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Main electrical panel inspection procedures & defects:
This article summarizes inspection of the building electrical panel, main panel, or electrical distribution and sub panels. This article series discusses procedures for safe and effective visual inspection of residential electrical systems including electrical panels and other components, when the inspection is conducted by trained building inspection professionals, home inspectors, electrical inspectors, and electricians.
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.
PANEL COVER - Residential Electrical Panel Cover Inspection and Special Safety Hazards - Look before you touch!
Information in this electrical inspection article series was presented by
Daniel Friedman - InspectApedia.com, and discussed by the Hudson Valley chapter of the American Society of Home Inspectors -
HVASHI Seminar 12 Sept 2002, Updated 2006, 2009, Jan 2014.
Match wiring gauge to ampacity - the following describe copper wire sized or gauges and the matching circuit ampacity or overcurrent protection that is required to be provided by fuse or circuit breaker:
14 gauge - 15Amps
12 gauge - 20 Amps
10 gauge - 30 Amps
8 gauge - 40 Amps
Exceptions to these wire sizes and fusing occur for special circumstances such as air conditioners whose motors produce a brief current surge during startup.
Aluminum electrical branch circuit wiring size guide: for a circuit of the same amperage, an aluminum solid conductor branch circuit wire must be 1 size larger than copper. Currently (since the 1970's) aluminum electrical wiring is used only in the form of multi-strand on single-use circuits like range /DHW/AC
older solid conductor AL may be present in next gauge.
Mains: Not in:
bathroom, clothes closet, kitchen cabinets, stairwells (same as service disconnect)
Should be at eye level, easily accessible, 3 ft. clearance
Subs: may be located almost anywhere (still some Sub panels are improper as above).
Finding hidden sub panels: Look for feeder-breaker/fuse or wires leaving main
FUSES vs CIRCUIT BREAKERS - Relative Electrical Safety of Fuses versus Circuit Breakers
more reliable to "blow" - not mechanical BUT
easier to over-fuse (reduce with S-type retro)
more convenient to reset
can be tested without having to then replace the safety device -
less likely to be over-fused (but over fusing still happens - compare wire gauge to breaker ampacity)
MAIN DISCONNECT INSPECTION - Electrical Panel Main Disconnect Inspection - Do you Pull the Main Fuse or Test the Main Breaker?
may be necessary to get some panel covers off
may be unsafe or may not want to shut power off to building
ask occupant/owner before killing power
report limitations to inspection
re-set accidentally-tripped breakers and leave note
OTHER ELECTRICAL PANEL DEFECTS - Other Electrical Panel Defects Discoverable by Visual Inspection
Obsolete and/or fused neutrals
Inappropriate support material
Unprotected panel openings/knockouts
Undersized panel - physical size/crowded
look at mains, breakers, bus, neutrals, wires
Rust or water -
look for rust, corrosion, overheating, subtle spots, suspect hidden damage to bus or breakers,
look for water trace marks on entering SEC
circuits not labeled
poor access to panel
panel upside down or switches on in down position (obsolete)
Panel used-with and not suitable for AL wiring
Continue reading at ELECTRICAL PANEL COVER SCREWS and warnings about removing and replacing panel covers, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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(Nov 15, 2012) Leo Duprey said:
I have a 200 amp main. How many and at what sizes of subpanels can I install from this main?(June 1, 2014)
Steve Pettifer said:
I have a 100 amp panel and when I turn on a breaker it reduces the voltage on one buss and increases the voltage on the other buss. I have checked all I can and even installed a new ground. How can that happen?
Sounds strange to me too.
If this is a 2-pole breaker it could have an internal fault.
If the voltage change is small the fault may be in how it's being measured.
(June 1, 2014) Anonymous said:
I had 225 volts on a 15 amp.
The last thing I tried was a 15 amp electric heater.
That gave me 50 volts on one buss and 190 on the other.
This is a sub panel but I have replaced the 100 amp in the main just in case.
I am checking voltage from each of the buss to the common buss.
Sounds as if we're either confusing volts and amps or you've connected or made a 120V circuit to a 240V circuit by crossing wires.
Not sure how you are measuring voltage nor if the procedure is safe, but in general you would expect to see similar voltage levels on each leg of a residential panel, around 120V.
You may be confusing amps and volts; current draw from plugging something in on a circuit shouldn't change the voltage level.
If you have abnormal voltage entering the panel you should call your electric company for diagnosis and repair assistance and
WATCH OUT: if the cause for voltage drop is in your equipment, wiring, or on your property there may be close-by serious safety hazards.
Safety Warning: Opening an electrical panel and approaching any live electrical wiring, devices, & equipment is a dangerous procedure that can damage electrical equipment or worse, cause electrical shock, or even death.
Such procedures should not be undertaken unless the person conducting the examination is trained and competent to avoid electric shock. If the inspector is not trained for this procedure s/he should never insert any instrument or tool into electrical equipment.
(June 1, 2014) Steve Pettifer said:
Starting to look like a break in the feed from main panel to sub panel.
Steve that's the sort of fault that left me worried about a site hazard - if wire is damaged and current is shorting somewhere; I've seen live electrical siding, porch railings, plumbing pipes, gas pipes.
Question: define open ground
(Sept 21, 2014) raul sanchez said:
What does open ground means
It means not connected to earth
In turn, that means "unsafe" since the grounding system cannot protect the occupants or wiring system if the grounding path is incomplete;
Question: a 8 space 16 circuit panel and a 16 space 16 circuit panel?
(Jan 23, 2015) curious said:
What is the difference between a 8 space 16 circuit panel and a 16 space 16 circuit panel?
Why would I choose one type over the other? (for what applications and/or their respective limitations)
I doubt that vendors are meticulous about terminology, but sometimes "space" means there is a spot to plug in a circuit breaker - the panel is unpopulated; while "circuit" may mean that a circuit breaker is included.
In any case, 16 spaces means you can plug in 16 circuit breakers with each breaker occupying one slot in the panel.
An 8-space panel that claims to offer 16 circuits must be using a form of circuit breaker that allows two breakers to fit into a single slot in the panel
Question: source of lists of electrical panel problems
(Jan 24, 2015) Mike said:
Where do you get your list for problem electrical panels?
40 years of building inspection, publications, research, teaching and attending professional conferences, talking with electricians and builders and home inspectors, reading research on electrical failures, IEEE, Holm Conference, ASME, and various trade associations. The original list of electrical distribution panel defects with which we started was scribbled out by a team of senior home inspectors, ASHI members, at a series of meetings intended to define an education curriculum as well as a basis for testing and certification exams for home inspectors. Ultimately ASHI decided not to follow the curriculum plans though of course, home inspection education companies lept into the gap.
But if you're saying you could add to the list, please do. None of us has a corner on the market of information.
Question: finding current with main breaker off
14 Feb 2015 LarryP said:
I have a strange measurement- with the main breaker OFF and all other panel breakers OFF if I measure between the HOT side of the main breaker and the NEUTRAL bus bar I get 30vac- ideas please
Watch out: this sounds very dangerous; I suspect a damaged service entry cable (with an internal short) or a damaged electrical panel or main breaker (some brands like FPE can remain "on" internally when switched off physically)
When I found the condition you described my brother-in-law, a theater electrician who didn't need to follow the NEC, had over-tightened BX connectors so that at many electrical boxes the neutral and hot were dead shorted. We found those quickly. But weaker shorts continued to trip GFCIs until we visited every single wire connection to find and re-make the faulty ones. Somewhere you may have a short hot to neutral or a failing electrical component giving the same effect.
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Recommended books on electrical inspection, electrical wiring, electrical problem diagnosis, and electrical repair can be found in the Electrical Books section of the InspectAPedia Bookstore. (courtesy of Amazon.com)
"Electrical System Inspection Basics," Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
"Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
"How to plan and install electric wiring for homes, farms, garages, shops," Montgomery Ward Co., 83-850.
"Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
"Home Wiring Inspection," Roswell W. Ard, Rodale's New Shelter, July/August, 1985 p. 35-40.
"Evaluating Wiring in Older Minnesota Homes," Agricultural Extension Service, University of Minnesota, St. Paul, Minnesota 55108.
"Electrical Systems," A Training Manual for Home Inspectors, Alfred L. Alk, American Society of Home Inspectors (ASHI), 1987, available from ASHI. [DF NOTE: I do NOT recommend this obsolete publication, though it was cited in the original Journal article as it contains unsafe inaccuracies]
"Basic Housing Inspection," US DHEW, S352.75 U48, p.144, out of print, but is available in most state libraries.
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