Question: I upgraded my service to 200 amp from 100 amp, but the new panel is 15 ft. away. In order to connect the existing wires to the new breakers in the new panel, will just a simple connection with higher guage wire and wire nut be fine or does it require a different approach?

thanks

Answer: 
Extending the existing circuits from the old panel to the new one with the same gauge wire is acceptable. There is no need to upsize the wire. If you have an excess of larger gauge wire that you are trying to use up then you can use it, provided the breakers in the new panel are listed for use with that gauge.

Your old breaker panel must not have open spaces when you are done. You can leave the old (disconnected) breakers, or you can buy blanks to fill in the openings. Note that some inspectors may not let you leave the old breakers. But using the old breaker panel as an oversized junction box is fine.
Not clear from the question. Is this about *service* or *branch circuits*?

### Service

Since you upgraded the *service*, you need larger feed wires to match. Instead of 2 sections of wire and a splice, get wires to go the whole distance.

If you are simply putting in a bigger panel (based on a previous question, I think you really do have upgraded service) then:

* you can splice inside the old panel
* you need to use appropriately sized connectors, which won't be wire nuts at that size

### Branch Circuits

It is quite common to reuse the old panel as a giant junction box. You can splice each connection inside the box with wire nuts. However, keep in mind that if you use conduit > 24" (and question says 15 feet), you have limits as to how many circuits per conduit. If you use individual cables, that's fine - but you can't just bundle them all together for the 15' run - they need to spaced out.