Question- Electric Windlass Wire Sizing

[Jon W]

I have a question regarding wire sizing for an electric windlass.

I'm installing my Maxwell HRC FF8-8 horizontal electric windlass. It has a 70A breaker/isolator and 600 watt motor.

My complete positive/negative cable circuit will be between 40-50 feet. From my ABYC chart, that circuit with 10% voltage drop can be a 4 AWG cable.

The Maxwell owners manual provides gauge sizes for a 10% voltage drop. For a circuit length between 33-56 ft it suggests using 3 AWG.

How do I determine the start up current under load of the 600 watt motor and the demand it will put on the circuit? Should I design for that, or for steady state and 10%?

Thanks for the help.

[Stu Jackson]

Might want to check again, I do not think they make 3 AWG:  https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity

[Jon W]

Hi Stu,

The cable size table in the owners manual has mm squared and AWG. They call for 1, 3, 5 gauge depending on circuit length. The 3 gauge is 26 mm squared. I couldn't find a 3 gauge, but guess it's their way of converting from metric to US gauge size.

[Ron Hill]

Jon : Call Maxwell and ask them about your installation.  They will ask you where the closest battery to the windlass is located. 
Depending on that length of that run, they'll recommend the gage of the wire.

A thought

[J_Sail]

Ron,
The info you refer to is basically what Jon quoted the manual. They may tell him more on a phone call, but they might just point to the manual. I think Jon is looking for additional guidance from someone experienced (hopefully an expert) who knows how much the motor will draw when starting under a load (i.e. anchored) and how much voltage sag is acceptable during starting. That would let him determine if he is okay with #4 AWG at his length or should increase to 2 AWG, with them commensurate cost, and difficulty in routing.

Regarding 3 AWG, Jon is correct, that is a common issue in converting from metric. For reference, each change of 3 gauge steps is an almost exact doubling in cross-section. Thus, 3 AWG is the same area as a pair of 6 AWG wires and is half of 0 AWG. So even though it is not commercially available, it is fine as a spec.

Hopefully MaineSail will see this and offer some sage advice.

[Stu Jackson]

Perhaps we can simplify this:

Using the West Marine chart, which is as simple a resource as any table, and derived therefrom, 45 feet is #2 wire, 50 feet is #1 wire for a 3% drop.  600 watts = 50A, which makes a 70A breaker reasonable.  The motor is the motor. 

Jeremy, would there be a startup higher load for a simple motor?  I doubt it, but please chime in, doesn't need Maine Sail.

ITWMB, I'd use #2 wire, just like many, many folks have done in the past.  It's between 3% and 10% voltage drop, with the engine running when the windlass is operating, thus with even higher voltage.

And you're right, #3 wire simply doesn't exist, so cross section means nothing because they don't make it.  In my engineering business, I would only specify what was actually produced by manufacturers.  Just kiddin' here...

[mainesail]

I have a question regarding wire sizing for an electric windlass.

I'm installing my Maxwell HRC FF8-8 horizontal electric windlass. It has a 70A breaker/isolator and 600 watt motor.

My complete positive/negative cable circuit will be between 40-50 feet. From my ABYC chart, that circuit with 10% voltage drop can be a 4 AWG cable.

The Maxwell owners manual provides gauge sizes for a 10% voltage drop. For a circuit length between 33-56 ft it suggests using 3 AWG.

How do I determine the start up current under load of the 600 watt motor and the demand it will put on the circuit? Should I design for that, or for steady state and 10%?

Thanks for the help.

I won't bore you with my diatribe on windlass engineers who apparently don't understand DC motors & perhaps Ohm's law. An allowable 10% voltage drop? Are you freaking kidding me? Apparently they have forgotten about battery voltage sag under high loads that are additive to this 10% VD........? Bottom line, the biggest wire you can fit will make your windlass happiest & perform the best...

A 700W 12V Lewmar V2 windlass can draw 294.6A just to start the motor turning (just had this fresh in my mind from two weeks ago). They ship it with a 90A breaker. They nuisance trip all the time because the motors 700W rating is with no load. This particular windlass is pulling 90-95A just to raise the anchor through the water. To pull the boat to the anchor, and no one ever does that of course, the windlass was drawing 115A - 120A... The owner was stumped as to why the factory supplied breaker nuisance trips...

[Jon W]

"Using the largest cable I can" comment got me thinking. I have left over 1 AWG from my electrical upgrade that I can use. Thanks, ironically in my case 1 AWG will save me money. May help the port list  .

The nuisance trip possibility is not good to hear. I'll call Maxwell next week and ask them about the real world loads to be expected.

Thanks everybody all the feedback really helped.

[KWKloeber]

Jon

Maybe this will help you

ken

[J_Sail]

Jon,
As you probably already know, you are free to put a larger breaker up to the ampacity of the wire, which is much higher than 70a. The only other consideration is that with motor loads (as I have posted before when discussing bilge pumps), it can be useful to have the breaker sized such that it will trip if you overload the motor so much that it's at risk of burning up its windings. Some motors have internal over-temperature protection, others don't. So if you call the manufacturer, you may want to ask if it is over-temperature protected, and if not, what they think is a good upper limit on a breaker to still get some protection against motor burnout if over loaded. For a 600w motor I would hazard a guess that 90-110a would be a good range to consider. (The manufacturer's tech support may or may not be sufficiently knowledgeable to help.)

Stu,
Yes motors draw much more current on startup (think back to the discussions of the short-term current for the starter motor on diesel engines).

In fact, motors draw more current anytime they are spinning slower than their no-load max rpm, whether it's due to starting inertia or mechanical load. When stalled (or starting from stationary) the current draw can easily be 3-10x. The wattage rating of a DC motor is for it spinning at full speed with no load. As you add mechanical load, the motor slows, and the current and wattage go up. So a 600w motor raising an anchor is going to draw more than 50A (and more than 600w) even after startup. I am not sure how much more Jon's winch motor will draw, though as I don't know how the winch is geared internally.

Jeremy

[J_Sail]

Upon further thought, a couple more tips:

1) I believe you would be very well served by a 100amp magnetic-hydraulic breaker of the type below:
https://www.bluesea.com/products/7144
or
https://www.bluesea.com/products/7187

Either should provide enough margin (and trip-delay-time) to avoid nuisance trips, but still provide reasonable protection against motor overheating damage if someone abuses it. The first one (187-series has a 5/16" stud and clearance to accept a 1/0AWG lug; the 285-series has 1/4" studs). Note that the delay curves shown on the two look more different than they are, due to different scales used. Both have better time delays than a Series-C breaker, which is less well suited to handle starting in-rush and momentary overloads.


2) Using your leftover #1AWG wire is a great idea. If you have enough for both the negative and positive runs in their respective colors that's great. If not, there is a very slight advantage to making the negative the heavier of the two. The windlass won't care, but when measuring voltages at various points, it's easier for a sailor to get confused when the drop on the negative is greater than the positive. On the other hand, it's even more important to follow proper color coding, so unless you want to do a lot of color tape application, the color of your leftover wire you have will dictate what it gets used for.