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Author Topic:   Fusing of Battery To Distribution Panel
coolarrow posted 12-30-2013 10:08 AM ET (US)   Profile for coolarrow   Send Email to coolarrow  
What size fuse would work best coming from the starboard battery to the distribution panel in a two battery system with a single engine?
jimh posted 12-30-2013 11:48 AM ET (US)     Profile for jimh  Send Email to jimh     
I believe you are asking about current protection for a circuit that is providing power to a secondary power distribution panel from a primary power bus or switch. Such a circuit should have over-current protection. It is more common that a circuit breaker is used than a fuse for this service. A circuit breaker can be reset when tripped, while a fuse, of course, has to be replaced. If you use a fuse, be certain to carry several spare replacement fuses.

Let us look at a commercial power panel for 12-Volt distribution and see what the manufacturer says. I turn to BLUE SEA SYSYTEMS. On my boat, I am using a secondary power panel PN5026:

http:/ / www. bluesea. com/ products/ 5026/ ST%20Blade%20Fuse%20Block%20 With%20Cover%20-%2012%20Circuit%20with%20Negative%20Bus/ featured

This panel is rated for a maximum of 100-Amperes. So the circuit feeding it should be fused for no more than 100-Amperes,

On my boat this panel is fed with a conductor of 8-AWG. A conductor of 8-AWG is rated at a fusing current of 472-Amperes. (See this listing for current capacities by wire size.) It would make no sense to use a 472-Ampere fuse, as the panel is only rated at 100-Amperes.

On my boat, the distribution panel presently has these circuits with individual fuses

20 - Cigar lighter socket
10 - Cabin lamp
10 - Cabin lamp
7.5 - Sump pump
7.5 - Marine Band radio transmitter
5.0 - Anderson PowerPole connector
3.0 - Chart plotter

That is a total fused current of 63-Amperes on the panel. The same power distribution circuit also feeds a circuit breaker bus that has three circuit breakers. I don't recall their exact ratings. I believe they are at least 5-Amperes. Let us add 15-Amperes to the total, giving 78-Amperes for the total fused current of the loads.

In my installation, this distribution circuit is protected by a circuit breaker with a rating of 50-Amperes. That is a rating that is about 65-percent of the total fused current of all the loads. I think this is a typical ratio. It would be very unusual for all of the loads on the circuit to be simultaneously operating at their maximum current. It is reasonable to use some lower value of current, a smaller percentage of the total. I don't know if there is a particular formula for calculating the over-current protection for this type of distribution.

To summarize, the over current protection for the conductor distributing power to a secondary panel should be

--equal or less than the maximum fused current rating of the conductor,

--equal or less than the maximum current rating of the distribution panel, and

--equal or less than the actual aggregate load on the panel.

In a typical installation, the current rating of the three components will be in descending order as listed above.

Your question cannot be answered because we have no information about any of these three components. Please give us:

--the conductor size that carries power to the secondary panel from the primary bus

--the current rating of the secondary panel

--the aggregate current of the loads on the secondary panel.

jimh posted 12-30-2013 12:05 PM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE: I noticed that on their website BLUE SEA SYSTEMS has an image showing their secondary power distribution panel installed in service on a boat. See

This is a panel rated for 100-Amperes "per block" and 30-Amperes "per circuit." In the (above linked) image, the panel has only nine of its 12 circuits connected. The total fused current of the nine circuits is 155-Amperes. In this application the total fused current of the circuits on the panel is 155-perent of the panel's rated maximum current. I don't know if that is typical of most installations.

In some literature linked from the product page at Wiring-Diagram-5026_5031.pdf

the manufacturer indicates the maximum fused current to the panel should be 125-Amperes. That is 125-percent of the panel's maximum current rating. For a comprehensive summary of the Blue Sea Systems fuse blocks, see ST_Blade_Fuse_Blocks.pdf

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