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 Starter motor amps		  New Topic  Topic Locked
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islander
Master Marine Consultant

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USA
3992 Posts
Initially Posted - 04/13/2018 :  07:01:22  Show Profile
So I got my new Tohatsu 9.8 and trying to figure out how many Amps it takes. On the starter it says .5kw. Went to an online kw to amp converter and for a 12v system it calculated it as 41.6 Amps. My question is what size fuse to use, 50?
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


jduck00
Captain

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313 Posts
Response Posted - 04/13/2018 :  07:22:24  Show Profile
That's what I would start with. I would also go with a slow blow being a starter motor. If it blows, go with the next step up. The amps will end up being the 500W divided by the voltage on the motor terminals during a start.

On my Yamaha 10, 50A should have been enough but I had to end up fusing up a bit. I would end up blowing the fuse if I really had to crank the motor for a while to get it to start. The blow time is a matter of heat and time.
Jeremy Duck
The Lucky Duck
1980 SKSR Hull # 1850
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dasreboot
Admiral

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803 Posts
Response Posted - 04/13/2018 :  07:48:53  Show Profile  Visit dasreboot's Homepage
whats the wire size? that determines the fuse rating.

http://bluesea.com/files/resources/reference/20010_Rev002.pdf
Todd Lewis
Eowyn 87 TR/WK C25 #5656
ARWEN 84 TR/SK C25 #4031
www.mainsailsailingschool.com
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islander
Master Marine Consultant

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3992 Posts
Response Posted - 04/13/2018 :  08:43:29  Show Profile
quote:
whats the wire size?

Heck of a good question. The motor comes with 8FT of battery wire but so far I can't figure out the size. Just these numbers on the jacket. My plan is to cut those wires long enough to feed through the transom then to a dual power post mounted above the little shelf at the back of the quarter berth so I have a disconnect point when removing the engine. From there another line to the battery that will have a battery mounted fuse.
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


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Dave5041
Former Mainsheet Editor

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3754 Posts
Response Posted - 04/13/2018 :  09:27:05  Show Profile
I have a similar set up that I think is a Minnkota. I have no leaks, heat or current issues with the # 6 cables and its very convenient for motor removal. I would recommend starting with no more than a 50A slow blow fuse. Most motors draw less than that under normal use, 30-40. The max current load rating is only reached when an electric motor is stalled, which might happen when you first hit the button or it can't crank the motor due to a mechanical problem. My Merc Bigfoot has the same 9.9 Tohatsu powerhead. You can always go to a bigger fuse, but the lowest rated fuse that is adequate provides the most protection.

Dave B. aboard Pearl
1982 TR/SK/Trad. #3399
Lake Erie/Florida Panhandle
Edited by - Dave5041 on 04/13/2018 09:49:32
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islander
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3992 Posts
Response Posted - 04/13/2018 :  13:53:40  Show Profile
The slow blow sounds like the ticket but if someone could point me in the right direction cuz I cant seem to find them for a battery post fuse block. All I get is the Blue Sea one pictured below and they don't seem to be slow blow. I'm going with 4gauge just because Wire is expensive and a little overkill isn't bad.
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


Edited by - islander on 04/13/2018 13:55:57
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Dave5041
Former Mainsheet Editor

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3754 Posts
Response Posted - 04/13/2018 :  19:01:33  Show Profile
More capacity never hurts. It depends on the fuse type. Standard blade fuses at Grainger (Littelfuse) come in 20,40,50, and 60. I'm not sure about Maxi blade fuses. Some terminal blocks have 2 fuses in parallel, each is half the amperage or one is fast and one is slow.

Dave B. aboard Pearl
1982 TR/SK/Trad. #3399
Lake Erie/Florida Panhandle
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jduck00
Captain

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USA
313 Posts
Response Posted - 04/14/2018 :  07:25:51  Show Profile
The Blue Sea 50A terminal fuse is close enough to slow blow. Its not specifically called out, but the TCC is what a slow blow fuse will look like.

https://www.bluesea.com/products/5177/MRBF_Terminal_Fuse_-_50A
Jeremy Duck
The Lucky Duck
1980 SKSR Hull # 1850
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islander
Master Marine Consultant

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USA
3992 Posts
Response Posted - 04/14/2018 :  10:35:22  Show Profile
Thanks Jeremy, This motor replacement and all the bits and pieces was definitely not in the plans so I want the right parts the first time. They don't take electrical parts back.
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


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Voyager
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5231 Posts
Response Posted - 04/15/2018 :  07:08:03  Show Profile
Scott, When I rewired Passage several years ago I started with a 50 A fuse for the Honda starter circuit. It worked for awhile but as you point out, if the engine got flooded or didn't start right away, the fuse overheated and blew. That told me the fuse was too small, so I put in a 60 A fuse and bought half a dozen spares. That did the trick.
Fuses are consumable items, so it doesn't hurt to have several spares.
And if you do eventually blow a 60A fuse, you've likely got a different problem.
Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain — Milford, CT
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islander
Master Marine Consultant

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USA
3992 Posts
Response Posted - 04/15/2018 :  08:31:28  Show Profile
Thank you Bruce! I wasn't getting a definite answer so I literally just ordered a 60 amp fuse as a shot in the dark. The cube fuses for the Blue Sea post mount aren't cheap at@ $20 a pop The mount they sit in are @$28. So just throwing $20 fuses at it until you get the right one could get a little expensive
. I did better on EBAY with the mount at $20 and the 60amp fuse at $13.
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


Edited by - islander on 04/15/2018 08:46:33
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glivs
Admiral

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822 Posts
Response Posted - 04/15/2018 :  09:43:35  Show Profile
I see where Scott’s 50A estimate comes from and understand that prolonged cranking may cause a fuse to overheat. I do not understand, however, seeming differences in systems. The starter on my Mercury 9.9 is protected by a 20A fuse under the cowling and a 35 or 40A fuse at the battery that I installed (I don’t recall which and am on travel so cannot look.) So...I was under the impression that Mercury and Tohatsu were the the same engine, or does that only apply to the power head? And why such a large difference in amperage requirements between different makes?

Service manual p.48:
The electric starting circuit is protected from overload by a 20 AMP fuse. If the fuse is blown try to locate and correctthe cause of the overload. If the cause is not found, the fuse may blow again.
Gerry & Leslie; Malletts Bay, VT
"Great Escape" 1989 C-25 SR/WK #5972
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islander
Master Marine Consultant

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USA
3992 Posts
Response Posted - 04/15/2018 :  11:52:54  Show Profile
ABYC rules say a fuse is to be placed no more than 7 inches from the battery.This is why I want the battery post fuse. Remember that a fuse only protects what ever is downstream from the fuse. The wire running from the battery up to the fuse is not protected.
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


Edited by - islander on 04/15/2018 11:55:14
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Dave5041
Former Mainsheet Editor

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3754 Posts
Response Posted - 04/15/2018 :  14:26:35  Show Profile
glivs was quoting the service manual. 20A does seem low, but a 20A fuse on a 6 A circuit is pretty high. None the less, I would be inclined to believe the service manual but keep the larger fuse at the battery. The start button on my old 2 stroker shorted and started to melt the plastic block by the time the fuse blew, but of course the 6 ga wire was fine.

Dave B. aboard Pearl
1982 TR/SK/Trad. #3399
Lake Erie/Florida Panhandle
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islander
Master Marine Consultant

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3992 Posts
Response Posted - 04/15/2018 :  14:49:15  Show Profile
The starter system has 2 circuits. One is the battery cables and is a high amp circuit. The other circuit is a low amp circuit consisting of the start button down to the solenoid. When you hit the start button it closes the solenoid to complete the high amp circuit from the battery to the starter. The 20amp fuse only protects the low amp circuit in case you have a short in the start button or the solenoid or the wires in between. It doesn't protect the high amp battery cable circuit. They do this so they don't have to have big heavy guage wires running up to a large start button. Same in a car.
.
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


Edited by - islander on 04/15/2018 14:57:15
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dasreboot
Admiral

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803 Posts
Response Posted - 04/15/2018 :  14:59:37  Show Profile  Visit dasreboot's Homepage
quote:
Originally posted by islander

quote:
whats the wire size?

Heck of a good question. The motor comes with 8FT of battery wire but so far I can't figure out the size. Just these numbers on the jacket.


Its 3/0 wire (bout 1/2 inch in diameter) from the chart. it can handle 225 amps inside of an engine room. You can put any fuse up to 225 amps on that cable. The fuse is sized for the wire size, not the expected load. It keeps the cable from overheating and burning if there is a short.
Todd Lewis
Eowyn 87 TR/WK C25 #5656
ARWEN 84 TR/SK C25 #4031
www.mainsailsailingschool.com
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glivs
Admiral

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USA
822 Posts
Response Posted - 04/15/2018 :  17:36:20  Show Profile
Thanks for the clarification on how the starter circuit functions...it now makes sense.
Gerry & Leslie; Malletts Bay, VT
"Great Escape" 1989 C-25 SR/WK #5972
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