Balancing electrical loads is an important part of laying out the circuits in a household wiring system. It is usually done by electricians when installing a new service panel (breaker box), rewiring a house, or adding multiple circuits during a remodel. In simple terms, an electrical service panel has two sides, and balancing the load is a matter of dividing the circuits evenly between the two sides so that the load, or power draw, is roughly the same on both sides. An unbalanced load occurs when there is significantly more power drawn on one side of the panel than the other. This can lead to overheating of electrical components and possibly overloading the panel.
Electrical Service Basics
Most homes have a type of electrical service called single-phase, three-wire. The service comes from the utility via two ungrounded ("hot") wires that carry 120 volts each, plus a single grounded ("neutral") wire. The wires connect to the home's service panel, and each hot wire provides 120-volt power to one of the two hot bus bars in the panel. The circuit breakers for the various household circuits (called branch circuits) snap into the panel and connect electrically to one or both of the hot bus bars. A single-pole circuit breaker connects to only one bus bar and provides 120 volts to a circuit. A double-pole breaker connects to both bus bars and supplies 240 volts to a circuit. Like the utility service wires, each branch circuit has one or two hot wires and a neutral wire. The electrical power leaves the panel along the hot wires and returns to the panel on the neutral. From there, the power goes back onto the utility grid via the utility service neutral.
Every circuit breaker has an amperage rating that indicates the maximum load the circuit can handle before the breaker shuts off to prevent damage from an overload. Single-pole breakers usually are rated for 15 or 20 amps. Double-pole breakers typically range from 30 to 50 amps or more. The amperage rating is the main factor used to balance the loads in the service panel. Another factor is the type of electrical equipment (appliance, outlets, lighting, etc.) served by the circuits and when that equipment is typically used. For example, a refrigerator runs 24 hours a day, 365 days a year and needs the most power for starting its compressor motor. By contrast, a whole-house fan (attic fan) has a relatively consistent power drawn and is used only during warm weather and usually at night or early in the morning.
To understand how balancing works, imagine that you have two 120-volt circuits with single-pole breakers. One circuit supplies a refrigerator that draws 8 amps; the other circuit supplies a chest freezer that draws 7 amps. Both appliances run all the time, year-round. To balance the load of the two circuits, the breakers should be on different hot bus bars, or "legs," of the service panel. That way, the amperage of the two circuits cancel each other out when the power returns to the utility on the neutral. In this case, the current on the neutral would be 1 amp: 8 – 7 = 1. If both appliances pulled 8 amps, the current on the neutral would be 0. The goal is to have the current on the neutral be as low as possible—for safety, energy-efficiency, and other reasons.
On the other hand, if you placed both circuits on the same leg of the panel, the loads of the appliances would add together, resulting in 15 amps of current returning on the neutral. That would be an unbalanced load and preferably avoided.
The leg or legs that each circuit draws from depends on where the breaker sits in the panel. In most panels, the breaker slots on each side of the panel alternate between the hot bus bars (legs). If two single-pole breakers are on the same side and are stacked one on top of the other, they will connect to different legs. If they're on the same side but have a slot in between them, they will connect to the same leg. Double-pole breakers take up two adjacent slots and connect to both legs. Each leg provides 120 volts for a total of 240 for the circuit. Because of this, double-pole breakers are automatically balanced, no matter where they are on the panel. Therefore, when you're laying out circuits for the house, the goal is to have roughly equal amperage draw on both legs of the panel.