Monday, November 4, 2013

Power supplies in an emergency - part 1

The preparatory explanation took much longer than anticipated, so I broke it up into separate posts.

Sunday morning, Christina and I were discussing purchasing an emergency generator as part of our emergency preparation.  Not everyone is as fortunate as mom and dad and able to incorporate back up generators into their house design, and so the rest of us need to plan for how to use our existing house in an emergency.  Fortunately, with a basic understanding of how your house is built, we can incorporate generators into our house.

First, a brief explanation of basic house design.  There is an outside power supply - the utility supply line.  It runs to a meter, then the "main breaker", then to a distribution panel full of smaller "circuit breakers".  
The main breaker serves as a master disconnect - a single place to break the power supply for the entire house.  On household breaker panels, it is typically physically location above the individual breakers and is rated for 100-400 amps.  From this, all your other circuits are powered.

The main breaker supplies (2) sets of 110 volt power.  This technical detail means that all 110 volt devices and outlets are connected on either circuit 1 or circuit 2.  Each circuit has multiple sub-circuits, represented by the various breakers.  If all the breakers are on - then the power can flow from one sub-circuit to another.   Any 220 volt devices (electric stove, dryer, air conditioner) straddles both circuits.
A few items are directly wired to their respective breaker, air conditioners, stoves, and laundry dryers typically are on a "dedicated" circuit, meaning there is only one outlet or device tied to that circuit.  Most outlets and devices are daisy-chained together to make a "circuit".

As a side note - you will notice, if you add up the breaker values of all the smaller breakers, they will total more than the main breaker.  Electric breakers are sized based on theoretical actual usage.  It is not likely any home will run the air conditioner, dryer, fridge, stove, and lights in every outlet at any given moment.  And on the smaller, 20 amp circuits, it is unlikely a homeowner will run enough lights, vacuums, etc. to overload a particular circuit at any given moment.

All circuits are complete circles - there is an "in" and "out" and the daisy-chain always returns the out to the utility supplier at some point - devices become part of that in and out by completing the physical circuit.  Until there is a complete circuit, there is no power in the lines.

Since everything is tied together at the breaker panel, that makes possible using a generator to power your home, i.e., by correctly inputting power into any outlet, all other outlets have access to that power.  There are some limitations which will be explained in later paragraphs.  There is negligible risk to overloading the circuits because a) there is no power in the wires until a device is actually turned on and b) the breakers will trip if the demanded load is too high.

A side note, it is technically safe to hold hot electrical wires, so long as there is no out.  Trouble is, there's virtually always an out somewhere.  The biggest out in the world is "ground" which literally means the dirt you stand on.  Literally.  The three prong outlets - the third prong is ground, a backup in case the out is broken somewhere, and that ground is usually right at the house.

When purchasing generators, the critical value to be concerned with is "amps".  All home devices use 110 (120) volts (devices like the dryer and air conditioner are using 220, but technically they are using two 110 power supplies - that's why they have one wire more than other devices).  Every device consumes a different amp load.  A toaster might use 3 amps, a light might use 0.5 amps, vacuum might use 4 amps, an air conditioner might use 40.  When enough devices are on the same circuit to push the amp load over the designed amount (usually 20 or 30) the breaker.

Generators are often sold in "watts" which is amps times volts.  Therefore, a 3,000 watt generator can produce up to 25 amps.  Once you determine the amp load you want (or your capacity), you can then decide the generator size.  There's no sense in purchasing a 6,000 watt generator, because generally your house doesn't have the capacity to receive 50 amps without modifications (assuming you won't plug other items directly into the generator, and as your home is safely designed and efficient system, I would recommend against that).  If you plan to eventually make the modifications, then you can go ahead and purchase the larger generator.

Wow . . . all that description, and we have not even started talking about the actual emergency preparation.  I suppose that gives me something else to write.  For next time.

dp



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