We have 200 AMP service.
Our circuit breaker panel is full. This is a ~20yr old sylvania panel and is in a good shape. I need to add two circuits to my garage and a wondering:
1. Should I replace two of my single circuit breakers with two twin circuit breaker such as http://www.amazon.com/exec/obidos/ASIN/B000H5T8C4/dvdhills-20? (I only need 20 amps on each circuit)
2. Should I change the service panel and buy a new one which has more spaces?
The new outlets won't add significant amperage overhead since they won't be used simultaneously.
Correction to Dr. Weasel:
Actually, according to the National Electrical Code, the load for a circuit should only be design for 80% of the rating of the circuit/breaker. However, the breaker can actually handle 100% of the load. Your 200-amp main breaker will protect the electrical service panel from overload if it is working properly.
What I would do in this case is pull two adjacent breakers out of the panel. Use those two slots to feed a 100-amp, 12-pole surface-mounted panel located on the wall near the existing panel. Feed the new panel with a smaller breaker, say 40-amps. Put the two breakers you pulled into the new panel. Now you have 10 extra breaker spaces.
You can find a 100-amp panel at a hardware store for around $50. However, you should always be aware of overloading the 200-amp panel. Normally, you'll never use more than 30-40% of the 200-amps, but you should still be wary.
Everything I described above is code legal.
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