Starter Cable Size Question

[ramblinrod]

Originally Posted by mitiempo

I agree. I believe ABYC requires 3% drop for motors. Even if they don't it is the better choice as the losses from connections will add to the drop.

Please show me the ABYC requirement to use 3% voltage drop for a motor.

[Boatwright]

Inrush current vs. running current in starter circuits:

Electric motors draw maximum current @ zero RPM.

Inrush currents of 300 - 400 amps are only seen for <1 second. Once the engine is turning over the current goes way down.

Typical running currents for small sailboat auxiliaries are around 100 - 150 amps for <10 seconds. With the new MRBF (marine battery fuse) requirement we have found that a 125 amp fuse works fine. For multiple seasons with a Yanmar 2GM, and a Volvo MD11 we have never blown a 125 amp MRBF.

The most common wire size in automotive starter applications is a #4, with some newer cars using a #2.

If you have to crank more than 10 or 15 seconds your problem is your engine tune NOT your starter.

For a 10% voltage drop, 12' round trip, and a 125 amp load, the wire size calculation yields a #4. Both fuse and wire will safely handle the brief inrush current.

Conclusion: Unless you have a much larger engine or very long runs, a #1 or #2 wire protected by a 125 or 150 amp MRBF is a bullet-proof setup. 3/0 wire is bulky overkill, and you will likely have practical problems routing and bolting up that large a wire to a sailboat auxiliary.

And back to the original question: Yes -- the negative wire needs to be the same size as the positive.

[Terra Nova]

Originally Posted by Boatwright

...The most common wire size in automotive starter applications is a #4, with some newer cars using a #2...

This is completely irrelevant to cruising boats, where a) most engines are high compression diesels, b) where being able to start your engine might be a matter of the life and death of your crew and c) where available battery power might likely be much less than a full charge.

People--just don't listen to such nonsense as trying to save a few dollars by under-sizing your starter circuit cabling. These are typically rather short runs (though often considerably longer than those in an automobile).

[ramblinrod]

Originally Posted by Terra Nova

This is completely irrelevant to cruising boats, where a) most engines are high compression diesels, b) where being able to start your engine might be a matter of the life and death of your crew and c) where available battery power might likely be much less than a full charge.

People--just don't listen to such nonsense as trying to save a few dollars by under-sizing your starter circuit cabling. These are typically rather short runs (though often considerably longer than those in an automobile).

Boatwright is absolutely correct.

One can always argue that bigger is better, but larger cable than required is simply not warranted.

In this case the goal is to use the lightest, least expensive cable, that will perform the job reliably under all conditions likely to be encountered.

If size 2 AWG cable turns the engine over at sufficient RPM to start, first time every time, under all reasonable conditions, without exceeding the maximum ampacity of the cable, there is no advantage to using heavier, harder to route, greater space consuming, more expensive cable.

Otherwise, one could argue that if 2 AWG cable is sufficient, 4/0 cable is better, so lets go with 1000 mm^2 cable as even better yet, but wait, why stop there?

In practice, we recommend drawing the line at the weight of cable required by the relevant standards authority. There is already safety margin included.

Production sailboats with factory wiring typically have 2 or 4 AWG cables running to the starter for auxiliary engines up to 40 HP.

[mitiempo]

Originally Posted by ramblinrod

Please show me the ABYC requirement to use 3% voltage drop for a motor.

ABYC calls for no more than 3% voltage drop for essential loads. Interior lighting is not essential but I think engine starting is essential.

[ramblinrod]

Originally Posted by mitiempo

ABYC calls for no more than 3% voltage drop for essential loads. Interior lighting is not essential but I think engine starting is essential.

Incorrect. ABYC requires cable selection for 3% voltage drop for circuits that will not tolerate greater. e.g. electronics which may brown out, navigation lights that may not emit required brightness.

For circuits that will tolerate 10% voltage drop, except for those explicitly stated, that is all that is required. Starters will (and have always) tolerated 10% voltage drop from even a nominally charged battery. (There may be exceptions such as something seriously wrong with the engine making it unusually hard to start, but that is another issue, and wiring is not meant to cover for every possible fault contingency, else, again if 2 AWG is compliant, why stop at 4/0?)

[Maine Sail]

Originally Posted by ramblinrod

Production sailboats with factory wiring typically have 2 or 4 AWG cables running to the starter for auxiliary engines up to 40 HP.

In the 60's 70's & 80's this was true but today even small Catalina's, Beneteau's, Hunters, Hanse and other production boats are all pretty much using 1/0 or larger for battery wiring including starting circuits. I was on two production boats yesterday a Hanse 400 (2/0) (Euro blue/brown) and a Beneteau First 36.7 (2/0) both were factory wiring. I was on a Catalina 32 on Friday which was also (2/0).

We can't forget that safety standard requirements now dictate house bank over current protection and if an engine can be started off either bank, as it can on most boats, then the cabling must be able to handle starting and the OCP will also be required to handle it without exceeding max amapacity. Fusing needs to be sized to not nuisance trip and this means 250A and larger for most small sailboat AUX engines. Builders are also offering factory inverter/charger installations which also most often dictate 1/0 and larger wire...

Ignoring the ABYC, Delco-Remy wants to see no more than a 0.5V drop in a 12V nominal starter circuit when a 500A load is applied to that circuit.

[ramblinrod]

My responses in red.

Originally Posted by Maine Sail

In the 60's 70's & 80's this was true but today even small Catalina's, Beneteau's, Hunters, Hanse and other production boats are all pretty much using 1/0 or larger for battery wiring including starting circuits. I was on two production boats yesterday a Hanse 400 (2/0) (Euro blue/brown) and a Beneteau First 36.7 (2/0) both were factory wiring. I was on a Catalina 32 on Friday which was also (2/0).

I will review this before responding.

We can't forget that safety standard requirements now dictate house bank over current protection and if an engine can be started off either bank, as it can on most boats, then the cabling must be able to handle starting and the OCP will also be required to handle it without exceeding max amapacity.

Correct, but irrelevant to the starter cable size discussion. The starter cable size is the starter cable size, regardless of the circuit details. If the starter can be operated form the house bank, then that is the starter cable, and it must be sized to suit the starter, or larger load that may be present.

Fusing needs to be sized to not nuisance trip and this means 250A and larger for most small sailboat AUX engines.

I disagree on the fuse rating. (See below.) ABYC does not requiring fusing for starter batteries, though I personally believe it is a good idea and recommend it to customers.

Builders are also offering factory inverter/charger installations which also most often dictate 1/0 and larger wire...

Correct, but irrelevant to starter cable discussion.

Ignoring the ABYC, Delco-Remy wants to see no more than a 0.5V drop in a 12V nominal starter circuit when a 500A load is applied to that circuit.

And here is the rub, lets look at some common current production small sailboat engine starters:

Yanmar 2YM15 - 4JH5E, rated 1.4 kW @ 12 Vdc or 116 FLA (full load amps).

So if the 4 cylinder 40 HP engine runs up close to FLA (it won't under normal circumstances), the 2 cylinder 15 HP will never see more than 60A nominal.

[mitiempo]

Quote "ABYC does not require fusing for starter batteries......"

Actually ABYC does not require fusing for DEDICATED start batteries. That means a battery that is used for starting only, usually with a simple on/off switch.

If the battery banks are wired through a 1/2/Both/Off switch the start bank is not dedicated to starting and needs fusing per ABYC and common sense.

In my opinion all banks should be fused - it is easily done.

[ramblinrod]

Originally Posted by mitiempo

Quote "ABYC does not require fusing for starter batteries......"

Actually ABYC does not require fusing for DEDICATED start batteries. That means a battery that is used for starting only, usually with a simple on/off switch.

If the battery banks are wired through a 1/2/Both/Off switch the start bank is not dedicated to starting and needs fusing per ABYC and common sense.

In my opinion all banks should be fused - it is easily done.

ABYC does not require a starting motor be provided overcurrent protection period.

If the max load on a house bank is 300 A and the cabling used is suitable for 300 A, then it needs to be fused for 300 A. That the starter circuit which may connected via any combining means, is only capable of handling 100 A, it does not have to be fused for 100 A (though as stated I agree it should be in my opinion too, this is not an ABYC requirement).

[Maine Sail]

Originally Posted by ramblinrod

My responses in red.



And here is the rub, lets look at some common current production small sailboat engine starters:

Yanmar 2YM15 - 4JH5E, rated 1.4 kW @ 12 Vdc or 116 FLA (full load amps).

So if the 4 cylinder 40 HP engine runs up close to FLA (it won't under normal circumstances), the 2 cylinder 15 HP will never see more than 60A nominal.

I work based on actual field measurements at the "as installed" level...


Here's a 1.4kW starter motor..

This is what the current graph looks like.


Here is average voltage (12.04V) & average cranking current from loaded starter to unloaded (286A). We already know the peak current hit around 640A... It also shows this motor starts, as one should, loaded starter to unloaded, in about .76 seconds.



I have a 1.2kW starter in my shop right now that eats 100A +/- spinning into free air (no load current) and draws about 375A for in-rush, again into free air (no engine load just the in-rush). Average cranking for this starter, which fits a small two cylinder diesel, is in excess of 200A..

If you run the numbers for Yanmar's starter cable requirements, based on their tech manuals, and convert to the closes AWG, you come up with a maximum allowable voltage drop of approx 1.75% or less than a 2% allowable voltage drop, in a Yanmar starting circuit.

This is quite a difference from a 10% allowable voltage drop for a Yanmar/Hitachi starter motor as being acceptable...



20mm Squared is closest to 4 AWG
40mm Squared is closest to 1 AWG

[Marqus]

Thanks Maine Sail, for taking the time to inject a real world data point into the thread.

Must say, very nice to see the Amp draw vs time graph, thanks. I've only ever measured using a max-hold function of a dmm.
(Does the Midtronics device use a typical current clamp?)

Your views on target voltage drops and cable thickness deserve to be given due weight since you back it up with coal face data.

[Maine Sail]

Originally Posted by Marqus

Must say, very nice to see the Amp draw vs time graph, thanks.

The difference in start duration, loaded to unloaded, changes and gets longer with the more voltage drop or battery sag you have.

Originally Posted by Marqus

I've only ever measured using a max-hold function of a dmm.
(Does the Midtronics device use a typical current clamp?)

Yes the Midtronics uses a DC clamp and also has two sets of test leads that can be used depending upon the diagnostics you are running..

Most DVM DC clamp meters are far too slow to capture and hold a peak or average cranking current. For that you need a very fast clamp meter, one capable of DC in-rush, such as a Fluke 376. The nice thing about the Midtronics tool is that it captures all of this for you but it is certainly not a worthwhile investment for a DIY..

For averages one can glean marginal data by cranking with the stop or fuel solenoid shut. Still won't be very accurate as the current is not stable during cranking, and you're looking at a slow response digital screen, but it can give you a ball park idea.

The Midtronics EXP-1000HD AMP KIT is an invaluable tool for me and saves both myself and my customers a lot of time and labor. It can give you cranking data but also conduct voltage drop tests, ripple tests, and all sorts of other testing useful for marine applications.

It does however SUCK as a battery tester for house bank batteries and is not even close to accurate for anything other than cranking ability. It is designed to measure a batteries ability to deliver impulse current not Ah capacity. None of the impedance testers, not just Midtronics, are good at predicting state of health or Ah capacity for deep cycle batteries.

[Marqus]

Thanks for the alert regarding the Max-Hold feature of commonplace DMMs. I had been using a mid-price DMM with a 400A DC clamp range to get an estimate of 250A for max inrush on my 1.2kW Bosch starter, but now suspect it could be an underestimate.

I have a digital oscilloscope (about the size of a small shoebox) which I could run off an inverter to potentially map my starter current in detail, but I would need to buy a stand-alone current clamp or a spare 500A shunt in order to do that. Quite fancy the idea of knowing the peak inrush for my starter motor.

[ramblinrod]

Originally Posted by ramblinrod

My responses in red.



And here is the rub, lets look at some common current production small sailboat engine starters:

Yanmar 2YM15 - 4JH5E, rated 1.4 kW @ 12 Vdc or 116 FLA (full load amps).

So if the 4 cylinder 40 HP engine runs up close to FLA (it won't under normal circumstances), the 2 cylinder 15 HP will never see more than 60A nominal.

Since this post, I have discovered that the starter power rating (1.4 kW) does not relate to the FLA rating, but rather the optimum efficiency point of the motor, and that the actual operating current may be higher.

Notwithstanding, I am not so concerned with instantaneous inrush current (the max peak current on initial start-up) as I am with the nominal current during normal and reasonable operation for the given application, and the size of fuse or breaker required to avoid nuisance tripping.

I would be shocked to see a 100 A fuse blow in the starter circuit of an Auxiliary diesel up to 40 HP or a gas engine up to 400 HP, unless there was something seriously wrong with the engine or starter circuit.