Anyone care to check my boat electrics calculation?

[PaulRainbow]

I may have used the wrong terminology Paul (maybe not!)
I am attaching a sketch of what I think I am doing based on this thread........
when I can resize it

That looks reasonable Dave. Can i suggest a couple of tweeks ? Forget the AIS or Glomex switches, but put the VHF on it's own supply. So run 2 wires from the busbar, one to the fuse holder, switch and 4 way fuse holder, as you have drawn it, then a second wire to a fuse, then straight to the VHF (no need for a switch). It's only one more wire, but saves two additional switches, whilst allowing you to leave the VHF on when everything else is off. The GPS, AIS and splitter will all be on at the same time, so it makes sense to have them on a single switch, as drawn.

Don't get hung up too much on the fuse rating, it's OK to be a little over rated. If you had all 4 items wired as per your sketch, i'd fit a 15a fuse before the switch and a 10a fuse for the VHF, the others can be smaller. If you go with my suggestion, fit 10a fuses for the VHF and the switch/4 way fuse holder and smaller ones for the GPS, AIS and splitter.

The fuse holder nearest the busbar would likely be better if it was a multi way one.

 

[PaulRainbow]

The easiest way is just to use a switch panel ,plenty on ebay.

Why, he only needs one switch ?

 

[FairweatherDave]

That looks reasonable Dave. Can i suggest a couple of tweeks ? Forget the AIS or Glomex switches, but put the VHF on it's own supply. So run 2 wires from the busbar, one to the fuse holder, switch and 4 way fuse holder, as you have drawn it, then a second wire to a fuse, then straight to the VHF (no need for a switch). It's only one more wire, but saves two additional switches, whilst allowing you to leave the VHF on when everything else is off. The GPS, AIS and splitter will all be on at the same time, so it makes sense to have them on a single switch, as drawn.
.

Thanks Paul, that all makes sense, but 2 questions....
(a) Can I use a common return. Or should the VHF have it's own return?
(b) A general question to all. I'm still not sure I get the argument for the VHF having it's own circuit. Part of it's safety value is in the DSC so the GPS info needs to come in. And will the (active) Glomex splitter still allow the VHF to work if it is without power? Obviously the AIS info would be less critical. I do appreciate it is easy to disconnect the coax from the splitter and stick it in the VHF directly. The radio will still transmit and receive without the DSC function I know. I am quite happy following the advice but even happier when I understand the arguments
PS I appreciate keeping the VHF circuit independent of other circuits, but this group of circuits is all VHF related?

 

[lpdsn]

(b) A general question to all. I'm still not sure I get the argument for the VHF having it's own circuit. Part of it's safety value is in the DSC so the GPS info needs to come in. And will the (active) Glomex splitter still allow the VHF to work if it is without power? Obviously the AIS info would be less critical. I do appreciate it is easy to disconnect the coax from the splitter and stick it in the VHF directly. The radio will still transmit and receive without the DSC function I know. I am quite happy following the advice but even happier when I understand the arguments
PS I appreciate keeping the VHF circuit independent of other circuits, but this group of circuits is all VHF related?

Good point. It's 'traditional' to have the VHF permanently connected but this is down to concern about RF noise rather than the reliability of switches. With the introduction of DSC radios (a couple of decades ago) the GPS became part of the circuit for effective emergency calls, and an active splitter just extends that further.

As I said a few posts back I have separate VHF & Instrument switches because that's the way the boat was built, but if I were designing it from scratch I reckon I'd go for a single switch. That said, I still would strongly argue for individual fuses.

With my set up I think if I were really desperate for power I'd leave the Instrument switch on to supply GPS data to the radio, but pull out the fuse for unnecessary instruments. I don't think in practice that's a likely situation though. I expect my batteries to be strong enough to give power in any emergency except for an emergency that takes them out (e.g. flooding or fire in the battery compartment). To get around that you're looking at a dedicated VHF battery which I guess should be a whole new thread.

 

[PaulRainbow]

Thanks Paul, that all makes sense, but 2 questions....
(a) Can I use a common return. Or should the VHF have it's own return?

Best practice, it should have it's own negative, in reality it doesn't matter much as long as the cable is suitably rated.

(b) A general question to all. I'm still not sure I get the argument for the VHF having it's own circuit. Part of it's safety value is in the DSC so the GPS info needs to come in. And will the (active) Glomex splitter still allow the VHF to work if it is without power? Obviously the AIS info would be less critical. I do appreciate it is easy to disconnect the coax from the splitter and stick it in the VHF directly. The radio will still transmit and receive without the DSC function I know. I am quite happy following the advice but even happier when I understand the arguments
PS I appreciate keeping the VHF circuit independent of other circuits, but this group of circuits is all VHF related?

Some good points. I wouldn't be concerned about the GPS for the DSC part of the radio, i was more thinking along the lines of leaving the VHF on to monitor incoming calls. The splitter point is very valid though, if that won't allow the VHF to receive without it being powered then it's not the best design and you are better off sticking to the sketch you posted, but i'd still omit the AIS and splitter switches.

 

[FairweatherDave]

Thanks Paul, lspdn, PeteK and the others. Really helpful. It's great going into the job with a bit of confidence.