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What are the Limiting Factors That Determine the Electrical Capacity or Size at a Building?

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What determines the actual limits of available amperage at a residential property? Residential Electrical Service Amps Limitations: This article explains the Limiting Factors that determine the electrical capacity (amps) or size at a building.

We describe five components to examine, the risks or shortcomings of relying on each of these individually to estimate the service amperage at a property, and how the service ampacity at a property is set by the lowest-capacity item among the five things we examine.

One of the most frequently asked questions at ASHI Education Seminars and Conferences is "How do I determine the service amperage?" This article series includes photographs and sketches that illustrate different ages and capacities of residential electrical panels, meter bases, and electric meters.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

LIMITING FACTOR sets AMPS - Checking the Electrical Service amperage - what is the limiting factor?

Definition of amps or amperage Carson Dunlop AssociatesThe safe and proper service amperage available at a property is set by the smallest of: the service conductors, the main disconnect fuse or switch, or the rated capacity of the electric panel itself. The inspector should consider all three of these and report any inconsistencies among them.

Sketch at left, courtesy of Carson Dunlop Associates, illustrates a definition of amperage or current flow. Details are at DEFINITIONS of ELECTRICAL TERMS

The main fuse/ circuit breaker (CB) is the only component which actively limits amperage at a property by shutting off loads drawing more than the main fuse rating. (Undersized conductors or service panels, if switched by a mismatched and oversized fuse or breaker, will permit more current to flow than their design intended, and can then overheat causing a risk of fire.)

Readers should also be sure to see AMPACITY - the LIMITING FACTOR. This article series explains how to estimate the electrical service size, (or "electrical power" or "service amps") at a building by visual examination of the service entry cables, electric meter and meter base, electrical service panel, main switch, and other details. One of the most frequently asked basic electrical questions is "How do I determine the service amperage?"

The main breakers or fuses are permitted to be smaller (have lower overcurrent protection) than the capability of the service equipment (panel) and conductors (entrance cable). For example, a 2AWG copper service conductor (rated for 125 Amps, or in Canada for 120 Amps) to a panel using a 100-Amp main (service disconnect) is not a defect.["Residential Electrical Inspection," Douglas Hansen, ITA class material, Ver. 6.1 Oct. 1991, Los Altos, CA] You may want to point out to your client that such a service can be upgraded to higher amperage simply by installing larger main fuses/breakers.

Note that the main disconnect may be a fuse, breaker, or switch which is physically separate from the distribution panel. This is more common in older equipment installations and in very large commercial high-amperage installations.

Watch out: for a new higher amps-rated electrical panel connected to old, smaller-capacity service entry cables. A common defect found in upgrades of older equipment is the installation of a new 100-Amp panel and main breaker while failing to replace the old 60-amp service entry cables. This is either incomplete work or work by an untrained person.

Keep in mind that by service entry cables we're referring to the cables from the mast head (point of attachment of overhead wires to the building) down to the meter and from meter into the main switch or panel. Overhead wires, being cooled by open air, may be of a smaller diameter determined to be safe by the utility company for open air use.

Changing overhead electrical wires, San Miguel de Allende (C) Daniel FriedmanElectrical Services providing less than 100 Amps are found in older properties but for a detached single-family dwelling these are considered obsolete and are a financing issue with some US lenders.[NOTE: NEC-230-42(b,1) and some FHA, VA, and private Banks in some states.]

Electrical services of 60 Amp capacity are still allowed in at least some Canadian areas and in the US for loads such as individual apartments. Inspectors should not write up 60-Amp services as a defect without first checking these points. In cabins or similar vacation structures in the U.S. and Canada, and in buildings in some other countries such as Mexico, much smaller amperage services are often brought into the building.

Home Inspectors should make clear to all parties that generally, electrical defects are unsafe conditions, can be quite dangerous, and deserve prompt attention.

One exception: an undersized service, if it is properly fused, will be inconvenient but is not itself inherently unsafe. For such systems the safety and fire risks come more often from the temptation of frustrated owners to subvert or bypass the safety devices the classic "penny in the fuse base" problem.

See SAFETY HAZARDS & SAFE ELECTRICAL INSPECTION PROCEDURES for examining Residential Electrical Panels.

AMPACITY - the LIMITING FACTOR - Summary: What is the limiting factor that determines the actual electrical service ampacity at a building?

Remember you're looking for the limiting factor in determining ampacity. Finding the electrical component in this list which has the smallest capacity means that you have found the "weak link in the chain" of components bringing electrical service to the building.

Particularly when inspecting older properties, where there is the chance that someone has upgraded some but not all of the components in this list, there may be an inconsistency such as the installation of a larger electrical panel without upgrading the main disconnect.

  1. Electrical service entry cable or "SEC" size and amperage rating (we do not generally consider the overhead or service lateral wiring)
  2. Electric meter (questionable but may assist, especially if the item's generation, age, and obsolescence can be determined)
  3. Electric meter base (questionable but may assist, especially if the item's generation, age, and obsolescence can be determined)
  4. Main electrical power switch fuse/circuit breaker ampacity
  5. Electrical panel rated ampacity (and bus design may support only 120V)

Ice on the electric meter and ice on the service entry cable at a building in Rhinebeck NY (C) Daniel FriedmanInspectors attempting to determine (or estimate) the ampacity of the electrical service at a building should report their determination.

If conditions limit your inspection, such as in the photo at left, you can't identify the capacity of all the components described in the text above. In that case you should report which determining components were or were not visible or identifiable.

We don't hesitate to tell our clients what the electrical service size (ampacity) appears to be from what we could see . But we make quite clear the difference between what we know and what we think.

Readers of this article should also be sure to review SAFETY HAZARDS & SAFE ELECTRICAL INSPECTION PROCEDURES for examining Residential Electrical Panels. T

The articles from which much of this online material originated appeared first in the ASHI Technical Journal, Vol. 2. No. 1, January 1992, "Determining Service Ampacity," Dan Friedman and Alan Carson, and the ASHI Technical Journal, Vol. 3. No. 1, Spring, 1993, "Determining Service Ampacity - Another Consideration," Robert L. Klewitz, P.E., with subsequent updates and additions to the original text ongoing to 2/19/2006. Reprints of the originals and reprints of the Journal are available from ASHI, the American Society of Home Inspectors

Articles on Determination of Ampacity & Voltage at Building Electrical Services


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