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ContinuousWave: Small Boat Electrical
Recommendations on Electrical Systems
|Author||Topic: Recommendations on Electrical Systems|
posted 05-01-2014 01:23 PM ET (US)
Many discussions of boat electrical systems seek guidance on suggestions for installation, mandatory practices, or regulatory compliance. I found a very good reference for this information from the United States Coast Guard. See
The PDF file (hyperlinked above) is part of Title 33 CFR, Section 183.401 to 183.460, and is a guide to addressing the provisions of the federal regulations. It warns that it is not a engineering manual for design of boat electrical systems. But it seems like a good resource to look for guidance.
Regarding over-current protection, the above literature also notes that conductors in the engine cranking circuit are exempt from requirements for over-current. See the illustration marked FIGURE 24.
posted 05-01-2014 01:48 PM ET (US)
The literature linked below is a very brief but informative paper on ignition protection. All electrical devices on a boat may require ignition protection if they are installed in the areas defined by federal regulations to require it. (See the above article for a source of the definition of those areas.) While attention is often focused on devices like starter motors or alternators, even simple electrical devices like a switch should be ignition protected if installed in certain areas. See
For example, a primary power switch, like a Blue Sea System model 5511s
meets the requirements of ignition protection. That particular component is certified to
--UL1500 Ignition-Protection Test for Marine Products, and
--SAE J1171 External Ignition Protection of Marine Electrical Devices
standards, and is suitable for installation in areas where ignition protection is required. You might not always be thinking of every component in the electrical system as being necessary to have ignition protection, but depending on where it the device is installed, ignition protection may be required.
posted 05-01-2014 05:37 PM ET (US)
Why are cranking circuits exempt from overcurrent protection?
Indeed I've never seen one in a boat or car.
posted 05-01-2014 08:31 PM ET (US)
Have you ever seen a fuse for several hundred amps?
posted 05-01-2014 10:10 PM ET (US)
My house has a 200 amp breaker. I would think the 240V wouldn't matter, current is current.
posted 05-02-2014 07:31 AM ET (US)
Chuck, I have 200 amp fuses between my console batteries and twin Optimax 150s. I have a spare 300 amp fuse as I was not sure how much current the engines would draw. They are not that big. This is fusing 00 gauge cable.
posted 05-02-2014 09:29 AM ET (US)
In automobile engine cranking circuits I have seen some indication of a "fusible link" being part of the wire conductor. I think that was just some part of the wire which would melt away if a very high current were to be drawn for an unusually long time. But that was perhaps years ago. I don't know if a fusible link is still common in automobile engine cranking circuits.
There is quite a difference in fusing depending on the voltage in the circuit that is going to be interrupted. If you want to interrupt a circuit with 12-VDC the fuse can be relatively simple because once the circuit starts to open the voltage across any air gap will only be 12-Volts, and 12-VDC is not able to jump across an air gap of much distance and start an electrical arc. If you want to interrupt 4,800-VAC, then you have to consider when your fuse elements opens there will be a rather spectacular arc, as 4,800-VAC will jump quite a large air gap. The fuse and its housing must take this into account. Usually some sort of mechanical arrangement will bring a insulating material into the path of the electrical arc, called an arc breaker. You have to stop the arc before it burns up the whole assembly.
Suppression of the arc is also important for use on a boat with gasoline fuel. If the fuse is to be located in an area requiring ignition protection, it must meet the ignition protection requirements.
posted 05-02-2014 01:46 PM ET (US)
The fusible link in cars has not been on the cable to the
starter in any case I've dealt with. That's always been a
big fat cable direct from battery to starter or solenoid.
The fusible links have been between the battery and ammeter
or smaller fuse blocks.
I once had a major short in my 240-Z that fried the fusible
posted 05-02-2014 03:51 PM ET (US)
Re fusible link not in cranking circuit on cars: this could be very true. I am going on memory, and just recall seeing them, not exactly what part of the circuit they were in.
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