goes in the water.
• Spend on a lid and insulation first. It lowers the wattage you need.

— Chris D., McEwen, Tennessee