Determining the Capacity of a Residential Electric Service

Feb 21, 2017

Determining the Capacity of a Residential Electric Service

This newsletter will help to provide an understanding of how to determine the capacity of a residential electric service. Quite often we are asked the seemingly simple question “What is the size of my electric service”? In most cases this is a simple question to answer, as long as one knows what to look for.

Volts vs. Amperes

First, it is important to understand that the capacity of an electric service is measured in amperage or current, not in volts. Amperage is the flow rate of the electrical current that is available. The more current capacity or amperage that is available, the more electrical appliances that can be utilized at a given time in the building. Residential electric service enters the building in two forms, 120 volts and 240 volts. These are nominal numbers, which means that the actual voltage in the house can vary. Often times, 240 volt electric service is referred to as “220”.

To help understand the difference between volts and amperes, electric service can be compared to the flow of water in a pipe. The amount of water flowing in a pipe is typically measured in a volume of water per a unit measure of time. As an example, 10 gallons per minute of water may flow through a certain pipe. This flow rate of water is analogous to amperage or current in an electric wire. Current is the measurement of the amount of electric service that is “flowing” through the wire at a given time. The pressure of the water flowing through the pipe is not a measurement of the amount of water, but rather the amount of energy that is generated by the water inside the pipe. Similarly the voltage carried by an electric wire is a measurement of the amount of energy that is being carried.

Once again, the amperage of an electric service determines its capacity and the voltage of the service (120 volts or 240 volts) is a determination of the form of the electric service that is utilized. In residential applications 120 volt service is utilized for lights, outlets, small appliances (such as microwave ovens, irons, toasters, clocks, televisions) etc. “220” volt service is utilized for larger electrical appliances such as air conditioning units, electric dryers, electric ranges, electric heaters, etc. Almost all modern homes will have 220 volt electric service capability in the house. There are still a few houses around that do not have 220 volt capability at the present time. These are usually old homes that have not had electrical upgrades for a period of many years. They are a rarity.

At the risk of oversimplifying, an easy way of determining whether a house has 220 volt or only 120 volt electric service would be to visually examine the overhead electric wire that connects to the house. The overhead wire is referred to as the service entry cable or service lateral. There are three wires, two “hot legs” and a separate neutral. The neutral is usually bare, meaning that you can actually see the metal wire. The hot legs are insulated, typically with a black rubber covering. This overhead wire is connected to the electric service cable, or “riser” for the house at the point where the overhead wire is attached to the building. If all three wires are connected to the service “riser” that runs down the wall of the house, you can typically conclude that there is 220 volt service for the house. This is because each of the “hot legs” carries 120 volts, together providing 240 volt or “220”, to the house. To the contrary, if one of the hot wires for the overhead service is not connected to the riser cable Determining the Capacity of the Service

The electric service capacities that one might see in residential buildings are 30 amperes, 60 amperes, 100 amperes, 125 amperes, 150 amperes, and 200 amperes. In a few instances the capacity is greater than 200 amperes, but this would only be the case for large modern upscale homes with greater electrical needs. We would offer the following in regard to these various capacities:

  • 30 Ampere. As discussed above, 30 ampere service has become quite a rarity. A 30 ampere service would only be of 120 volt capability. Those rare cases where 30 ampere service is found would be small older homes that have been lived in by the same family or person for a period of generations, and the need for modernization or upgrade has not presented itself. This service is considered inadequate for modern living.
  • 60 Ampere. This is typically the lowest capacity for a 120/240 volt service. This capacity is considered to be marginal at best for modern living. Quite often, 60 ampere service also includes the presence of an old fuse panel, as opposed to the more modern circuit breaker panel.
  • 100 Ampere. A large number of existing average size homes have 100 ampere capacity electric services. Average size homes with gas or oil heating systems and hot water systems generally do not need electric service of greater than 100 amperes capacity. Of course, this can also depend on the electrical usage of the occupants, and the use of other electrical appliances.
  • 125 Amperes. These are very rare and will be discussed at the end of this document.
  • 150 Amperes. Common practice is such that this has become the typical minimum that might be installed in modern construction for a single family home.
  • 200 Amperes. This is becoming the norm for modern single family residential construction. In many instances it is not a necessity, but it is installed with new construction.

What is the Capacity?

To put in it simple terms, the capacity of the electric service in a house is determined by three factors, the capacity of the service entry cable (the cable feeding the house), the capacity of the main electric panel, and the capacity of the main disconnect. In most cases these three factors are the same. In other words, it is very common for a 100 ampere capacity cable to feed a 100 ampere capacity circuit breaker panel with a 100 ampere capacity main disconnect.

Capacity of the Service Entry Cable.

Sometimes the actual capacity of the service entry cable is printed directly on the cable. Unfortunately this is not common, but when looking at some cables you will see “100A” or “150A”. This easily identifies the capacity of the cable. More often, the capacity of the cable can be estimated by the size of the cable. Once again, at the risk of oversimplifying:

  • 60 ampere capacity service entry cables are between 3/4 inch and 7/8 inch wide
  • 100 ampere capacity cables are approximately 1 inch wide
  • 150 ampere capacity cables are approximately 1-1/4 inches wide
  • 200 ampere capacity cables are typically 1 and 1/2 inch wide.

The width of the cable can vary depending on whether it is a copper (older) or aluminum cable, and also depending on the material of the outer jacketing.

Panel Rating

The rating of a panel is usually indicated on the label inside of the panel door. These labels will typically indicate “200 ampere max. capacity” or “100A maximum capacity”.

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).

Multi-family Buildings

It is often times the case that multi-family residences have separate or individual electric services for each apartment. In these cases it is common for a single large service entry cable to feed a number of electric meters. Each individual meter then feeds each individual electrical panel. The capacity of the service for each apartment is determined by the cable feeding each of the individual panels and the rating of the individual panels and their disconnects.

Split Buss Panels

Confusion arises when the use of split buss panels comes into play. These types of panels were utilized with great frequency around the 1950’s and 60’s. They are not equipped with a single disconnect. This can cause confusion since the size of the main disconnect often determines the capacity of the service. For a split buss panel the size or capacity of the service is determined by the size of the cable and the rating of the panel (since there is no single main disconnect). It is very common for split buss panels to have a rating of 125 ampere maximum capacity. It is also common for these panels to be fed with a 100 capacity cable. This service would be considered of 100 amperes capacity (as dictated by the cable). 125 ampere capacity services are rare due to the fact that 125 ampere capacity service entry cables are a rarity.

Electrical codes for existing services do not require that a panel be equipped with a single main disconnect. This is why split buss panels remain in use today. They are not utilized for new installations, but many of the panels are still in use. Electrical codes do limit the number of main disconnects to six. This is commonly referred to as the “6 throw rule”. What it means that one must be able to turn off all the electric service to the house with no more than 6 main disconnects.

I hope that this discussion provides you with some general understanding regarding how to determine the capacity of an electric service. Please feel free to call our office if you have specific questions in this regard. We are always willing to help.

Douglas J. Burgasser, P. E.

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