Aboutcleggsan Expertise All technical areas of Electronics Engineering.
Experience BSEE, MBA, Design, R&D, University Research.
Senior Life Member of IEEE. Life Fellow of AES.
Organizations IEEE, Consumer Electronics Society, Audio Engineering Society.
Broad teaching experience; work experience mostly in consumer electronics and conversion from analog to digital technologies. Pioneer in digital audio at all levels.
Expert: cleggsan Date: 6/23/2008 Subject: Sizing cable for garage
Question I'm building a garage that is approximately 185 feet from our home circuit
breaker box to where I intend to put a box in the garage. From there I have
to run about 25 feet to the garage door opener (3/4 hp) and I'll put in four
or five lights and an equal number of outlets. I don't plan on using any
heavy duty equipment but might find myself once a year borrowing a table
saw.
My main question is what do I need to run from the house to the garage.
Inside the garage I had planned on using 12 gauge.
Answer You must know the wattage requirements more precisely, I think.
For example, a 15 amp circuit (at 120v ac) will deliver safely 1500W of juice before the breaker goes. (Note: The National Electrical Code suggests the breaker trip currents be derated to 80% to give some headroom. I advise you to use a standard box and circuit breaker and any other means to comply with building codes in your area.)
A 3/4 electric motor will have a big start current, but once running will draw around 750 watts or 1/2 of the 15 amp circuit. That would leave you 7.5 amps for the lights and such.
So, you need to determine, based on your projected usage and lights if the 7.5 amps remaining is sufficient.
According to most resistance charts for extra long runs of a 15 amp service a 10 gauge wire is recommended. For example, 10 gauge has a resistance of 1 ohm per thousand feet - or 2 ohms total since there are two wires in a complete circuit. In your case you are delivering about 8 amps rms or 12 amps peak for a voltage drop in a 200 ft run of less than 2 volts. This would be tolerable since in only fully loaded cases would you get that much drop. (Assuming you are using electrical grade copper wire).
If you went to 12 ga you would end up with a peak voltage drop inside the garage of 4 or 5 v - which is too much.
So, I recommend you go with 10 gauge wires from the home to the garage. If you decide to go with a 20 amp service you will need to drop down to 8 gauge wires.
Hope this helps.
c
PS: Please do not depend on my answer to get you past your building inspectors; every city/municipal agency seems to have their own unique requirements that they impose on their poor citizenry. However, I think the above recommendations are safe and will deliver a well regulated voltage to the garage area.
