Calculating Winch Amp Draw

I’m getting closer to installing a transom-mounted davit. It’s looking like the Dinghy Butler won out. It comes with a Terra 25SR winch that has a pulley system that reduces the lifting force by 1/2 or 2/3 (I’ll verify once I know).

The dinghy and all equipment would weigh an absolute max 600 lbs—likely less. The davit is a classic transom-mount that pivots near the swim platform and lifts the dinghy up and forward until it’s resting near the transom.

I’m looking for a way to calculate the load the winch will be handling, which (along with wire run distance and the other usual factors) will determine the amp draw and the wire size and circuit protection I’ll need.

I’m not sure how to extrapolate the amp draw from the table below, factoring in the reduction from the pulley arrangement. Is it simply a matter of determining the mechanical advantage from the pulleys and applying it to both of the values in the Terra table below? If the pulley advantage is 2:1, can I assume 50% less work by the winch = a 50% reduction in amp draw?

Thanks in advance.

If it were me, I would get a healthy battery and measure the load with the davit and dinghy all set up. A DC ring / clamp style multi meter should do the trick. I would add atleast 50% to the amp draw to cover voltage loss and amperage increase between wire run and battery bank voltage.

If you really want to do the math, it's not a straight lift, but pulleys add friction which increases load. Also if the water isn't flat, the transom rising and falling will effect momentary amp draw. Then there's the question of whether you change dinghies or outboards.

On second thought, I think I would wire it for near the maximum capacity (100 or 130 amps) and then adjust the fuse size to match the load.

Ted

Allow for pulley/line friction, say 30%.

My main concern is should you size for a stall or near-stall event at a higher load, like if the line got snarled.

That seems overkill. Sizing and tripping at around 100 amps would provide a torque limit effect if the tackle can handle it. Others here may chime in/know better then me. Recc a breaker over a fuse, I've been through that negative experience!

I am a retired engineer, so bear with me if I get a little nerdy and technical:

Looking at the video of the davit the lifting line pull is near horizontal and it levers the dinghy up and forward as you say. That levering will significantly reduce the load, at least 1/2 from the looks of it.

You say your winch has a pulley system that reduces the load further by 1/2 to 1/3. So the total force reduction is 1/4 to 1/6.

As others have noted, pulleys have friction which increases the load. Overall I think you would be ok if you figured the load on your winch was 1/3 of the weight of the dinghy.

Looking at the amperage chart, it indicates the draw will be less than 49 amps but more than 25 amps. But to be safe I would size the wire for 100 amps with a 0.5V drop. If your round trip wiring path from the battery to the switch to the windlass back to the battery were 20' then the required wire size is #4.

You generally size the fuse to match the wire's ampacity not the load. #4 wire has an ampacity of 160A, which coincidentally is just about the maximum that the winch will draw during normal operation. So size the fuse for 160A.

David

It is true that the purpose of supply-side circuit protection is to protect the wiring, not the device.

But, when over-sizing wire, I don't think it makes sense to over-size the circuit protection -- doing so negates a big benefit of over-sizing the wiring, which is providing more current-carrying and thermal headroom in the event of a problem. The same is also true if one is buying thicker wiring to provide a greater margin of safety for peripheral corrosion and damage at connectors, etc.

If the wire is over-sized and the circuit protection is not, when the overloading happens, the wire still has a lot of current-carrying capacity left, possibly making up for some "real world inefficiencies" such as a hot engine room, older connections, etc. This headroom is the reason I oversize wiring.

But, if the circuit protection is over-sized, then the current will get higher than it otherwise would, consuming the headroom, and waiting however long it takes to bake the higher ampacity breaker into tripping. This might be a minor risk -- but there really isn't an offsetting benefit. At any rate, in my mind, to some extent, unless I've left the circuit protection the same, buying thicker wire has cost more money, but not bought a greater margin of safety from an overload.

Sometimes I end up with the bigger breaker because I am "future proofing" something with plans to add more of a load later. But, this is explicitly balancing marginal amounts of present safety for marginal amounts of future convenience. At least in my thinking.

If I've got multiple devices, I set the breaker and wiring for the maximum total load -- and add device-side protection for each device aligned to its own demand.

I'd like to claim I have device-side circuit protection for all of my devices to protect their internals in addition to the external wire -- but I obviously don't. I have it for some things (radios, electronics, pumps, etc), but not for many things (lighting, 120VAC devices).

Windlasses are a weird case because they have very bursty energy demands. In these cases, I have always liked the idea of oversizing the wire and setting the breaker accordingly, but putting a fuse to spec near the device. That way, if one really wants to cheat for a short bit on a hard pull, knowing the risk to the windlass, one can swap out the fuse and the wire is ready.

So, in my world supply-side circuit protection is to protect the wire -- including any headroom that was designed in. And, device-side circuit protection is to protect the device from cooking itself if it fails.

Any rate, just thoughts from the peanut gallery.

Also keep in mind that winches are usually rated for line speed/amp draw/capacity differently for each layer on the winch drum. Absent that info, I would assume all numbers are best case which usually means the first layer. Everything else takes more power.

Ken

Assuming that Terra 25sr is a Superwinch, I found a lot more info here:

https://cdn.shopify.com/s/files/1/0742/2113/files/Terra_25_Syn_TIM.pdf?18090140891352743535

Also wanted to mention there is quite a bit of loss in a pulley so it’s not a direct reduction of effort.

From all the data, if I were setting it up for myself, I’d rig it for 75 or 100 amps.

Ken

These are really thoughtful replies, thanks.

Seems to be to be a consensus on oversizing the wire—which I try to do—but not so much agreement on circuit protection.

The dc buses at the main panel are fed with dual 4/0 cables from the house bank, so I don’t worry much about that end of things. This would be a 55-ft round trip from the main panel breaker—near the forward ER—to the transom and back to the negative bus in the aft ER.

When I plug the values for 100-amp draw into Blue Sea’s circuit wizard, I get 3/0 as the recommended size. (3/0 for voltage drop; #3 for ampacity) I don’t foresee operating the davit for more than 60 seconds at a time, but I doubled that to be conservative.

I’m probably being dense, but are some of you recommending sizing the circuit for 100 amps, with 100-amp breaker at the panel, and a slightly larger fuse near the load?

Another option: I have a spare fuse block coming out of the house bank (visible near top, between yellow-capped remote and red manual battery switches). All this, including negative DC bus, is in the aft ER, shortening the round trip to 36 ft or so.

Coming off that fuse with a battery switch and running the circuit to the transom only drops me to 2/0 cable, with 3% voltage drop. But would that be legal in lieu of a circuit breaker?

Main advantage would be running less cable. Also, it would be less convenient to wander down to the ER every time I wanted to operate the davit.

Only one fuse, at the main panel.

What does the chart give for voltage drop with 2/0 at 100 amps?

Ted

I recommend sizing the panel circuit protection for the minimum of the load and the wire, which should be the load.

My next best recommendation is to size the panel circuit protection to the wire. And, if the device draws significantly less current than the ampacity of the wire add a fuse or breaker sized to the lower value of the davit near the davit.

Most crane applications utilize solenoids to power the winch. These solenoids only need a few amps. On my boat there is a breaker on the panel in the pilothouse that provides power to the solenoids and the remote control. So, deactivating the crane is done by flipping the breaker to the solenoid power.

Ted

I have measured a half a volt drop in several windlass installations at normal load and about 1 volt at just before stalling and they worked fine. So I wouldn't be too hung up on 3%. Also remember the 100A design criteria that most of us are focused on is 1/3 of actual amperage.

So with a 55' run at 100 amps you get a voltage drop of 1.4 V for #4 (too much), .9 V for #2 (which is what I would recommend) and .56 V for 2/0 (which is overkill IMO). At the normal load of maybe 35 amps the drop is .3V for #2 wire which is better than most windlasses.

Use only one fuse near the source, ie where the new wire ties into the 2/0 or at the battery terminals. It probably makes little difference whether it is 100A or 160A, as it will never trip unless there is a jam.

David

O C Diver said:

Only one fuse, at the main panel.

What does the chart give for voltage drop with 2/0 at 100 amps?

Ted

Click to expand...

It’s an online calculator, so I inputted 3%. It spit out 2/0 @ 100 amps.

O C Diver said:

Most crane applications utilize solenoids to power the winch. These solenoids only need a few amps. On my boat there is a breaker on the panel in the pilothouse that provides power to the solenoids and the remote control. So, deactivating the crane is done by flipping the breaker to the solenoid power.

Ted

Click to expand...

I need to do more reading. I think a solenoid comes with the winch.

gkesden said:

I recommend sizing the panel circuit protection for the minimum of the load and the wire, which should be the load.

My next best recommendation is to size the panel circuit protection to the wire. And, if the device draws significantly less current than the ampacity of the wire add a fuse or breaker sized to the lower value of the davit near the davit.

Click to expand...

Part of my issue is not being clear on what the load will actually be with pulley reductions, friction and potential hang ups.

DavidM said:

I have measured a half a volt drop in several windlass installations at normal load and about 1 volt at just before stalling and they worked fine. So I wouldn't be too hung up on 3%. Also remember the 100A design criteria that most of us are focused on is 1/3 of actual amperage.

So with a 55' run at 100 amps you get a voltage drop of 1.4 V for #4 (too much), .9 V for #2 (which is what I would recommend) and .56 V for 2/0 (which is overkill IMO). At the normal load of maybe 35 amps the drop is .3V for #2 wire which is better than most windlasses.

Use only one fuse near the source, ie where the new wire ties into the 2/0 or at the battery terminals. It probably makes little difference whether it is 100A or 160A, as it will never trip unless there is a jam.

David

Click to expand...

#2 would definitely be my preference from a workability perspective.

Thanks again, all.

DavidM said:

I have measured a half a volt drop in several windlass installations at normal load and about 1 volt at just before stalling and they worked fine. So I wouldn't be too hung up on 3%. Also remember the 100A design criteria that most of us are focused on is 1/3 of actual amperage.

So with a 55' run at 100 amps you get a voltage drop of 1.4 V for #4 (too much), .9 V for #2 (which is what I would recommend) and .56 V for 2/0 (which is overkill IMO). At the normal load of maybe 35 amps the drop is .3V for #2 wire which is better than most windlasses.

Use only one fuse near the source, ie where the new wire ties into the 2/0 or at the battery terminals. It probably makes little difference whether it is 100A or 160A, as it will never trip unless there is a jam.

David

Click to expand...

This is exactly what I would suggest too.

Ken

This is how the Dinghy Butler wiring is described in a good, but unofficial, installation video.

From battery to fuse to solenoid to winch, back to solenoid to ground. (photo also shows wireless receiver.)

Any compelling reason to have this circuit switched (or originating on a panel breaker), or does the solenoid perform that function adequately/safely?

The wiring from the solenoid to the winch appears to be #6. If I need #2 to get from the panel or house bank to the solenoid, is the #6 undersized?

Sorry to have so many questions but I only want to do this once.

(I do plan to cap/insulate the positive lugs.)

Is your remote control wireless?

If not, do as I suggested in post #12.

If the winch is reversed by switching the polarity of the wires at the solenoid, I would mount the fuse and then the solenoid next to where you will draw power (batteries or distribution panel). That eliminates a long run that is always energized. My crane is wired this way.

Regarding wire size, there isn't a chance I would use #6. Why don't you price out #2, 1/0, and 2/0. And see what the cost difference is. There is no down side to bigger wire. Nobody ever said, "I wished I'd run smaller wire so I could have a larger voltage drop ".

Ted

O C Diver said:

Is your remote control wireless?

If not, do as I suggested in post #12.

If the winch is reversed by switching the polarity of the wires at the solenoid, I would mount the fuse and then the solenoid next to where you will draw power (batteries or distribution panel). That eliminates a long run that is always energized. My crane is wired this way.

Regarding wire size, there isn't a chance I would use #6. Why don't you price out #2, 1/0, and 2/0. And see what the cost difference is. There is no down side to bigger wire. Nobody ever said, "I wished I'd run smaller wire so I could have a larger voltage drop ".

Ted

Click to expand...

Thanks, Ted. Yes, it’s wireless. Good thoughts on location. To be clear, I’m not considering #6. I was asking if the existing #6 on the harness that comes with the winch is too small.

DavidM said:

I am a retired engineer, so bear with me if I get a little nerdy and technical:

Looking at the video of the davit the lifting line pull is near horizontal and it levers the dinghy up and forward as you say. That levering will significantly reduce the load, at least 1/2 from the looks of it.

You say your winch has a pulley system that reduces the load further by 1/2 to 1/3. So the total force reduction is 1/4 to 1/6.

As others have noted, pulleys have friction which increases the load. Overall I think you would be ok if you figured the load on your winch was 1/3 of the weight of the dinghy.

Looking at the amperage chart, it indicates the draw will be less than 49 amps but more than 25 amps. But to be safe I would size the wire for 100 amps with a 0.5V drop. If your round trip wiring path from the battery to the switch to the windlass back to the battery were 20' then the required wire size is #4.

You generally size the fuse to match the wire's ampacity not the load. #4 wire has an ampacity of 160A, which coincidentally is just about the maximum that the winch will draw during normal operation. So size the fuse for 160A.

David

Click to expand...

David I correct. If the anchor gets stuck the motor will draw it maximum current. You dont want to be blowing fuses at that time.

bayview said:

David I correct. If the anchor gets stuck the motor will draw it maximum current. You dont want to be blowing fuses at that time.

Click to expand...

Well this is a dinghy winch, right? If the dinghy gets jammed I do want the fuse/breaker to blow if I sit there with my hand on the switch for too long.

Same thing with a windlass and anchor. And you shouldn't try pulling an anchor out of the mud with the windlass. You should move the boat forward with the engine and then break it out when near vertical with a little punch from the engine before hauling it up with the windlass. The windlass should only be used for lifting the chain and anchor once the anchor has been broken out.

David

I think you are "stacking" a lot of assumptions to be on the safe side.
Pulley: halves the effort (and doubles the run time. I would suggest 5-7% loss, at most, depending on bearing type.
Wire: 3% is unnecessarily low drop. Starters commonly have 20% drop in freezing weather and 10% is considered acceptable in warm weather.
The idea of running something like 2/0 for that size intermittent use motor seems a waste to me.
PS: undersized fuses have a drop of their own. Not so breakers.

angus99 said:

Thanks, Ted. Yes, it’s wireless. Good thoughts on location. To be clear, I’m not considering #6. I was asking if the existing #6 on the harness that comes with the winch is too small.

Click to expand...

How long is the #6 wire? Depending on length and amperage, it may be an acceptable wire size. My preference would be to run from the winch to the solenoid without an additional junction.

Ted

O C Diver said:

How long is the #6 wire? Depending on length and amperage, it may be an acceptable wire size. My preference would be to run from the winch to the solenoid without an additional junction.

Ted

Click to expand...

Maybe 5-6 ft of #6 after I shorten the factory harness. It would be an uninterrupted run from the solenoid to the winch. Any breaker or fuse would be on the battery side of the solenoid.

bayview said:

David I correct. If the anchor gets stuck the motor will draw it maximum current. You dont want to be blowing fuses at that time.

Click to expand...

I stepped on the up bottom while the anchor was fully retracted just last weekend. I am quite thankful that the 100 amp fuse blew rather than tearing up my bow pulpit. As mentioned above pulling the anchor out of the mud is best left to forward motion as opposed to the windless.

DavidM said:

Well this is a dinghy winch, right? If the dinghy gets jammed I do want the fuse/breaker to blow if I sit there with my hand on the switch for too long.

Same thing with a windlass and anchor. And you shouldn't try pulling an anchor out of the mud with the windlass. You should move the boat forward with the engine and then break it out when near vertical with a little punch from the engine before hauling it up with the windlass. The windlass should only be used for lifting the chain and anchor once the anchor has been broken out.

David

Click to expand...

Thanks for clarifying, David. It’s a davit, not a windlass.

Brooksie said:

I think you are "stacking" a lot of assumptions to be on the safe side.
Pulley: halves the effort (and doubles the run time. I would suggest 5-7% loss, at most, depending on bearing type.
Wire: 3% is unnecessarily low drop. Starters commonly have 20% drop in freezing weather and 10% is considered acceptable in warm weather.
The idea of running something like 2/0 for that size intermittent use motor seems a waste to me.
PS: undersized fuses have a drop of their own. Not so breakers.

Click to expand...

Now we’re talkin’. I inputted 10% voltage loss with a 55’ run from a 100-amp breaker, 105C, and the Blue Sea calculator spits out #3 wire. I’d go to #2, I guess, to be conservative.

angus99 said:

Maybe 5-6 ft of #6 after I shorten the factory harness. It would be an uninterrupted run from the solenoid to the winch. Any breaker or fuse would be on the battery side of the solenoid.

Click to expand...

How will you join the #6 to the #2, a heat shrinked but splice?

If it's manageable, I would replace the last 6' of #6, eliminate the junction, and have the wire size the same for the whole way. That additional 12' is negligible in the overall cost.

Ted

O C Diver said:

How will you join the #6 to the #2, a heat shrinked but splice?

If it's manageable, I would replace the last 6' of #6, eliminate the junction, and have the wire size the same for the whole way. That additional 12' is negligible in the overall cost.

Ted

Click to expand...

I assumed the wires coming out of the winch terminated internally and I was stuck with #6, but—looking at the manual—they actually bolt to studs. So, yes, I can use lugs to run nothing but #2. Nothing like reading the instructions!

The only terminations will be at the breaker, the solenoid, the winch and the DC negative bus.