Electrical Wiring in the Home/Interlock breaker


Thank you for taking the time to answer my question.

I currently have a portable generator that feeds a manual 6 breaker transfer switch. I would like to remove the current transfer switch and install an interlock breaker (one that backfeeds the panel while forcing the main to be shut off to prevent engergizing the line). I have four questions about installing the interlock breaker.

1) I need a breaker space at the top of my panel, on the left side. Currently, the space is taken up by a 2-pole 30A breaker for my hot water tank. As I understand, the heavy draw appliances should be closest to the top. Will moving the hot water tank breaker down to the 5th and 7th space be an issue since the interlock breaker is not used?

2) On the same leg as the hot water tank is a 2-pole 30A breaker for A/C. This is in the last two spaces on that side. Should this be moved up, below the breaker for the hot water tank? I don't know if it matters, but on the opposite leg, starting at the top are the following 2-pole breakers: sub panel (60A), stove (60A) and dryer (30A).

3) In order to install the interlock breaker, I would need to open up spaces by installing slimline breakers. I have a Siemens G3040B120 panel with 150A service. I believe that I am allowed 10 slimline breakers based on the model of the panel. As long as the correct breakers are used, and the panel allows for their use, are there any guidelines for using these types of breakers?

4) My current transfer switch uses 10/3 with ground. Is this sufficient for a 5500 watt generator? I will not be using the stove, hot water, a/c with the generator. Only general lighting and television or furnace as needed.

Thank you again for your time.

In order:

1. I am not sure who made the comment about heavy draw being near the top, but with the design of the buss bar, the draw top to bottom could only be measured with an extremely sensitive meter, so the location is insignificant.
2. There are of course two 120 volt power legs in a single phase panel. Every other breaker space on the same side is the opposite leg, thus with a double pole breaker located on either side will catch both incoming legs of power.
3. You have to be careful using slim line breakers, personally I would never use them. I know it may be personal preference, but the breaker is half the size of a standard breaker, so the switch mechanism is also half the size. I would prefer to add a sub panel and move at least one of the double pole devices to this panel in order to open a spot in the main panel for a breaker to feed the sub, but if you do use the slim breakers, keep in mind the two closest together will be connected to the same power leg, so in order to provided 240 volts there is a special 4 pole slim breaker and the top and bottom will be one 240 circuit to catch both power legs and the two inside will be the other to catch again both power legs. Also since these breakers are half size, I would use it to power the lower draw devices such as the water heater and the dryer.
4. The formula to determine amp draw is total watts divided by voltage, so a 5500 watt generator at 240 volts is just under 23 amps, so 10/3 will be sufficient as it is rated for 30 amps.

Hope this helped, any other questions feel free to ask. Thanks J
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J Cook


General electrical wiring, including but not limited to, residential, panelboards, control wiring, low voltage wiring, Heating and A/C control wiring, single and three phase wiring.


I have been doing electrical since high school. I have been licensed by the State of North Carolina for ten years and am currently the Building Maintenance Superintendent for a municipality. I have extensive control wiring experience.

Refrigeration Service Engineers Society.

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