W32:
Try not to blind yourself with science :-)
Gord May has given you all the “hardcore” stuff you need to know – as you have already acknowledged. MaggieDrum and Alchemy are worth listening to :-)
But you ask explicitly about how “we” would go about wiring a new construction from scratch. So here are some things that I consider essential desiderata. I base them on observations made over half a century in many “production” boats and in several “owner fitted” boats.
Running wires through PVC pipe and placing it behind the furniture in order to conceal it is obviously folly. Do that, and replacement/repair/augmentation will require the use of dynamite to remove the furniture to get at it. Were I fitting out a bare
hull, all wiring would be run in wire gutters designed as an integral part of the
cabin TRIM, e.g a removable fake “shelf” running along the deckhead/houseside joint.
Provided there is, as there should be, a master switch on the main
electrical panel – in the case of TP, at the nav station – there is no reason that ALL below-decks illumination should not be on a “ring circuit” that is live at all (needed) times, "branch circuits" to be provided for by terminal blocks at appropriate locations. Local switches on the lamps will take care of local requirements.
Fittings/electrical devises ex chandlers come with “tails” with bare ends that need to be connected to circuits. “Butt”-connectors are a no-no! For my
money, the
installation needs to be such that removal/repair/replacement of the fitting may be performed easily. Take, for example, a simple sidelight mounted on the
deck house: The “tails” are taken through a grommeted and sealed hole in the house side. Generally, the tails are hardwired within the fitting – which is sensible enuff. On the INSIDE of the house they need IMO to be taken to a TERMINAL BLOCK, so they may be easily disconnected without having to interfere with the permanently installed circuit. In the case of incandescent lights (if any are still used) the polarity is of no consequence, but with LEDs polarity matters. The tails on LEDs are therefore of different colours, commonly red and black, or they are marked with a deeky little label on the positive lead. However,
current wiring practice for low voltage (12VDC) is that “return”/”ground”/”negative” should be yellow, and that is therefore what should be used in the permanent circuit(s). In this example, the terminal block should be hidden behind a tastefully designed detachable panel in the ceiling. Both the permanent circuit and the tails from the fitting should be furnished with “captive” (as opposed to "plain") fork terminals. I prefer fork terminals to eye terminals because to disconnent a fork terminal, you don't need to remove the screw completely from the terminal block, risking the loss of it. The means of attaching the terminals to the wires and sealing the joint against moisture and
corrosion is described in the material Gordon May provided.
Ideally there should never be any kind of circuit run through the
bilge. However, that
rule must necessarily be broken in regard to bilge pumps and their “float” switches. The tails provided by the manufacturer of these devices are never long enuff. The proper means of lengthening them is to make a proper SOLDER joint, also described in the material provided by GM. The lengthened wires may then be taken to a terminal block fitted above any level the bilge
water could conceivably reach.
In TP I have made a mount for
pump and switch so they can be lifted from the bilge as an entity. Complete
removal is facilitated by having the wiring taken to a terminal block immediately next to where the mount is held to the floor by studs and wingnuts immediately under the
deck sole where. They are easily reached.
The sniffer-element on bilge sniffers will die if it gets wet or in any other way contaminated. As it must of necessity be deep in the bilge to do its job, there is a case to be made for mounting it, also, as I've just described for the bilge pump(s).
As a boat gets older, there WILL come a time when the
mast needs to come down. It is sensible, therefore, to make provision at the building stage for taking all wiring leading to mast-mounted devices through the deck in a manner that facilitates this task while at the same time prevents ingress of
water through the fitting. As it is utterly impossible to prevent water from getting inside a
mast, and as getting inside a mast is an expensive proposition, it is of the utmost importance that any wiring joints within the mast be absolutely above reproach.
At the “business end” of circuits, it is usually obvious what each wire does. That is not so at the “panel end” however. It is essential, IMO, to draw a schematic of the wiring at the panel end, and ALSO to mark all wires so they are readily identifiable. Bits of masking tape with scribbled notations wrapped around the wire just don't cut it! A simple easy way of doing this marking is to use short “rings” of shrink tubing in various colours fitted next to the panel end of each wire. A green wire with the ring combination “red/black/red” might indicate the feed for the
cabin lights, for instance, and the combination “red/red/red” on a brown wire might indicate the feed for the
bilge pump. With three rings you'll have more permutations than the boat will have circuits. The schematic will, of course, be annotated with the ring combinations, and it will live with the boat's other important
documentation.
A practice often seen is to mount the stern light on the “pushpit” (or a “combo”light on the pulpit) leading the wiring through the tubing. IMO that is asking for trouble. Running wire in such a way that replacing it becomes problematic is downright silly, and it is rare that a better placement of the fitting cannot be found, thus giving a more sensible wire-run.
You are already aware, as others have mentioned, that you start your “system design” by establishing a “power budget”, i.e. making a list of the draws of EVERY device you are going to install on the boat. The draw of each device will determine the gauge of wire you need to use for it. GM's material includes a chart for the determination of that. I like to go to the next heavier gauge. As has been said many times, the fuse is there to protect the WIRE, not the device, and should be sized accordingly! The sum total of all draws over a given time is what determines what your generating capacity should be. Your battery capacity should be enough to sustain those total draws for a time long enuff to take you from one charging opportunity to the next. The batteries don't care how the juice is generated as long as they get it. Therefore any combination of generating devices will satisfy the requirements as long as in total they turn out enuff juice to furnish your total draws PLUS an increment to recharge low batteries.
In TP – which has a measly 20HP Beta
diesel - I don't distinguish twixt “house” and “starter” batteries. The draw to start such a baby
engine is very little indeed, so little in fact that within a few minutes of the engine starting, and the alternator “kicking in” at something like 1,400
RPM, the power used to do it has been replaced! One of our members has written a very excellent treatise on precisely that topic, giving actual test data proving the rectitude of the contention! Obviously TP has a standard “1-2-both-off” switch. To even the load on the batteries I use #1 on odd dates and #2 on even dates for the “hotel” functions when no charging is taking place. For starting, and when charging is available, I use “Both”. That simple!
TP lies afloat all year and plugs into a 30Amp shoreside circuit. She has a 20Amp ProMariner charger. Her Alternator is the 30A machine native to her engine. Her battery capacity is 2 x 27-series deep draw batteries for a total capacity of something like 240A/hrs of which no more than 50% should ever be expended. No fancy stuff at all, because I really don't have any interest in "toys" :-). If, when on the hook, I get "twinges" that the battery in
current use for
hotel functions may be "getting low", I simply fire 'er up long enuff to satisfy the current needs. The ongoing draws are very little normally. Just
LED lights and the odd squirt of water. The solenoid shut-off for the
propane is a bit of a hog, but is obviously only active while
cooking is happening. Heating is accomplished by means of putting on an extra sweater and the long skivvies with the loden insides:-)! Air-conditioning happens by opening the hatches. Hot water? A kettle on the
stove does the job. Showers? Take a dip in the briney!
Oh, speaking of "when on the hook": The capstan, miserable little toy that it is, is a hog. I never run it, up or down, without having the engine at 1,5K or so for the five minutes or so that it's in use, so that the alternator output is more than the capstan
consumption. The engine needs to run for reasons of sound seamanship in any event when you are playing with your ground tackle.
I know I'm terribly old-fashioned, but there you have it ;-)!
TP