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Electrical Systems 101

Panel Sizes

Contrary to popular opinion, the size of the electrical service to your house is not necessarily the rating stamped on the electrical panel cover. That marking is the maximum capacity of the panel. The panel, the main circuit breakers (or fuses), and the service entrance cable (SEC) must all have a 100 amp capacity for you to have a 100 amp service. For example, a 200 amp panel with a 100 amp SEC and a 100 amp main circuit breaker means the house has a 100 amp electrical service. The size and material (copper or aluminum) of the SEC determines its capacity. In most cases, the SEC capacity is the limiting factor that determines the electrical service capacity.

Most houses built in the last 30 years have at least a 100 amp electrical service, although many smaller, older homes have been getting along adequately with a 60 amp electrical service as long as they have gas appliances, gas equipment, and no central air conditioning.

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Ground Fault Electrical Circuits

A ground fault circuit interrupter (GFCI) is a safety device installed to protect you when using electrical devices in wet locations. When using an electrical device under normal conditions, the same amount of current (amps) flows in the “hot” wire and the “neutral” wire. The ground fault circuit interrupter measures the difference between the current in the “hot” and neutral wires. If your body becomes the path of least resistance, the current flows through you, reducing the current in the neutral wire. If the difference in currents in the “hot” and neutral wires is more than 5 Milliamps, the ground fault circuit interrupter senses this and shuts off the power to that outlet in approximately one-fortieth of one second.

Ground fault circuit interrupters come in 2 types: ground-fault electrical outlets and ground-fault circuit breakers. Both have the same function. Ground fault electrical outlets are the ones with the “test” and “reset” buttons on them. Ground fault circuit breakers are the ones in your electrical panel with a “test” button on them. Both types should be tested once a month to make sure the safety device is still working. Just because there is power at a ground fault-protected circuit doesn’t necessarily mean the safety device is working. It must be regularly tested. Approximately 20% of the ground fault safety devices that we test are not working.

Ground fault safety devices were originally required only in bathrooms beginning in about 1980. Gradually the code requirements have changed to include all “wet” (within 6 feet of a water source) locations in and around the home, such as bathrooms, kitchens, laundries, garages, whirlpools, jacuzzis, and exterior outlets. Several electrical outlets can be connected to one ground fault safety device.

Over Current Protection Devices

Current protection devices (fuses and circuit breakers) are designed to protect the wires connected to them. They limit the current (amperes) flowing through the wire to prevent it from overheating. An overheated wire could melt its insulation and cause a fire. Current protection devices come in two types: Circuit Breakers and Fuses. Each size fuse or circuit breaker is made to be connected to a particular wire material and size. Fifteen amp fuses or circuit breakers should be connected to a #14 solid copper wire. Twenty amp fuses or circuit breakers should be connected to a #12 solid copper wire. Thirty amp fuses or circuit breakers should be connected to a #10 solid copper wire, etc. Oversized fuses or circuit breakers are potentially dangerous. If a 30 amp circuit breaker or fuse has a #12 solid copper wire connected to it, a potentially dangerous situation exists. If 25 amps of current were flowing through the wire, which was only rated to safely handle 20 amps, the wire will overheat and could cause a fire. Since only 25 amps are flowing the 30 amp circuit breaker will not trip to protect the wire. Oversized fuses or circuit breakers should be corrected by a licensed electrician.

When we encounter a situation with oversized fuses or breakers, we frequently hear “It’s been like that for years, and we’ve never had a problem!”. It was only luck that a fire didn’t start.

Aluminum Branch Circuit Wiring

Circuits that use single solid aluminum wiring are considered a significantly higher fire risk than copper-wired circuits. Aluminum wiring is not considered a safety hazard on the multi-stranded wire used for major and minor appliance circuits or feeders to panels and subpanels.
Solid aluminum circuit wires were used in tract housing built between 1964 and 1975. Some high-rise housing also utilized this wiring.

Aluminum wiring expands more when current flows through it than copper wiring. Consequently, aluminum wires can tend to work loose where they are connected to panels and devices. The loose connections can arc and cause a fire. Aluminum wire also appears to cause a chemical reaction where it connects to some receptacles and switches. Aluminum oxide forms at the connection points. Aluminum oxide is very resistant to electrical current flow. The aluminum oxide heats up and creates an intermittently hot connection. One in five houses with aluminum wire in the general electrical circuits has an intermittent hot connection according to government research.

Popping, snapping, or heat from electrical devices is a warning. The U.S. Consumer Product Safety Commission (CPSC) says the safest way to repair aluminum lighting circuits is to use “cop alum” connectors. This should only be done by an electrician that specializes in this type of work. Many professional electricians are not familiar with the required procedures. This process is relatively expensive but cheaper than a new house.


Homeowners should turn all circuit breakers off and on once per year to clear the contacts of any corrosion which may form. The surface of the circuit breakers should be periodically felt for “hot spots”. If abnormally hot spots are detected, a licensed electrician should be contacted to investigate and make corrections. Make sure the service entrance cable (SEC) has no tree branches touching it. Remove them if necessary. The service entrance cable is attached to the service head. Make sure the service head is still properly secured to the side of the house. If not, resecure it. Your electrical panels are safety devices. Occasionally you may need to get into it quickly to turn off a circuit breaker. Every panel should have three feet of clear space in front of it at all times. (This is actually a code requirement). Find another place to store those items!