Blowing Fuse on Fish Finder ??.

Ok new problem now.
I just installed a Hummingbird Helix 7 unit. Wired it to my main bus. When I turn on my other GPS Unit the Helix goes dim nd erratic.
I was told that interference from other unit is causing it. Was told to wire it by itself.
So I wired it directly to the engine battery switch and ground. using the correct gage wire.
Now if with the Helix on and start the engine it blows the fuse .I had a 3amp fuse and also use da 5amp fuse ( didn't have another 3amp) and it blows .
How or why is voltage going to and blowing the fuse when I start the engine.
Help Please ;
Bob

never ever have your humming bird on while you start the engine. start the engine........then power up the hummingbird. (same advice that is in the owners manual)

Never run lights and other accessories on the engine circuit. Do you have a fuse block and grounding bar under the dash. It just sounds like you need to upgrade the house wiring. Pull two heavy guage wires directly from the battery to the fuse block and grounding bar then you can fuse all the components individually and uprgade to 8 or 12 station fuse block for future appliances. If you upgrade to a accy circuit from he battery make sure it is fused at the battery as well.