Thanks For the thorough reply Tim, valuable and extensive detail there, and some food for thought. I do understand the operation of the acr and the goals/theory of overcurrent protection, as well as voltage drop/cable ampacity more than my already-too-long post might infer. Thanks for the -qualified - validation of the approach. Did not know the abyc 7" rule, nor that fusing the house bank is now 'code'. (Amazing that it wasn't...). I will review the abyc standards on this whole area, and also measure the battery to destination drop in the existing system. This will tell me the margin in which my question exists. Regardless, Am going to rethink, incorporate, and document standards-compliant circuit protection as well, to your point, this only makes sense. Since this means recabling the house bank I will oversize the cable anyway, assuring that he extra few feet of cable are immaterial, drop-wise. A slippery slope as usual but a good one. great stuff, very helpful, and thanks again for taking the time.
Dave. Sent from my iPhone > On Jul 10, 2016, at 8:42 AM, Tim Goodyear <timg...@gmail.com> wrote: > > Hi Dave, > > Your ACR doesn't function as you describe; it is just a relay, with no > charging intelligence built in except that it will only close when it senses > charging (a voltage above a certain level for a certain time) on one or both > battery banks. That shouldn't matter too much if both banks are the same > battery chemistry and the same age, but they will both "see" the same voltage > (minus wiring voltage drops) when the relay is closed, so make sure it's > appropriate. > > You are heading in the right direction on voltage drops and overcurrent > protection (fuses), but voltage is crucial to battery charging and you don't > want to lose your boat to an electrical fire for the sake of a couple of > fuses... > > Voltage drop will depend on current (how big is your alternator / battery > charger), wire length (total of positive and negative lengths) and wire size > - if it's going to be longer than recommended, increase the size of the wire. > Blue Sea Systems probably have a calculator on their site or you can search > for ABYC voltage drop tables. > > Under ABYC standards, connections of less than 7" do not need to be fuses, > neither do engine cranking circuits, but if you don't fuse other circuits you > are asking for insurance / survey trouble if things go wrong... Blue Sea > have a great on-battery fuse (MRBF) that you could mount directly on the > studs of the ACR without a lot of effort (and now required for your house > bank). You are protecting the wires, not the devices with these fuses, > preventing short circuit current from causing them to ignite, so if the ACR > wires are a different size than the battery cables you need different fuses. > > https://www.bluesea.com/products/5191/MRBF_Terminal_Fuse_Block_-_30_to_300A > > I hope that helps! > > Tim > > Tim Goodyear > Ex-35-3 > >> On Jul 10, 2016, at 8:17 AM, Syerdave--- via CnC-List >> <cnc-list@cnc-list.com> wrote: >> >> Hello all, >> Wondering about the practical vs the theoretical here. >> Am upgrading the DC system, adding a Group 24 dedicated start battery, and >> connecting the two group 31 in parallel to double the capacity of what will >> become the dedicated house bank. Am adding a blue sea switch and ACR. >> FYI - an ACR is a device that upon starting the engine essentially 'chooses' >> to top up the start battery fully before allowing the the house bank to be >> charged. >> Anyway, the manual states that the acr should be connected as close as >> possible to the battery (to prevent voltage drop from compromising its >> function to any degree) and both of its leads fused based on the wire gauge. >> All makes sense, especially when the batteries are located At a distance >> from the switch or ACR. however.... >> The simplest installation is directly adjacent to the switch, not the >> battery. The leads to the ACR would be very short and well protected - less >> than a foot. (The battery cables themselves are not fused!) In the 33-2, >> the distance to battery is still very short - less than eight feet of cable >> for sure. >> >> In this situation, I can only see an infinitesimal difference in voltage >> drop by locating the ACR a few feet closer to the battery, really, no >> meaningful difference. With a lead length measured in inches the positive >> wires to the ACR probably do not need to be fused. (the ground wire IS >> fused, protecting against failure of the device itself.). >> >> Has anyone got any experience with this that would tell me otherwise? >> Thanks! >> >> Dave Syer 33-2 >> _______________________________________________ >> >> This list is supported by the generous donations of our members. If you like >> what we do, please help us pay for our costs by donating. All Contributions >> are greatly appreciated! _______________________________________________ This list is supported by the generous donations of our members. If you like what we do, please help us pay for our costs by donating. All Contributions are greatly appreciated!
