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Ice on electric meter box (C) 2010 HankeyandBrown.com Sources of Moisture Intrusion and Corrosion in Residential Electrical Services

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This article discusses moisture movement methods, provisions of the National Electric Code intended to reduce the potential for intrusion, and three home inspections of wet electric service panels, each with a different method of water entry: flow along an overhead service drop, flow through an underground conduit, and condensation induced by humid air entering a panel in an air conditioned basement.

Photographs are included to illustrate the causes and paths for the moisture intrusion.

References to other cases are included, along with references to further information on the movement of moisture in buildings and corrosion in electrical panels.



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Moisture Intrusion and Corrosion in Residential Electrical Service Panels

Also see CORROSION in ELECTRICAL PANELS and a complete approach to finding and correcting building moisture & leak problems see MOISTURE CONTROL in BUILDINGS

Roger Hankey, ASHI® Certified Inspector #269, October 2010

Article Contents

Field study of sources of moisture & causes of corrosion in residential electrical panels.

ABSTRACT: The intrusion of moisture at electric service panels has the potential to cause corrosion and water damage to electrical equipment. While the design and installation of most systems keeps them dry, roughly 10% of residential systems suffer from moisture intrusion and corrosion1.

Home inspectors are tasked with examining electric service panels, typically in existing homes, for potential home buyers. This article discusses moisture movement methods, provisions of the National Electric Code intended to reduce the potential for intrusion, and three home inspections of wet electric service panels, each with a different method of water entry: flow along an overhead service drop, flow through an underground conduit, and condensation induced by humid air entering a panel in an air conditioned basement. Photographs are included to illustrate the causes and paths for the moisture intrusion. References to other cases are included, along with references to further information on the movement of moisture in buildings and corrosion in electrical panels.

Introduction to Water Problems in Electrical Equipment

Moisture intrusion means water, whether liquid, frozen, condensed or vaporized, that penetrates into a structure or behind the outside cladding of a structure.2

The presence of water, except in plumbing components designed to provide, receive, store, or remove water, and on exterior components designed to shed water, has the potential to damage building systems and components. Given the widespread presence of water in the environment, building components and systems usually are designed to reduce the potential for moisture intrusion.

How Water Moves in buildings - Basic Building Science

Manufacturers of electrical equipment recognize this omnipresence of water and specify product installation methods and maintenance to reduce the potential for moisture intrusion and remove water that enters the system. Success in diagnosing moisture intrusion begins with understanding the ways moisture moves. The four moisture transport mechanisms predominant in building science are:

Liquid flow water movement is primarily influenced by gravity and wind. This method of moisture movement requires an entry point and path for the water flow to reach electrical components.

Capillary suction causing water movement is an important factor in moisture movement in many porous building materials but rarely occurs on non-porous metallic or plastic electrical components. Capillary suction can dampen a basement foundation wall. Many interior service or distribution panels are mounted on foundation walls. If the panel is not isolated from the foundation wall by an air space and a suitable mounting board, the moisture in the foundation wall can corrode the panel.

Moisture movement from air flow can occasionally be a factor leading to condensation in electrical equipment. The air flow can be caused by air pressure differentials, convection currents such the “stack effect” of rising warm air being displaced by falling cooler air, or by mechanical equipment such as fans and blowers.

Vapor diffusion (water vapor moving directly through permeable solid materials) is the moisture transport method that moves the least amount of water in a building, and is rarely a factor in introducing water into electrical systems which are composed primarily of non-permeable materials.4

Building & Home Inspection Standards vs. Building Moisture

Home inspection standards: The American Society of Home Inspectors® (ASHI) Standards do not require the inspector to determine compliance with regulatory requirements, (Sec. 13.2A8). However, a general knowledge of long standing regulatory requirements can serve as a basis for understanding why identifying and reporting moisture intrusion and corrosion in electrical equipment is an important aspect of a home inspection.

Wet or corroded electrical equipment are “components inspected that, in the professional judgment of the inspector, are not functioning properly”, as stated in ASHI® Standards 2.2C1, and therefore must be reported. The Standards also require opening the electric service panel.

The National Electric Code & moisture intrusion:

The National Electric Code (NEC) requires metallic electrical equipment be protected against deteriorating agents, including corrosion. NEC section 100 defines terms including:

Rain tight: Constructed or protected so that exposure to a beating rain will not result in the entrance of water under specified test conditions.

NEC 110.11 states “Unless identified for use in the operating environment, no conductors or equipment shall be located in damp or wet locations”; and NEC 110.12(C) states “There shall be no damaged parts that may adversely affect safe operation or mechanical strength of the equipment such as parts that are broken; bent; cut; or deteriorated by corrosion, chemical action, or overheating.”

NEC 300-6 (A) states: “Ferrous raceways, cable trays, cable bus, auxiliary gutters, cable armor, boxes, cable sheathing, cabinets, metal elbows, couplings, fittings, supports, and support hardware shall be suitably protected against corrosion inside and outside (except threads at joints) by a coating of approved corrosion-resistant material such as zinc, cadmium, or enamel. Where protected from corrosion solely by enamel, they shall not be used outdoors or in wet locations…”

Vulnerability of Indoor Electrical Components to Moisture

Many interior metallic electrical components are protected solely by enamel and therefore are not designed for contact with water. Moisture in electrical equipment can lead to:

4 Case Studies of Moisture Entry into Electrical System Components

Electrical service components, City of Bloomington MN Inspections Dept, R HankeyThe following cases illustrate three ways moisture can enter electrical components.

[Click to enlarge & see any image in higher resolution]

Electric services in the Twin Cities area of Minneapolis-St. Paul, Minnesota are nearly all installed using conduit.

Typical suburban homes have an underground service lateral from a ground mounted transformer to a meter box on the house near eye level and a raceway (conduit) descending to a short distance above grade with a 90 deg. elbow and a raceway through the rim joist to a main panel in the basement.

Others have a similar arrangement with the meter and service panel on either side of an attached garage wall.

Typical urban services have overhead service drops to a service mast head high on the wall or above the roof.

Drawing from City of Bloomington, MN Inspections Department

[Click any drawing or photo for an enlarged, more-detailed version.]

Drawing: City of Bloomington, MN Inspections Department

Most electric service installations incorporate good workmanship and follow generally established practices. Therefore these systems stay dry. The infrequent presence of moisture in electrical panels creates a challenge for those who investigate water intrusion.

While multiple adverse conditions often lead to the moisture intrusion, those conditions are often subtle variations from normal. The first indication of trouble is usually found at the service panel

Water Entry into Electrical Panel Case #1 - Leaks at SEC

Electric panel bottom corrosion (C) Roger Hankey

 

Case #1 is an example of these subtle variations. The subject was a St. Paul, MN house built in 1911. The overhead electric service was wall mounted with a fused main panel in the basement, circa 1960.

The obvious adverse condition is the corrosion on the bottom of the panel.

Upon close view there is also corrosion at the connections of the white neutral wires and the bare grounding wires above the “D” handle main disconnect.

Photos in this article (C) HankeyandBrown.com.
Electric panel water entry (C) Roger Hankey

[Click to enlarge any image]

Notice a water drop on the connection bar and the stained paper label. An earlier initial examination at the overhead service drop did not suggest any concerns.

Given the presence of water and corrosion on the neutral bus, a closer second inspection of the service drop was done.

The service drop was a braided triplex cable descending from a utility pole to the rear wall. Two black ungrounded conductors each had a slight drip loop.

The white grounded conductor was slightly shorter and entered the mast head horizontally rather than from below.(Photo below-left). Looking into the end of the mast head you can see how readily rain would follow the in-sloping white wire into the interior of the service entry conduit.

Electric service masthead leaks (C) Roger Hankey Electric service masthead leaks (C) Roger Hankey

A further inspection of the service revealed staining on the bottom of the elbow where the conduit penetrated the wall. Our next photo (below, left) shows the under-side of the conduit elbow cover.

There was no drain hole in the bottom of this elbow.

Therefore, the probable moisture path was along the service drop, into the mast head on the white neutral conductor, down the mast, through the meter box, through the elbow and into the main panel to the neutral bus and finally onto the bottom of the panel.

Electric conduit bottom leak stains(C) Roger Hankey

Photo (left) shows staining on conduit elbow under-side. [Click to enlarge any image]

The inspection report recommended a qualified electrician correct the drip loop, examine all the service components, and make any needed corrections to the service and fuse panel.

NEC references are not needed or helpful in an inspection report. However, they can help inspectors understand generally established design parameters and installation practices. Installations in substantial compliance with the NEC are less likely to have moisture intrusion.

NEC 225.22 Raceways on exterior surfaces of buildings or other structures shall be raintight and arranged to drain. (NEC frequently refers to conduits as raceways.)

NEC 230.52 Where individual open conductors enter a building or other structure, they shall enter through roof bushings or through the wall in an upward slant through individual, noncombustible, nonabsorbent insulating tubes. Drip loops shall be formed on the conductors before they enter the tubes.

NEC 230.53 Where exposed to the weather, raceways enclosing service-entrance conductors shall be raintight and arranged to drain.

NEC 230.54 Lists requirements of an overhead service that reduce the potential for water intrusion:

Water Entry into Electrical Panel Case #2 - Snow Melt

Water flowing over main circuit breaker (C) 2010 HankeyandBrown.com

Case #2 of water leaks in an electrical panel involves an early April 2008 inspection of a rural property with a house built in 2006.

Heavy wet snow fell two days before the inspection. The snow was rapidly melting due to mild sunny weather.

The electric service panel was located on the basement wall adjoining an attached grade level garage. The first concern found was a wet floor near the service panel.

Water was flowing out of the service panel! [We moved our © notice up so that you can see the puddle of water on the bottom of the electrical panel enclosure.]

Using gloves and great care (see SAFETY for ELECTRICAL INSPECTORS), the inspector removed the electrical panel cover, revealing water flowing from the service entrance conduit, onto the main breakers, cascading down the branch circuit breakers, and out the bottom of the panel.

Watch out: touching wet electrical system components or standing in or near wet areas during electrical panel inspections is dangerous, risks shock or electrocution, and should not be performed without the proper training and protective equipment.

Electric panel water entry points (C) Roger Hankey

The challenge in this case was to find the source of the water, which was draining into the electrical panel at about cup per minute.

The underground electrical service came to a electric meter box on a plywood panel erected about 20 feet from the attached garage (see next photo, below-left). The site near the service sloped toward the base of the meter stand.

The service then returned to the ground in a conduit. The sun had warmed the meter stand and melted the snow around its base. Further, a puddle of water had formed along the base of the garage wall.

These conditions were consistent with snow melt flowing subsurface above the frost line to a break or opening in the buried conduit permitting water entry and conveying water into the service panel.

Exterior electric service entrance stand (C) 2010 HankeyandBrown.com

The photo at left shows the free-standing electrical meter monitoring the underground electrical service discussed above.

An interesting aspect of this case, aside from the adverse condition found, was that during a call to the seller to alert him to this condition, he replied: “Oh, I thought I corrected that last year.”

The inspection report recommended a qualified electrician correct this condition as soon as possible.

 

 

 

 

 

 

Water Entry into Electrical Panel Case #3 - Negative Air Pressure & Moisture Condensation

Condensation on electric service panel (C) 2010 HankeyandBrown.com

Case #3 of water intrusion into an electrical panel began with condensation found on the inside service panel cover door

This inspection was conducted on a hot humid day in July 2010. The panel was in a semi-finished basement of a house built in 1987.

Central air conditioning was keeping the basement very cool. The only atypical condition found outside on the underground service entrance was a loose cover on the conduit elbow where the conduit entered the building by passing through a floor framing rim joist.

Removing the electrical panel cover revealed condensation throughout the panel including water dripping off wires and the neutral bus (yellow arrows, photo at below-left).

Condensation inside electric service panel (C) 2010 HankeyandBrown.com

Our photo at left shows a drop of water hanging from the center of the red wire in the top of the electrical panel.

The bottom of the electrical panel (next photo below-left) was wet and corroded. A closer inspection of the service entrance conductors found no sealant in the open end or in the conduit elbow at the building exterior.

The air handler and loose return air ducts in the HVAC system created negative air pressure in the basement which drew in the hot humid exterior air via the joints in the electric meter box and the opening on the conduit elbow.

When this warm air met the cool service panel interior surfaces, condensation formed throughout the electrical panel and dripped to the bottom.


Corrosion in bottom of electric service panel (C) 2010 HankeyandBrown.com

The recommendation in this case was to have a qualified electrician fully examine the service and make all necessary corrections.

Two NEC sections, if implemented, reduce the potential for moisture intrusion and corrosion:

NEC 230.8 Where a service raceway enters a building or structure from an underground distribution system, it shall be sealed …

NEC 300.7(A) Sealing. … where passing from the interior to the exterior of a building, the raceway or sleeve shall be filled with an approved material to prevent the circulation of warm air to a colder section of the raceway or sleeve.

Poorly installed duct seal in an electric service panel (C) 2010 HankeyandBrown.com

Not the best repair: Our photo at left shows a different application of duct seal putty inside of an electrical panel. However, like the little Dutch boy with his thumb in the dyke at the very last possible point, stopping air, moisture. This duct seal photo is not the same panel as the panel with the condensation (above), and while the two methods of sealing the conduit (interior and exterior) can be compared, they were not done on the same electric service.

Stopping water from entering an electrical panel, or any leak building leak for that matter, is usually better accomplished outside the building where the leaks originate.

See National Electrical Code Provisions Reduce Electrical Panel Water or Moisture Leaks, Corrosion for NEC code sections relevant to this moisture source.

For more technical advice on finding and correcting hidden air leaks in buildings see AIR BYPASS LEAKS and AIR LEAK DETECTION TOOLS.

Because condensation in an electrical panel or in / on any building interior surface is also a function of the relative indoor humidity, also see HUMIDITY LEVEL TARGET

Actual repair: Shown in our two photos below, the electrician correcting of this case applied putty known as “Duct Seal” around the conductors inside the service entrance elbow (below left) and cut a drain slot in the bottom edge of the elbow (below left, bottom of photo).


Electric service conduit elbow sealed and bottom notched (C) 2010 HankeyandBrown.com Typical underground electric service in Minnesota (C) 2010 HankeyandBrown.com

Also see SEWER GAS ODORS where we describe sewer gas leaks through electrical conduit.

Water Entry into Electrical Panel Case #4 - Mouse Infestation in the Electrical Panel

Case #4 of moisture intrusion into an electrical panel is an underground service feeding twin 200 amp panels on the rear wall of an attached garage at a 1994 built residence inspected in December 2010.

Gap in electric service conduit connection (C) 2010 HankeyandBrown.com

The service entrance conduit for this electrical panel (photo at left) had pulled out of the elbow leaving a gap of about an inch.  The movement of this conduit is consistent with soil compaction and probably occurred within three years of construction.

Corrosion on electric service panel (C) 2010 HankeyandBrown.com

The interior wall of the garage had large junction box which contained the “Y” connections that fed power to two 200 amp distribution panels and the current transformers that drove the solid state electric meter.

The bottom of the junction box had rusty corners.

There were stains on the wall between the right panel and the junction box, and corrosion on the panel cover at the main breaker of the right panel.

 

 

 


Mice in electrical panel (C) R Hankey

The gap in the service conduit was large enough to permit mice to enter the elbow and pass into the junction box.

Rodent nesting material was found in the bottom of the junction box and the right distribution panel.

Mice in electrical panel (C) Roger Hankey

The mouse urine had corroded the threads completely off the cover screws in the bottom of the right panel.

The bare copper grounding conductor in the lower right of the right panel had turned green where it was in contact with the nesting material.

Two dead mice were found: one on the left side of the right distribution panel and one in the junction box.

Mice in electrical panel (C) R Hankey

The recommended action was to have a qualified electrician clean out the box and panel as soon as possible, examine all components for damage, and make any needed repairs, including connecting the separated conduit on the exterior to prevent future mouse or insect infestation.

Other Sources of Water Intrusion / Corrosion in Electric Panels

Other conditions which can lead to water intrusion and corrosion in electric panels as well as other forms of damage include:

Service conduit pulled out of meter box by soil compaction (C) 2010 HankeyandBrown.com Electric meter box pulled down by soil compaction (C) 2010 HankeyandBrown.com Roof ice dam affecting electric service (C) 2010 HankeyandBrown.com

A challenge that faces building and home inspectors is that corroded electrical panels are likely to be dry during the inspection and the conditions which created corrosion are typically elsewhere in the system.

To avoid the ice dam problems such as shown on the building at left,
see ROOF ICE DAM LEAKS


Ice on electric meter box (C) 2010 HankeyandBrown.com

Ice accumulating and meltwater dripping off a roof has covered the electric meter and meter pan in our photo at left.

In warm weather, clues indicating the presence of these troublesome moisture sources may not be apparent.

Water intrusion Inspection limitations:

Precisely because the visual inspection of a building on a particular day cannot be expected to detect problems that may appear only during other weather conditions, home inspection standards such as those published by ASHI® do not require a home inspector to determine the source of the moisture intrusion. This task is left to electricians or to building forensic experts,.

But these examples suggest potential points of moisture intrusion.

Other potential sources of moisture and corrosion in electrical panels include: 

Watch out: extreme caution should be used when examining electrical equipment where water or moisture is present. When in doubt, leave further investigation of wet or corroded electrical services to qualified electricians with proper tools and the permission to shut the system down.

See SAFETY for ELECTRICAL INSPECTORS for more advice.

Electric service and fuse box in a shower stall (C) 2010 HankeyandBrown.com

 

Watch out for homeowner or other amateur modifications that affect the electrical system and electrical safety.

This potentially lethal electrical installation combines what was probably an original location for an indoor electrical meter and a later project to "add-on" a bathroom shower.

Even inspecting this electrical system could be extremely dangerous.
See SAFETY for ELECTRICAL INSPECTORS.

National Electrical Code Provisions Reduce Electrical Panel Water or Moisture Leaks, Corrosion

The following NEC sections, if implemented, reduce the potential for moisture intrusion and corrosion:

230.8 Where a service raceway enters a building or structure from an underground distribution system, it shall be sealed …

300.7(A) Sealing. … where passing from the interior to the exterior of a building, the raceway or sleeve shall be filled with an approved material to prevent the circulation of warm air to a colder section of the raceway or sleeve.

-- © Roger Hankey, text and illustrations adapted with permission from the author. Roger Hankey is principal of Hankey and Brown home inspectors, Eden Prairie, MN. Mr. Hankey is a past chairman of the ASHI Standards Committee. Mr. Hankey has served in other ASHI professional and leadership roles. Contact Roger Hankey at: 952 829-0044 - rhankey@hankeyandbrown.com. Mr. Hankey is a frequent contributor to InspectAPedia.com.

Most Common Water Enter Points into Electrical Meter Bases and Panels

Research of a large number of homes documented at CORROSION in ELECTRICAL PANELS found that at many electrical meters and panels the common leak points for water entry were found originating at the masthead, at frayed leaky service entry cable (SEC) insulation, at a poor seal of the SEC at its entry to the electrical meter base, or where the SEC penetrates the building wall passing to an electrical panel located lower than that pint, such as in a basement.

For those cases, the best repair would have been proper SEC installation details so that water was not entering the conduit and following the SEC cable in the first place.

In good building science it's generally better to keep water out of where it is not wanted than to let the water in and then let it run out.

White stains or "water tracking" marks sometimes found on SEC wires entering the top of an electrical panel or at other wiring in an electrical enclosure or conduit may be caused by mineral deposits or debris left from water running down inside the SEC cable jacket or conduit into the enclosure or conduit. Examples of these conditions are found in photographs that track water moving inside the SEC, visible at CORROSION in ELECTRICAL PANELS. - Ed.

Also of interest:

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Continue reading at CORROSION in ELECTRICAL PANELS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see HOW TO TURN OFF ELECTRICITY in a building that has been wet or flooded

Or see ELECTRICAL INSPECTOR SAFETY PROCEDURES important basic safety procedures, clothing, and equipment for home inspectors and electrical inspectors.

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

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