Balancing Generator Output Through a Sub-Panel

A power outage can be a scary event. After a disaster subsides, damage is tallied, and worries about family, food, and shelter are put to rest, you might consider the possibility of setting up a backup generator to build in some stability and comfort for the next close call.

But the cost of such equipment ranges widely—from a few hundred to several thousand dollars, so it's easy to get overwhelmed by the idea and let it fade away over the next few weeks. It's a real possibility, though. Here are some considerations you can take into account while making your decision.

Planning For a Generator

Getting a good deal is usually at the top of the priority list, and the first thing to establish before shopping is the specific power need of your family, which it can differ greatly between households. Buildings use different methods to for heating, furnaces, cooking, cooling, and cleaning appliances.

Take an audit of heavy energy consuming devices, like refrigerators, fans, air conditioners, TVs, computers, and video game consoles. Then prioritize which of these are most important to keep up and running during a crisis.

Calculating the power load of these devices will allow you to define the size of the generator you need. Figure 1 lists some electrical appliances that could qualify as priority items, though of course it's up to each household to decide what belongs on the list.

A 30 Amps (8000 Watts) generator could supply enough power for most people to run the necessary appliances around the house. If you decide you need to keep a 5000 Watt water heater online, 22 of the 30 Amps will be required just for the that, so a larger generator would probably be better.

Transfer Switch Detailed

A transfer switch is designed to isolate the generator from the main panel and bring power to specific circuits in the home. As shown in Figure 3a, it has two busbars, each one connected to a 120V line from the main panel during normal operation, or from the generator during power outages. In the diagram below, the heavy dotted black outline represents the busbar connected to the black wire going in, and the heavy dotted red outline represents the busbar from the red wire coming in. 230V would be measured across the two. Each breaker opposite each other plugs in on the same busbar and the next pair, up or down, to the other busbar.

Importance of the Breakers and the Load Assigned to Each

A total load of 1800 watts on one breaker will require 15 amps while 2400 watts will draw 20 amps and require 12 gauge wiring. So if you determine a 20 amps plug should be available in the kitchen but the current ones are only wired on 14 gauge wires, one will have to be rewired, or a new plug should be installed.

It’s a good idea to treat the appliances such as the fridge, freezer, and the water pump (in rural areas), and whatever system heats the house, separately from the appliances usually needed for just a few hours per day, such as lights, kitchen appliances to process food, alternate heat source, hairdryer, clothes washer, etc.

Dividing the Load

Cooking and using a hairdryer or portable heater can add an extra 3000 to 4000 extra watts during peak time, but this can be efficiently coordinated with some planning—they don’t all have to run at the same time. Furthermore, the kitchen counters should be accommodated with two 20 amps plugs from different breakers and busbars to greatly improve the situation.

Calculate the wattage of your prioritized devices on each breaker so you don’t exceed its capacity. With a little structuring, the generator’s performance can be greatly optimized, potentially reducing the required size.

Breaker wiring projects should be completed by a licensed electrician, but it's always a good idea to know what you're looking for as a homeowner, since you understand the specific needs of your household.