You have two options:
- use tandem breakers
- Install a sub panel
There are caveats with both routes. When using tandem breakers on a 120 volt system (i.e. with a neutral present), you want to avoid something that is called a multifeed. This is, two circuits on the same phase sharing the same neutral. When using tandem breakers, its very easy to inadvertently do this.
If you are in the US, you will see two feeders from the meter, one of them probably has some red tape on it, the other is black. These are your phases. Normally, every other breaker is on a different phase, but tandem breakers put both circuits on the same. Take care that both circuits attached to a tandem have their own neutral (white wire).
While you might be able to install a tandem breaker yourself (I highly recommend calling a qualified professional), you will surely want an electrician to install a sub panel. They aren't much different from a regular panel except:
- Ground and neutral conductors are isolated, not bonded on sub panels
- You want to watch your loads
- You'll want to be careful about where you place the breaker that feeds a sub panel, so that you don't develop a hot spot on the main panel bus. For instance, you don't want a 100 amp sub panel breaker right in the middle of a water heater and an air conditioner.
In either case, I really recommend calling an electrician.
This sounds like pure luck that you haven't had problems before. Niall has the ratings right: My microwave says 1100W power output, and 1500W max power use (which is 13A), and a 1/3HP sump pump is 9.5A (or 10A for a 1/2 HP pump), with a peak around 15A when it first starts up. If this is a 20A circuit, you're likely over the limit, and if it's a 15, you're definitely over.
After rain is very easy to explain: your sump pump is coming on more often. The starting surge of sump pump is around 5A, and the microwave probably has a surge of at least 2A when it starts up.
Basement lights are easy to figure out, just add up the wattage of the bulbs (and current in Amps = # Watts / 115V). Likewise for anything plugged into the basement outlets.
Since the sump pump turns on and off at random times depending on the water level, it's likely that any time it tries to start while you have the microwave going it's tripping the breaker. When the microwave is on a lower power setting, it may not cause the same problem.
If possible, your best bet is to run a new circuit for the microwave: by the sounds of it, it's been retrofitted, and someone just picked the nearest circuit. I would try to find where it was added (assuming unfinished basement or at least access to the ceiling or other relevant areas), and add a junction box and a new line back to the panel, and give it its own 20A circuit.
Next best is to run a new circuit for the sump pump. You'll still have the issue with anything plugged into the basement outlets, but at least that's more controllable.
Best Answer
It is fairly simple to add an additional breaker to a box but of course there would be some sort of minimum charge to come out and also the cost of the breaker, so probably around $100.
BUT that assumes the actual line for the sump pump/microwave (the one you want to put on the new circuit) is all by itself. I have seen some interesting wiring in old houses where one thing is wired to the next which is wired to the next and eventually all of it goes back to the same breaker. The electrician might need some time to figure out how the microwave and/or sump pump are currently wired before being able to propose a solution. If this is the case then I would expect a much higher bill (since this is no longer a trivial "install a new breaker" job).