
ContinuousWave Whaler Moderated Discussion Areas ContinuousWave: Small Boat Electrical Fusing of Battery To Distribution Panel

Author  Topic: Fusing of Battery To Distribution Panel 
coolarrow 
posted 12302013 10:08 AM ET (US)
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 12302013 11:48 AM ET (US)
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 overcurrent 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 12Volt distribution and see what the manufacturer says. I turn to BLUE SEA SYSYTEMS. On my boat, I am using a secondary power panel PN5026: This panel is rated for a maximum of 100Amperes. So the circuit feeding it should be fused for no more than 100Amperes, On my boat this panel is fed with a conductor of 8AWG. A conductor of 8AWG is rated at a fusing current of 472Amperes. (See this listing for current capacities by wire size.) It would make no sense to use a 472Ampere fuse, as the panel is only rated at 100Amperes. On my boat, the distribution panel presently has these circuits with individual fuses AMPSCircuit That is a total fused current of 63Amperes 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 5Amperes. Let us add 15Amperes to the total, giving 78Amperes for the total fused current of the loads. In my installation, this distribution circuit is protected by a circuit breaker with a rating of 50Amperes. That is a rating that is about 65percent 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 overcurrent 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 12302013 12:05 PM ET (US)
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 http://assets.bluesea.com/images/products/5026_install.jpg This is a panel rated for 100Amperes "per block" and 30Amperes "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 155Amperes. In this application the total fused current of the circuits on the panel is 155perent 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 http://assets.bluesea.com/files/resources/instructions/ WiringDiagram5026_5031.pdf the manufacturer indicates the maximum fused current to the panel should be 125Amperes. That is 125percent of the panel's maximum current rating. For a comprehensive summary of the Blue Sea Systems fuse blocks, see http://assets.bluesea.com/files/resources/sales_sheets/ ST_Blade_Fuse_Blocks.pdf 
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