Electrical wiring problem

If the gauge of the wire is smaller than 12, say 14 which is very common for 15 amp circuits, then the wiring may go before the breaker will trip.  The narrow gauge wire won't allow a large enough draw through the breaker itself to trip it, as all the resistance is elsewhere.

I have seen cases before where people have substituted higher amp breakers for lower ones.  Outlets and things catch on fire without tripping the breaker.

You could have two (split) circuits that should be connected to opposite sides of the transformer supplying power.  If they are both on the same side you could have twice the normal amperage going through the common wire.  They must be rewired immediately and not used until then, or, you could catch the house on fire!!!!!!  Call an electrician!!!  Don't trust general contractors because unfortunately, they don't understand what I am talking about!!!

You can read following article: Electrical Wiring and Circuitry: Understanding the Basics

Firstly, you should have an electrical tester while changing these things, or better yet, hand it to a qualified technician. They are the ones who know what to change, where to place things over. And, preventing fire is and advantage if you'll call their help.