Before you size wire for a lift that struggles: a boat sitting too far back makes a sound lift labor the way a weak supply does, and moving it forward costs nothing. On an older lift the load can be off left to right as well as front to back, and parking will not fix that one. Rule the load out first →
What to do about it, cheapest first
If the run is over the line, or close to it:
- Check the joints first. Free. Every splice, terminal and outlet, panel to motor. A loose or corroded one adds drop the calculation cannot see. A clean run that is too thin is still too thin.
- Convert to 240 V. Half the current, a quarter of the drop, usually on the wire already in the ground. It needs a 240 V circuit and a motor to suit; we can order a motor set up for 240 V if the job is planned that way, and we sell the GFCI a conversion needs. It does nothing for a bad joint.
- Pull heavier wire. This page gives the size. It fixes drop and ampacity, and it is the only answer when 240 V would still be over the line. It does nothing for connections.
- Live with a run between 3% and 4%. Inside BH-USA’s limit, so it works. The margin left will not cover the next heavier motor, so settle for it only if this is the last motor this dock will see, and measure it under load first.
The last three are an electrician’s job, and so is the first anywhere past the outlet. Take them the size from this page.
Minimum copper conductor
Smallest copper conductor that carries the motor and holds voltage drop inside 3%, by run length.
| Run | 1 HP @ 120 V | 1 HP @ 240 V | 1.5 HP @ 120 V | 1.5 HP @ 240 V |
|---|---|---|---|---|
| 50 ft | #12 | #14 | #10 | #14 |
| 100 ft | #8 | #14 | #8 | #12 |
| 150 ft | #6 | #12 | #6 | #10 |
| 200 ft | #6 | #12 | #4 | #10 |
| 300 ft | #4 | #10 | #3 | #8 |
Aluminum runs one to two sizes larger. #3 is a real size but an uncommon one — #2 is what a supply house will stock. Where BH-USA’s own chart calls for a heavier conductor than the code minimum, this follows their chart — which is why the 1.5 HP at 240 V column runs heavier than the code alone would ask.
Two motors on one run
A double lift, or two lifts fed from the same circuit. Sized per NEC 430.24 — 125% of the largest motor plus the full load of the other — and cross-checked against BH-USA’s own chart.
| Run | 2 × 1 HP @ 120 V | 2 × 1 HP @ 240 V | 2 × 1.5 HP @ 120 V | 2 × 1.5 HP @ 240 V |
|---|---|---|---|---|
| 50 ft | #8 | #10 | #6 | #10 |
| 100 ft | #6 | #10 | #4 | #10 |
| 200 ft | #3 | #8 | #2 | #8 |
| 300 ft | #1 | #6 | 1/0 | #4 |
| 400 ft | 1/0 | #4 | 2/0 | #2 |
The larger of the code calculation and BH-USA’s own chart. Note the 120 V column: two 1.5 HP motors at 400 ft wants 2/0, and BH-USA will not put a size against that run at all. At those distances the answer is 240 V, not heavier wire.
Why so many boathouse runs on Lake Sinclair come up short
Part of it is the lake. Georgia Power does not permit a free-standing boat lift on Lake Sinclair — their rule, not ours — so a lift here hangs inside a boathouse, and the boathouse sits at the water’s edge while the panel sits in the house. The supply has to cross the whole yard to reach the motor, down the slope and out along the dock, and that is how a run lands in the part of the tables above where the sizes get heavy. Long runs at 120 V are where voltage drop lives.
A lot of the docks on this lake were built twenty to twenty-five years ago, and the lift that went on them was usually a 3/4 HP motor on a 2,800 to 4,800 lb plate. The boats of the day suited that.
Boats have got heavier since. A lift sized for what people run now draws more current than the one it replaced, so a supply that was adequate for decades can turn out to be marginal the day a new motor goes on — even though nothing about the wiring changed. The usual sign is a motor that runs fine empty and stalls part way up with a boat on it.
That is what this calculator is for. Tell it what is coming off and what is going on and it will show you the difference between the two on the wiring you already have.
The honest version of this is rarely “your wiring is bad.” It is usually that the run never had much margin, the old motor was living inside what margin there was, and the new one spends it.
Two things BH-USA tells you never to do
Both are the obvious workaround when a lift is a long way from a panel, and both are in their guide as a flat prohibition rather than a caution: never run a boat lift on an extension cord, and never run one off a generator.
The reasoning is the same as the rest of this page. A cord long enough to reach is a long run of thin wire added to a run that was already long, and it lands on the far end where it does the most harm. A generator has its own problems, and a motor with no thermal protector is not the thing to find them with.
Worth saying because it is not obvious that a temporary fix is a warranty question. In our experience a winding burnt this way gets treated as misuse rather than a manufacturing fault.
What we fit, and what it replaces
Older docks around here typically ran a 3/4 HP motor on a 2,800 to 4,800 lb plate. What goes on now:
- 6,500 lb plate with the 1 HP heavy duty. Cast iron worm gear, 96 tooth, on a 1/2 in. aluminum backplate — which is what we use, though it also comes powder-coated or galvanized. Ten-inch pulley, single belt.
- 7,500 lb plate with the 1.5 HP. Bronze worm gear on steel, galvanized or powder-coated. No aluminum option at this size.
- 8,500 lb plate with the 1.5 HP. Bronze worm gear, dual belt pulley.
All of them run a 5:1 reduction from the motor to the pulley and take the same 34 in. drive belt, which comes with the plate.
The other limit: fifteen minutes
The calculator sizes the wire. It cannot size the way the lift gets used, and that turns out to matter just as much.
These motors are TENV — totally enclosed, non-venting. Sealing a motor against lake air and wasps also seals in its heat, and there is no fan on the back to move it. So the nameplate carries a run time instead of the word “continuous.” BH-USA’s own Equipment Guide puts it plainly: the motors are “totally enclosed non-venting (TENV), meaning all thermal protection has been removed, giving these motors a maximum 15 minute run time.”
Both motors we fit read Time: 15 MIN on the nameplate. Both also read Thermally Protected: None.
That second one is the one to read twice. There is no thermal cut-out in these motors. Nothing opens, nothing clicks, nothing gives you a warning and resets itself in ten minutes. Run past what the motor can shed and the winding is the part that gives — and in our experience that gets treated as misuse rather than a defect.
What fifteen minutes actually means
Fifteen from cold, not fifteen out of every hour. A short-time rating is measured starting from a motor sitting at ambient, so a lift that has already been up and down twice does not have a fresh fifteen minutes in it.
On the gap between runs, BH-USA’s motor guide says the motors overheat with “about 15 minutes of continuous running or continual start and stop without at least 5 minute cool down between each cycle.” The gear plate wants its own rest as well — their guide asks for three to four minutes after a lifting cycle before running it again.
In ordinary use none of this is a constraint. A boat goes up in well under a minute. You would have to be doing something out of the ordinary to get near fifteen minutes — and the out-of-the-ordinary things are exactly the ones people do: jogging the lift up and down to line up bunks, running it from the house on a remote, holding a spring-loaded switch through a long adjustment.
“It trips” is two different faults
Worth separating, because they get confused constantly and only one of them is a wiring problem.
- It trips on the first lift of the day, with the boat on it. That is the supply. The motor pulls harder as the weight comes on, the voltage sags, and it either bogs audibly or trips. This calculator is for that one.
- The first lift is fine and it quits after several. That is heat, not wire. The run may be perfectly good. The motor has been asked for more cycles than it can shed between them, and the fix is time rather than copper.
A meter settles it. Put one on the outlet and lift the boat. If the voltage holds up and it still quits after repeated runs, stop looking at the wiring.
For the electrician: 430.22(E) applies, and changes nothing
A hoist is short-time duty and these are 15-minute rated motors, so Table 430.22(E) permits sizing the conductor at 120% of nameplate rather than 125% of the Table 430.248 value. For the 1 HP that is 15.6 A instead of 20 A. It is a real permission and it is a smaller number.
It does not reach, though not for the reason you might expect. Ampacity really does decide the size on shorter runs — out to somewhere between 57 and 166 ft, depending on the motor and the voltage — and on those runs both rules land on exactly the same conductor, so the permission has nothing to give. Past that crossover the size is set by voltage drop instead, and at 150 ft the answer already carries two to three times the ampacity either rule asks for. Shrinking a requirement that was not the binding one does not shrink the conductor. Every size in the tables above is identical under either rule, at every length from 1 to 400 ft, and there is a test in the build that fails the day that stops being true.
One difference worth naming: BH-USA heads their chart 115 V and 230 V where this page says 120 and 240. That is the same supply described by its older nominal figures, and it errs the safe way — a size worked out at 115 V is if anything heavier than the same run needs at 120.
Two other reasons to leave it alone. BH-USA publishes its own wire chart, and 110.3(B) makes it binding wherever it is the heavier of the two. And older editions of the Code printed the caution directly beneath this very table — “For long runs, it may be necessary in order to avoid excessive voltage drop, to use conductors of sizes larger than the minimum.” The modern printing dropped the cue, not the physics.
What this is and is not
It sizes a dedicated branch circuit for the lift over a known distance, which is how a lift should be fed. It assumes nothing else is on that circuit.
If your boathouse also has lights, a TV, a fridge or an outlet or two, those want their own supply. Once there is a small panel out there feeding several things, the run from the house stops being a branch circuit and becomes a feeder — a different calculation with a load assessment behind it, and squarely an electrician’s job. This tool will still tell you what the lift alone needs, which is a useful part of that conversation, but it is not the whole of it.
Use it to know what to expect and what to ask for. The install is an electrician’s call, and if you do not have one we are happy to point you at somebody.
Common questions
How long can I run the lift for?
Fifteen minutes, and that is the manufacturer’s number rather than ours — both motors read Time: 15 MIN on the nameplate. They also read Thermally Protected: None, so there is nothing inside to stop the motor if you go past it. BH-USA’s motor guide asks for at least five minutes of cool-down between cycles — that figure is in the older guide rather than on the current spec sheets, which publish the run time and no cool-down number.
Raising a boat takes well under a minute, so this is not a limit you meet by using the lift normally. You meet it by jogging it up and down to line up bunks, or by leaving a remote or a spring-loaded switch running while you do something else.
It trips after a few lifts, but the first one is fine. Is that the wiring?
Probably not. That pattern is heat rather than voltage. A motor that is short of voltage struggles when the weight comes on — so it fails on the first lift with the boat on it, not the fourth. A motor that is fine cold and quits after several runs has been asked for more cycles than it can shed between them.
Put a meter on the outlet and lift the boat. If the voltage holds up and it still quits after repeated runs, the run is not your problem and heavier wire will not fix it. Give it longer between lifts.
Does the run-time limit change what wire I need?
No, and it is worth being clear about why. The wire is sized from what the motor draws while it runs, which does not depend on how long you run it for. The fifteen minutes is a limit on operating the lift, not on the circuit.
It cuts the other way too: a correctly sized run buys you no extra minutes. Good wire and marginal wire will both cook a motor that is held on too long.
What size breaker should the lift be on?
BH-USA specifies a dedicated 20 amp circuit for a lift, and dedicated is the important half of that — a lift sharing a circuit with anything else is a different problem from the one this page solves.
The number on its own is not the answer, though. A breaker protects the wire, so the conductor has to suit it, and on the runs common around here the size this calculator gives you is set by voltage drop and comes out well above anything a 20 amp breaker needs. The two have to be decided together.
Which is why this is an electrician’s call rather than ours or yours. Bring them the conductor size from this page and BH-USA’s requirement, and they have most of what they need.
How do I measure the voltage drop myself?
With a meter and a boat, in about five minutes. This is the one number on this page you do not have to take on trust, and it is worth more than the prediction above it.
- Read it at rest first, at the outlet the lift is on. It will almost certainly look fine — 120-something. That is the trap, not the answer.
- Read it again while the lift is raising the boat, with the boat actually on it. BH-USA is explicit that in most cases low voltage will not show up until the load is on the hoist.
- Take the difference, and turn it into a percentage of the resting reading. That percentage is what this page has been predicting.
- Compare the two. If your measured drop is close to the predicted one, the wire is the whole story and the number above is your answer.
BH-USA’s own procedure takes three readings rather than one — at the main panel, at the lift switch, and at the motor — which tells you not just how much you are losing but where.
If the measured drop is much worse than predicted, that gap is the finding. The arithmetic on this page assumes clean connections and the wire it was told about. A drop well past prediction usually means a bad joint, a corroded connection or a splice nobody mentioned — and heavier wire will not fix any of those. Reading at an outlet with a meter is a homeowner job; readings at motor terminals or inside a panel are an electrician’s.
Is 12-2 twelve gauge or two gauge?
Twelve. The first number is the wire size and the second is how many conductors are inside, so 12-2 is 12 AWG with two conductors plus a ground.
It is worth getting right, because the two readings are nowhere near each other and the mistake flatters the wrong answer. Real 2 AWG is a heavy conductor that would sail through most runs on this lake. Actual 12-2 on a long run can sit well past the limit — the same numbers that look comfortable read as a problem once the size is entered correctly.
One more thing the marking does not tell you: if the jacket says AL or ALUM it is aluminum, which carries less current for the same size. Set the material to match.
My lift worked fine for years. Why would the wiring be a problem now?
Because the motor changed, not the wiring. Older lifts ran smaller motors that drew less current, and the run was sized for those. A motor matched to a modern boat pulls more, and that is what finds the weakness in a run that had been perfectly adequate until then.
Is 240 V better than 120 V?
At the same horsepower a motor on 240 V draws half the current, and voltage drop over a given run falls accordingly. Whether that is worth doing depends entirely on your situation — most docks here run 120 V and always have. Put your own numbers in above and the comparison will show you what the difference actually is on your run rather than in the abstract.
How do I know if my wire is copper or aluminum?
Look at the bare end of a conductor. Copper is the color of a penny. Aluminum is silver-gray. It matters because aluminum carries less current for the same size, so an aluminum run typically needs to be one or two sizes larger than the copper answer.
How do I measure the run?
The distance the wire actually travels from the panel to the motor, one way — not the straight line across the yard. Follow the path it takes, including any drop down a wall or run along a joist. If you are estimating, round up; it is the safer direction to be wrong in.
Why does it ask for the boat’s weight?
Only to pick the motor and show how much of the plate you are using. Weight does not change the wire size — conductors are sized from the motor’s rating, not from what it happens to be pulling at the time. But a boat close to the plate’s rating is exactly the setup where a marginal supply shows itself, so it is worth knowing where you sit.
My boathouse has lights and a fridge too. Does that change this?
It changes what the run from the house has to carry, yes. A lift should be on its own dedicated circuit — it is a motor, it draws hard when it starts, and sharing a circuit with other things is how you get nuisance trips. Once a boathouse has a small panel feeding several loads, the run out to it is a feeder rather than a branch circuit, and sizing it properly means adding up everything on the end of it. That is an electrician’s calculation. What this tool gives you is the lift’s share of it, which is a genuinely useful number to walk in with.
I have two lifts on one slip. Do I size for both running at once?
For the conductor, yes. The code sizes for everything on the circuit running together and an inspector will expect that, because nothing stops two people pressing two switches at the same moment. In practice it is unusual to raise both lifts at once, so the voltage drop you will actually measure is the one-lift figure — which the tool shows you as well. Size for the worst case, expect the ordinary one.
Can I use this for a lift that is not a Boat Hoist USA?
The motor rules are the same for any single-phase motor of the same horsepower, so the answers hold for a 1 or 1.5 HP lift motor generally. The nameplate figures and the plate pairings shown are for the BH-USA motors we sell and fit.
If it turns out to be the lift
We can take it from there
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