I've seen a lot of 100w or 90/100w headlight globes for sale in fact they seem to be easier to obtain than the standard globes.

I've always wondered that if a 100w globe (or 90/100W) is fitted in place of the standard 55W globe (or 55/60W), is the standard wiring capable of handling double the current?

Has anyone done this and had any problems with the rest of the electrical system?

I have to job of fix at least 10 cars a year that have these fitted. The headlight circuit in most cars is barely capable of handling the load imposed by the standard globes.
You will end if with a combination of burnt connectors, wiring, and/or headlight switch. The globes will also burn with less intensity than the lower wattage globes due to voltage drop.
To correctly install these higher wattage globes you must fabricate a heavier wiring harness with decent relays, and only then will you get the desired effect. Of course if you have plastic lenses then you can kiss them goodbye.
Your best bet is to look for a +100% range in a decent brand of globe (phillips, osram, tungsram, etc). Whilst initially they are not as cheap as the 100/90 globes, they are as bright if not brighter, fool proof, and tend to last longer.

I agree 100% with the_ash about overloading the circuits. I fitted 100 W globes to the high beam to out TM Magna about 18 years ago. One of the original globes blew, so I fitted a 100 W to replace it. It was so good that I went out and bought another. When I changed to the TF Magna, they came along too, and they went with the car a few weeks ago when we bought a new Outlander.

I was well aware of the potential for overlaoding of the circuit, but we had no troubles. When it came to the low beam, the TF was dismal, to say the least. so I put in a pair of +75's and they were much better, although low beam was still rather poor. I didn't want to try my luck using 90/100W bulbs in the car becasue from my experience with my Pajero, they didn't seem to last very long.

Our lights don't get that much usage (at night anyway). We use low beam all the time during the day saying to other drivers "I am here, don't hit me!". When we do drive at night, there are lots of very large grasshoppers in our area and we need as much light as we can get. The Pajero has a 72W LED light bar on it and that is very good for what we want.

To correctly install these higher wattage globes you must fabricate a heavier wiring harness with decent relays, and only then will you get the desired effect. Of course if you have plastic lenses then you can kiss them goodbye.
Think of the safety factor.................
Totally agree ash seen plenty of burnt switches........nothing worse than going into a tight bend on a steep mountain road at night and your switch decides to fail at that point...total darkness no moon steep drop on one side!!! You get no warning of the failure either in the nights leading up to it!!
That was personal experience in my first few years of driving!! Its not worth the risk Imo. Do it correctly!!
Remember Electrical FIRE is also a factor!! Probably would void insurance if that occured. Vee

One of the Vic boys installed them.

Burnt the crap out off his plug and wiring. Not with it

It's been proven that the stock wiring on Magnas is too small to cope with even the factory 55w bulbs, let alone 100w bulbs.

Philips and Osram make a range of bulbs that are stock wattage but produce noticeably more light than standard bulbs. They are available cheap from www.powerbulbs.co.uk

Red Valdez has it spot on, you can install some relays and from that use thicker wiring, this will close the bottleneck and help any bulb you put in the magna perform better.