Carnation Construction
Contact Dave
T-06-06 Page loading indicator

Home Site Map - Techniques - Electrical -

Electrical Backup Power

Star Designing your electrical system so it can switch to generator power in a blackout.


Backup Power

General principle

You cannot just feed power from a generator back into your house wiring.  This would be very dangerous and may electrocute power workers who are working to restore utility power to your house.  It is necessary to have proper certified changeover switches that switch from utility power to generator power via a completely off state.  You could use a huge switch into your main panel for this, but a much better idea is to use lots of smaller switches on the circuits that you will actually need during a power outage.

Example things that need power during an outage include:

Well water pressure pump
Well submersible pump
PC and home network for internet
Fans etc for heating system
One or two small electrical heaters

Septic systems don't need to be powered during a power outage, assuming the outage only lasts a maximum of a day or two because the first tank has enough extra capacity without being pumped into the second tank.

In practice you will want to keep the total power usage to under about 4000 watts so that you can buy a mainstream economical generator.  Here are some guideline power usage figures...

Power usage guidelines


It is very important to get a generator that has a 230V (ie dual pole) output.  Here's one that I think is a good choice.

Champion generator 46514 Details are here .

I recommend buying two economically priced generators rather than one more expensive one so that you have redundancy rather than a single point of failure.

Generator Transfer Panel

Once you have decided which circuits will need power,  you wire from the generator transfer panel.  The generator transfer panel looks like and behaves like a subpanel.  The unit chosen needs to be able to accommodate arc-fault breakers.

TransGen Wiring Diagram

The best units are made by GenTran.  They do internal and external versions with various power handling capabilities.  Here's one I think is a good choice for outside use...

GenTran R200660 20-Amp 6-Circuit Outdoor Generator Transfer Switch for Generators up to 5000 Watts

Details are here (including installation instructions).

The instructions actually make it sound harder than it really is.  You feed the transfer panel from a 60A dual breaker in the Main Panel.  This goes via four 6-guage wires to the Transfer Panel.  Because the Transfer Panel is a Sub-Panel, the earth and neutral is kept separate.  All things that you want to be available for backup power you wire to the breakers in the transfer panel.

GenTrans generator panel 

For use inside your house use an internal version.  Only the internal versions have power balancing meters, so are the best choice for household circuits.  Here's what I think is the most suitable model...

GenTran 200660 PowerStay 20 Amp Manual Generator Transfer Switch  
http://www.amazon.com/GenTran-200660-PowerStay-Generator-Transfer/dp/B0000CCXUF/ref=sr_1_1?s=home-garden&ie=UTF8&qid=1327204181&sr=1-1       $273 + tax

Generator Transfer Internal Panel 

Here's the GenTrans wiring details (both internal and external) (click image for higher resolution)...

GenTrans wiring details

An interesting point is that all the circuits that you decide need power during an outage will always be powered by the generator transfer panel even when not using a generator.  To cater for the fact that you will not be as power conscious when not in a power outage, the generator panel can provide more power when not in generator mode.  The GenTran units shown above are fed with a dual pole 60 amp breaker from the main panel.  You need to make sure the maximum current you ever use when not in a power outage is below that dual pole 60 amps.

Exterior connection box and cable

If mounting inside then you will need a connection box on the outside of your house (the generator MUST be used outside because of exhaust fumes).

GenTran 20 Amp 125/250-Volt Non-Metallic Generator Power Inlet Box NEMA L14-20 with Spring-Loaded Flip Lid
http://www.amazon.com/GenTran-250-Volt-Non-Metallic-Generator-Spring-Loaded/dp/B004BSF2QO/ref=sr_1_13?s=home-garden&ie=UTF8&qid=1327168737&sr=1-13         $48

Generator connection box 

When wiring an L14-20 (or an L14-30) plug or socket, G is (green) ground, W is (white) neutral, Red and Black are the interchangeable Live1 and Live2 and go on the X and Y terminals.

Reliance Controls PC2020 20-Feet 20-Amp L14-20 Generator Power Cord for up to 5000-Watt Generators
http://www.amazon.com/Reliance-PC2020-Generator-5000-Watt-Generators/dp/B000HS2LAC/ref=pd_bxgy_ol_img_c         $48 + tax

GeneratorCable L14-20 

For the generator you may need to put an L14-30 plug on the end of the wire (instead of an L14-20 plug).  The wiring for an L14-30 plug is as per this diagram (from: http://www.smps.us/portablegenerators.html)...

L14-30 wiring diagram 

Other details and my plot specific details

In my case because of the configuration of the wiring around my property, I need two transfer panels and two generators, ie two separate setups.  My main panel and my electricity meter are mounted outside on the driveway.  There are feeds from there to the well house and various out buildings, as well as a feed to the subpanel in the main house.  I use an external generator transfer panel and generator to power the well house and out buildings during a power outage.

External generator transfer panel in yard

Inside the house, next to the internal subpanel that does lighting and electronics, I use an internal generator transfer panel fed via an external connector box and another generator.  Note that the other house subpanel that powers the heavy current appliances does not have any power backup so none of these will be able to operate during a power outage.

The two generators are both 4000W and are interchangeable and movable.  If one fails then one generator will be used most of the time for the house, but moved to the outside transfer panel for 10 minutes whenever the water pressure has dropped (and the faucets stop working).  Note that even when both generators are functional, it is only necessary to run the generator on the external panel that feeds the well house occasionally and this will save gas.

Take heed of the fact that you can never exceed dual pole 60 amps even when not in a power outage.  Here are the calculations for the external generator transfer panel...

Well submersible pump             6A 230V (Dual pole)
Water pressure pump               10A 230V (Dual pole)
Power tools in the well house    14A 110V (Single pole)
Well house heater                     14A 110V (Single pole)

The Well house needs a 30A dual pole breaker (possibly could be 40A).  That leaves only a 30A (possibly 20A) dual pole for everything else fed from the external generator transfer panel.

During a power outage it is necessary to keep the well house power usage to under 20A dual pole or the generator circuit breaker (in the transfer panel or on the actual generator) will keep tripping.  If this happens then you will need to in the well house sub-panel turn off the breaker to the submersible pump and just rely on the stored water in the tank.  If the tank runs dry then turn off the breaker to the pressure pump and let the submersible pump have power for a while.

The septic system does not need to be fed from a generator transfer panel on the assumption that a power outage will be less than a day or two.  To allow for a more serious longer outage it is worth putting an inline L14-20 plug and socket arrangement so you can manually plug in a generator for an hour every couple of days to pump sewage from the first tank to the second tank and out to the drain field.

It is not practical to have your garage (with lots of power tools etc) powered from the generator transfer panel.  Instead you should pick a suitable 20A single pole (ie 115V) power outlet in the yard that is powered by the generator transfer panel and then run an extension cord if you need a particular power tool during a power outage.  If it is an out-building without power tools then it is practical to feed it from the Transfer Panel.

A typical household electric hot water heating boiler is too much current to be powered from an economical mainstream generator.  In order to provide at least some hot water during a power outage (and assuming you do not have natural gas or propane available) one option is to have a small electric boiler, eg a 6-20 gallon unit.  It is good to use a 115V 15A unit (ie single pole) so that this can be used on one pole while the other pole is used for eg an electric oil filled radiator (also 115V 15A).