Portable Generators and Separately Derived Systems

September 20, 2017 admin 6 comments

Portable Generators have a wide range of uses and have become very efficient and reliable when those needs have come into play over the years.  One of their more common uses is being a Separately Derived back up power system for electrical power outages and emergency situations.  They range from your common house hold portable generator for selective loads to large trucker-trailer portable generators to supply power for municipal builds or hospitals.

For your standard home residence  these systems can give the customer a cheaper option compared to larger standby generators with automatic transfer switched systems.

The difference between a Standby generator system and Separately Derived generator system is the Standby Generator is fixed in place and is an integral part of the electrical system and the Separately Derived system is a power source derived separately from the electrical system.

Selecting your Generator

What are you looking for when selecting a portable generator?  There are several things to consider when you are trying to decide:

  • How much wattage do I need? What do you want the generator to power on in the event of a power outage?
  • Are you going to be able to use a manual start or do you need an electric start generator?
  • If you decide to get an electric start, you may want to purchase a battery charger to keep the battery at optimal levels.
  • Where are you going to store the generator so that it is readily accessible when it is needed?
  • Are you going to be able to maintain and test the generator periodically?
Portable Generac Generator back up power

Usually a portable generator for a residence is used to power only essential loads, such as the refrigerator, water heater, general outlets for t.v.'s or electronics, and lighting.  The size of the portable generator you would need for this would be around 7,500 watts of generated power.  This will deliver enough power for a 30 Amp 240 Volt supply of power to your existing electrical system.

With this amount of power you will be able to operate the majority of all your branch circuitry in the residence.  Of course with the mindset to be energy efficient and only use power and lighting when needed.  What you can use based on this 7,500 watt system:

  • General use Outlets throughout home
  • T.V.'s and Electronics
  • Microwave
  • 1 Refrigerator
  • 1 Freezer
  • Water Heater
  • Lighting
  • Small Motoring equipment

This can vary depending on how much power is actually being consumed by your individual loads and when they are operated.  You can essentially turn everything off, then use your power for your Air Conditioner when going to bed.  Just don't open the fridge til morning!

What if you want your air conditioner on as well?  You can definitely have your portable generator power any array of loads throughout your property. They make larger wattage generators and if you purchased one from 10,000 to 12,000 Watts this will basically cover an average sized house, except the dryer and range.

If you decide to get an electric start portable generator, we recommend purchasing an automatic battery charger that has the function of cutting off when the batter is fully charged.  That way the battery is at an optimal level when needed.

We do recommend having an area that is dedicated to the back-up generator so that it is readily accessible during the event of a power outage and can be easily maintained and tested.

As far as maintenance and testing, you would just need to use the same type of maintenance for any small engines.  Making sure it has oil and the filters are clean and breathable, etc. The main thing you will need to note when turning the generator on for testing is making sure the output power is within a 115 volt range.  If it is not, then refer to your manufacturers recommendations.

Manual Transfer Switch

The manual transfer switch is the most popular and practical way to make your portable generator into a separately derived system to provide back up power for your electrical system.

Here is what is included with this system:

  • 7500 Watt Manual Transfer Switch that has 10 transferable circuits.  It also has a visible way to read your wattage.  There are smaller and larger transfer switches to fit any setup or circuitry.
  • 50A exterior power inlet that connects to the Manual Transfer Switch.
  • 50A power cord to connect your portable generator to the power inlet.
  • Re-wiring of existing electrical panel for selected circuits that will be transferring power.
Manual Transfer Switch portable generator back up power

Installing this system makes the whole process of providing power through emergency means very simple.  Here are the generic steps to do when a power outage occurs.

  1. Manually move the generator outdoors within range of the 50A generator power inlet.
  2. Install the 50A power cord from the inlet to the generator 240V outlet equivalent.
  3. Start the portable generator and turn on the main generator breaker so it can start transferring power.
  4. Go inside to your Manual Transfer Switch and turn all your breakers on to the generator selection.
  5. Enjoy your own personal generated power.

This is the most streamlined way to provide power to your system and selected loads using a portable generator.  You don't need to worry about turning anything off or back feeding anything in your main electrical panel.  The manual transfer switch does everything for you and it's very safe and convenient for you to operate.

You will also be able to use the other outlets on the portable generator, just be careful to use the right extension cords when using the power.  Make sure everything is electrically sound with all your cords and connections.  Also use your best judgement and don't exceed the power load capability of your generator.

There are other types of systems for a portable generator to provide back up power.  You can have an automatic transfer switching system, as well as a whole house separately derived system.  These are specific to each project and usually would require a consultation to decide the best option for your scenario.

Don't Back Feed the System

The biggest concern with emergency back-up systems is making sure there is a disconnection between the two potential sources of energy attached to your electrical system. One source is from your local power generating facility and the other source is from your emergency back-up generator that's at your residence.

There has to be a means to disconnect this flow of electricity so that it does not back feed the system and create a hazardous electrical potential down stream with opposing forces or causing electrocution.  Not only do you back feed the wiring from your house, but you also energize your neighbors system, and the power pole your are getting your power feed from.

The power that is back fed from your generator also travels through the pole mounted transformer and transforms the generated voltage up to a much higher voltage.  This higher voltage potential is one of the biggest concerns for the people working on this equipment and trying to restore power to your neighborhood.

You also don't want the power to be turned on from their side when they repair the problem.  This would cause both power potentials generated from each end to collide with each other, causing a lot of damage.

If you decide to connect your portable generator to your system through means other than a legitimate transferable system, then please use the right precautions.  We do not recommend this option and it is not legal by code standards.


If you want to do this option, we don't recommend it, but we want to make sure everyone is safe and has power during an emergency situation.  So we offer this service in extreme conditions only and to make sure we help everyone be safe in an emergency situation.

We can fabricate or purchase a cord for your portable generator that will connect into one of your existing equipment outlets.  This will allow you to have power to your whole house and you can turn off all your breakers except your selective loads.  We then will make the proper precautions and make it impossible for you to back feed your system so everyone is safe.

Thanks for reading this.

Offshore Electric offers Standby and Separately Derived Generator systems through Virginia.  Feel free to contact us if you have any questions or want pricing.

(757) 726-7160

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