Electrical Service Main Disconnect or Main Breaker: This article explains how to estimate the electrical service size, (or "electrical power" or "service amps") at a building by visual examination of the main electrical service disconnect switch, and related details.

In this article series on determining the ampacity and voltage provided by different residential electrical equipment, we also include photographs and sketches illustrate electrical panels, meter bases, and electric meters.

How to find the Main electrical service disconnect ampacity

In determining the service amperage at a property a key component to examine is the main disconnect switch such as we illustrate at page top and at left, thanks to Carson Dunlop Associates ' sketch.

We look for an amperage indication right on the main disconnect switch. If no markings are present, document that observation.

Markings in or on the panel box about ampacity indicate what current limiting device (such as a fuse) size could or should be installed, not what fuse or breaker size is actually present.

[Click to enlarge any image]

Look at the main electrical disconnect switch fuse/breaker for this ampacity number.

Beware: I [DF] have found buildings with mismatched SEC and main panel switch, with errors in both directions: in one instance a new 100A SEC was installed but connected to an old 60A service panel (safe but not delivering 100A to the building).

In a second case, a new 150A panel was installed but connected to an old 60A SEC (unsafe)

Split Bus Electrical Panels: Multiple Main Disconnects

Watch Out: some electrical panels, especially models using a split-bus design may not have a single main disconnect switch in the panel. There may be multiple "main" disconnects in the panel, usually grouped together.

Our Federal Pacific Electric panel (photo at left) appears to be one of these models [click to see an enlarged detailed view of this panel]. By its internal label (not shown) this panel was rated for 200 Amps.

Several of these switches may power individual high-ampacity electrical circuits in the building (air conditioner, electric range, electric water heater, for example) and one will power the lower bus in the panel to which branch circuits are wired for lighting and electrical receptacles.

Also check for a separate individual main disconnect switch between the panel and the electric meter.

Details are at SPLIT BUS ELECTRICAL PANEL HAZARDS

See more split bus panels

at FRANK ADAM ELECTRICAL PANELS

Also see FEDERAL PACIFIC FPE HAZARDS

On some fuse systems the main fuses, usually a cartridge type, may be visible and the fuse ratings may be read on the fuse itself. [Figure at left] There are varying opinions among inspectors about pulling out a fuse block to read a hidden fuse value.

Home inspection educator and writer Mark Cramer reminds us that you can usually see the color of the fuse through the little hole. Also note that you generally can't put a fuse that's too large into a block. For example, you can't put a 150 amp fuse in a 60 amp block.

However I have seen smaller fuses inserted in larger-fuse blocks using an adapter, and I've seen copper tubing and copper pipe inserted where a fuse belonged!

So if you can't see the fuse, it's risky to assume the right thing is in place. In Canadian and in some US panels fuses can be changed individually and may be more visible.

In an occupied building we do not pull a fuse or shut off any component without permission.

While these devices are designed to be pulled by the homeowner and thus fall under ASHI's definition of "normal user or operator controls," we've seen and had reports of accidents including broken fuse pullout blocks, arcing, loss of power, and angry owners whose computers or kidney dialysis machines were in operation.

And if we find a horrible rusty fuse or breaker panel such as the fuse panel shown at left, we would be reluctant to handle it - this equipment is unsafe and likely to be easily pushed "over the edge" into collapse simply by touching it.

We do pull fuses in unoccupied buildings and may pull fuses in an occupied building with permission, provided in both cases our visual inspection of the equipment does not reveal any obvious unsafe condition.

On the occasion when we pulled a fuse which disintegrated all parties agreed that it was better to discover the need for that repair now than in an emergency when power was needed.

If you find a fuse smaller than the maximum permitted by the conductors and panel equipment you should inform your client. But this is not a defect any more than it is unsafe to screw an 60-watt light bulb into a light designed to permit 100 watt bulbs.

Watch out: the opposite is not OK - a fuse that is rated at larger capacity than permitted for the wire size, such as a 30Amp fuse on a #14 wire branch circuit, would be unsafe. #14 copper wire is rated for a 15-Amp circuit and should be protected by a 15-Amp fuse or circuit breaker.

If you cannot see the actual fuse, you may be able to see a rating on the fuse holder itself. However be warned that there are methods (not recommended) for modifying fuse holders to carry fuses other than those intended. You may even find pieces of copper pipe used as "jumpers" to replace the original fuses. These modifications are unsafe. If you do not pull the fuse and cannot see the ampacity, report this limitation to your client.

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Continue reading  at SE CABLE & BRANCH CIRCUIT WIRE SIZES vs AMPS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• BLOWN FUSE REPLACEMENT
• ELECTRICAL INSPECTOR SAFETY PROCEDURES important basic safety procedures, clothing, and equipment for home inspectors and electrical inspectors.
• MAIN ELECTRICAL DISCONNECT
• SAFETY HAZARDS & SAFE ELECTRICAL INSPECTION PROCEDURES for examining Residential Electrical Panels.

Suggested citation for this web page

MAIN DISCONNECT AMPACITY at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to ELECTRICAL INSPECTION & TESTING

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