The Voltage Range in Your Home
It’s easy to get confused when we talk about the voltage range that a home electrical system supplies to our devices. For a long time, most people called the power from your home outlet “110 volts.” Similarly, “220 volts” was used for larger household appliances like electric ranges and clothes dryers. These designations for home AC power are actually out-of-date. They don’t accurately describe the voltage range that comes into your home from the electric utility. So, why the confusion?
These days, almost every residential customer can get 120 volts from their wall outlet. However, power is typically delivered into your home at a nominal voltage of 240 volts. Inside the transformer on the utility pole, the power is divided into a split phase system, with each line having a nominal voltage of 120 volts. Nominal voltage is the voltage that the line is designed for; however, in the real-world, the tolerance for voltage fluctuations is −5% to +5%. This leads to an actual voltage range of anywhere from 114V to 126V from your outlet and a voltage range of 228V to 252V for your full-phase appliances. Now, you might look at these voltage ranges and think that a difference that large could potentially pose a hazard to you or your electric devices. However, I can assure you that this is completely normal and is taken into consideration when the circuits are designed.
We talked briefly about the 240 volts that is supplied to your home from the power company. At the transformer, the single phase power from the utility company is split into 3 wires: 2 line wires and a ground. This is known as a single-phase three-wire or split-phase system. Normally, your lamps and other 120V devices are connected between one line wire and the grounded center, while electric stoves, dryers, and other appliances are connected to both line wires and the ground. This way, each half can balance the other when electrical loads increase. Our Quick 220® voltage converter combines these halves and produces a convenient outlet with a voltage range of 228V to 252V without the need to call an expensive electrical contractor.
Capacity of Main Disconnect
Most modern panels have a single main disconnect. Often times this disconnect is labeled as the “main”. The capacity of the disconnect is labeled directly on the disconnect. It will typically indicate “100A”, “150A”, or “200A”.
As you can see from the above, if you were to see that a panel is rated at 150 amperes maximum, and it is equipped with a 150 ampere main disconnect and it is fed by a 150 ampere cable, you can conclude that the service is of 150 amperes capacity.
There are instances when the three determining factors are not equal. For example, if a 100 ampere capacity cable is feeding a 150 ampere capacity panel with a 150 ampere capacity disconnect, the service would technically be considered a 100 ampere capacity service. The cable would be the limiting factor. In addition, this would be an unsafe condition since the cable would not be of great enough capacity to withstand the potential for 150 amperes of current that would be permitted by the panel and disconnect. The cable would be considered as undersized and replacement with an appropriately sized cable would be recommended for safety reasons. To the contrary, a 150 ampere cable feeding a 100 ampere capacity panel and disconnect would be considered a 100 ampere capacity service and it would also be considered safe (the cable can be oversized but it cannot be undersized).
HOW TO DETERMINE SIZE FOR A HOME ELECTRICAL SERVICE
When replacing an old electrical service entrance or designing an installation for a new home, one must be aware of current and future energy needs. Think about the myriad of kitchen appliances we take for granted today that hadn’t been imagined a few decades ago. Who would have thought spas and hot tubs would be common luxuries for many? Most RV’s require 50 amp 240 volt circuits to adequately power their needs. Where will you be plugging in your car in future years? Home electrical services must be sized to meet these demands. It is recommended that no residence be served with less than a 100 amp rated service entrance. This becomes a requirement when the load exceeds 10,000 watts or has 6 or more 2-wire branch circuits. Service size requirements vary according to area, so we recommend that a careful check be made before installing a minimum-sized service. Standard-sized services for residential applications are 100, 200, or 400 amps. The general rules of thumb for determining service sizes are:
- 100 amps will service a range, water heater, general lighting, and receptacle outlets. Usually recommended for workshops, vacation cabins and larger pump systems. This amount of ampacity is really too small for a residence unless gas heat and appliances are used.
- 200 amps will take care of range, dryer, water heater, general lighting and receptacles, and a considerable amount of electric heat, while the…
- 400 amp services are designed for use in larger homes that require over 20kW (20,000 watts) of electric heat plus all the aforementioned items.
We have drawn out a chart to help you determine your service requirements. It is based on Articles 220 of the 2005 National Electrical Code. The chart is very liberal in that it uses a 100% demand factor on all of the heat load as well as on some of the appliances that might otherwise be figured at a lesser amperage. Minimum service size can be found by adding up the total wattage that will be used, counting the first 10 kW at 100%, and using a 40% demand factor on all the rest. Once the calculated demand is determined in terms of wattage, divide that by 240 volts to convert it into amps. This would be your required service size. Pick from the standard services the one that is just above your amperage requirement.
Service Switch
A disconnecting means or “disconnect switch” must be provided in the service entrance to disconnect all the wiring in the building from the electrical power supply. This may be accomplished with a separate switch called a service switch or main disconnect. Many years ago it was common to install a separate switch next to the fuse box to shut the power off. In modern residences, the service switch is actually the main breaker housed inside the breaker box which shuts off power to all the other breakers in the box.
The size of the service switch or main breaker depends on the electrical load served. Service switches come in standard sizes of 30, 60, 100, 150, 200, 400 and larger. The National Electrical Code requires a minimum 100-amp service size for all residences, although many newer houses typically use a 150 amp or larger service.
Fuse Box vs. Circuit Breaker Panel
In most homes, the main service panel uses circuit breakers that control and protect the individual circuits. Circuit breakers are specially designed safety switches that prevent individual branch circuits from drawing more power than the circuit wires can safely handle. Virtually all homes built since the early 1960s use circuit breakers as the power distribution method. Older homes also have circuit breaker panels if their electrical service was updated after 1960.
Where an electrical service was installed before the early 1960s and has not been updated, it may use a different style of power distribution—a fuse panel, which protects individual circuits with screw-in or cartridge fuses.
The use of fuse panels and circuit breaker panels for residential wiring follows a historical pattern:
- 30-amp fuse panel: Installed before 1950, these service panels provide only 120-volt current. Such a service provides insufficient power for modern usage and generally needs to be updated.
- 60-amp fuse panel: Installed from 1950 to about 1965, 60-amp fuse panels provide 240-volts of power, but are still insufficient for most homes. An update is usually needed.
- Circuit breaker panel: Since the early 1960s, homes have generally been wired with circuit breaker panels that provide 240-volt current. Early services may provide 60-amps of power, while large houses built today may have 200 amps or more of power. Homes with 60-amp or 100-amp service often require an electrical service update during major remodeling or expansion projects.