Electrics on a 1959 Dominator

If it were mine, and the fact that it isn't original, I would choose to modernise to 12v and do the voltage control, fuses,  decent lights, indicators etc thing.
12v bulbs are available anywhere for example.
I do understand that there is a following for the 'keep it original/6v' route, but availability of parts and personal safety of having lights that actually have a decent output trumps that as far as I'm concerned. 
One question: what type of alternator have you got? Resin encapsulated stator? Should be OK. If it is the original 1959 item, then maybe not.

I'm sure Al O and others will be able to guide you as well, but Grant Tiller has a lot of wiring diagrams on his site that will almost enable you to pick and choose what you want of your wiring.

Good luck, 
George 
 

You need a fuse in the live supply from the battery , usually the negative side , traditionaly about 35 amp .  A 59 bike is likely to be coil ignition , charging controlled by a PSR8 type switch  . As said better to convert to 12v and use a modern rec/reg  unit mounted under seat area . Pictures of what you have would help.

... as mentioned above, a voltage regulator is needed along with a fuse. If you study the Tiller wiring diagrams, and get to really understand how wiring works, it makes it a lot easier to design/modify what you need: for instance, indictators, heated grips , LED bulbs etc.

The alternator I had with visible coils died in1967 and was replaced with a resinfilled stator. They didn't withstand vibrations, eventually short circuits in the coils. Though there is still 3 wire, 6V alternators available, I prefer a RM21. Having 12V and a rectifier/regulator will make things easier in the future.

Most of above words 'of wisdom' are correct. The alternators can fail even if they are encapsulated. The three wire ones can be used for 12V. BUT 3 wires is NOT 3 phase. 3 phase must have 9 pole pieces.  Robert- do not worry about what you have, decide what you are going to do. Georges word above are the ones to follow.
The original 6V did NOT have a regulator, regulation was achieved by the PRS8 switching the alternator and a degree of allowing the 6V battery to gas (fuss and fume!).
12V needs regulation, we used to have Zener diodes-still viable but no longer available. We know have 12V regulator/rectifiers, (I sell them.)
!2V  allows more choice of battery (you need the biggest you can get in the battery box 9Ahr?) more choice of bulbs, (even  LED and indicators if you must). More choice of ignitions- I sell one for the 18D2.
After a re-read of Georges words I must correct a point-"12v gives better lights" A regular miss information.
If you just change to 12V from 6V then the extra voltage overcomes the bad wiring/connectors due to age British weather, corrosion etc. 6V can be made to work properly IF you are prepared to understand the electric system properly and fit parts accordingly!

Just a small point, but it is slightly advantageous to put the main fuse in line in a sole return (earth) lead near the battery.  Protects against the rather common accident where we short the battery live terminal to the frame with a screwdriver or spanner.

20 amp continuous rated fuse is big enough.  

... generally put them, but of course not on the Electra......

"Convention is to install in the consumer circuits rather than earth."

Why? 

Not that I'm claiming to know what consumer circuits are.

Recommend that you read the info sheets Alan has written on his website
http://www.aoservices.co.uk/

The only reason to install a fuse in the battery outgoings side is when you have multiple "consumer" circuits to protect. As you see in a domestic distribution board. As an example for a classic motorcycle; lights,rear brake light, indicators,, auxiliary power outlets; all could be protected by appropriately lower fuse sizes.  

The fuzed ground leg would have helped me with my commando. After a humped back bridge incident and a perished battery retaining strap.
 

Don't bring in domestic distribution system, that deals with AC at lethal 240 volt levels, plus a safety independent EARTH circuit to true ground.

Stick with DC at 6 or 12 volt with a battery and a frame earth return path as used in our classics.
(Edit - I  should have mentioned this is doesn't apply to electric start, which was not generally available in the 1950/60s or the bike in this thread.)

A fuse should protect the cables against overload (overheat and fire) so if you wanted each circuit could have its own fuse (calculated by load) in the feed from the battery. And, if you want to fly over hump backs, you could also add a large one in the earth return cable capable to carry full load under normal use but still avoid burning your bum.

Yes, I know it over complicates things but then the old brain can get confused between autos, classic bikes, aircraft and domestic - they all have their own ways of calculating cables sizes and fuses.

With an electric starter the fuse can't be on the ground side.
On our kickstart bikes it doesn't matter which side of the battery the fuse is fitted.
Except when you short circuit the battery as happened to Jonathan.
The most common cause for a blown fuse is a short circuit. A single fuse will protect.
A more complex circuit diagram as seen on modern bikes means more things that can go wrong.

Stans words above are an important issue-forget Mains distribution it is to be treated as 'another world'.
We deal with vehicle electrics mostly 12V (6V is to be treated with the same respect)
Even Mikaels idea of 'kickstart' bikes is a poor analysis.
These are the basic facts TAKE NOTE. The fuse in any circuit is there to stop the Energy in the battery setting fire to your bike. On our old machines this is mostly taken care of with a single fuse in either battery line. BUT do NOT put further wires to the battery terminals these will bypass the the fuse and you have potential trouble.
IF you have coil ignition and want to be safe then it would be a good idea to have two fuses, one specifically for the ignition and the other for the rest of the bike. (this means when a light circuit blows the fuse, we do not stop 'forward motion'.
The above is the rule. Now we brake the rule. ANY electric start cables must NOT be fused ie battery to solenoid, solenoid to starter, and battery to earth.
TAKE NOTE!!
Now an answer to Andy. Initialy you do not need to change the Alternator. The original RM19 (or such) will give 12V. Yes you fit the modern regulator/rectifier in place of the original rectifier. Yes you use the PRS8 switch but you can simplify as per my web site (aoservices.co.uk)  

Edit: I had started typing this before I saw Alan's post.  I've ended up repeating some of his points.

If we're on a general conversation about fuses on old bikes now, the purpose of the main fuse is to stop a short circuit setting the wiring on fire or making the battery burst.  

As mentioned, there is something to be said for the late 1960s Meriden practice of putting the main fuse in the battery's return lead. A downside of that is it can confuse a subsequent owner or mechanic.  I think Al Osborne pointed that issue out to me and it's valid.

The main fuse rated current should be significantly larger than any anticipated load, so that it will never blow in service, unless the battery is short circuited.  We don't want to build needless breakdowns into our bike.

Lower current rated fuses in individual circuits may be desirable. An example is a fuse in the live feed to the light switch. A short circuit in the lighting system then would blow the lighting fuse but not the main fuse.  You lose your lights, but the bike still runs. You also have a clue where to look for the problem.

The situation is complicated by the fact that old type glass fuses and their holders with biro pen springs are paltry pieces of junk and themselves cause breakdowns by corroding and shaking to bits.  Originality is great, but these modern blade fuses are more robust.

The original fuse holders on our bikes were the Lucas 'clear' plastic that held a 1 1/4" glass fuse. The wires had to be soldered into each end terminal.-Adequate.
I have been supplying a Bulgin (manufacturer) very similar device that incorporates grub screw attachments. But has become very expensive.
There is a 1 1/4 glass fuse holder on the market with RED wires on it-rubbish. You end up with the wrong coloured wires if you use it on the live side of your battery. And extra connectors- any surplus connectors are always to be avoided.
The Blade fuse holder above is ok shame about the wrong colour. And you could end up with RED wires on BOTH terminals of your battery-disaster.

I sometimes change a reasonably short wire's colour with heat-shrink.

... this type which although not waterproof (are any?) at least allow me to choose the wire colour myself.https://www.almsolutions.co.uk/index.php?main_page=product_info&cPath=89_128&products_id=2391