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ELECTRICAL INSPECTION, DIAGNOSIS, REPAIR
ACCURACY vs PRECISION of MEASUREMENTS
AFCIs ARC FAULT CIRCUIT INTERRUPTERS
ALUMINUM SECs & WIRING
ALUMINUM WIRING HAZARDS & REPAIRS
AMPS VOLTS DETERMINATION
AMPERAGE MEASUREMENT METHODS
AMPACITY - the LIMITING FACTOR
APPLIANCE EFFICIENCY RATINGS
BACKUP ELECTRICAL GENERATORS
BACK-WIRED ELECTRICAL DEVICES
BOOKSTORE - ELECTRICAL
BUILDING SAFETY HAZARDS GUIDE
Cadet & Encore Heater Recall
CHALLENGER ELECTRIC PANELS
CIRCUIT BREAKER FAILURES
CIRCUIT BREAKER SIZE for A/C or HEAT PUMP
Classified CIRCUIT BREAKER WARNING
CORROSION in ELECTRICAL PANELS
CORROSION & MOISTURE SOURCES in PANELS
CUTLER HAMMER PANEL FIRE
DEFINITIONS of ELECTRICAL TERMS
Definition of Amps, Electrical Current
Definition of Electrical Circuits, shorts
Definition of Volts
Definition of Watts
How a Building Gets 240V and 120V
How many Watts in a Circuit
Definition of AC Alternating Current
Definition of DC Direct Current
Definition of Electrical Ground Terms
Definition of Electrical Potential
Definition of Ohms, Electrical Resistance
Definition of Power Factor, Real Power
EFFICIENCY of 120V vs 240V EQUIPMENT
DIRECTORY OF ELECTRICIANS
DMM Digital Multimeter HOW TO USE
ELECTRIC METERS & METER BASES
ELECTRIC MOTOR DIAGNOSTIC GUIDE
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRIC PANEL AMPACITY
ELECTRIC PANEL INSPECTION
ELECTRIC PANEL MOISTURE
Electric Power Frequency Table
ELECTRICITY BASICS, HOW IT WORKS
ELECTRICAL CIRCUIT ID, MAP & LABEL
ELECTRICAL CIRCUITS, SHORTS
ELECTRICAL CODE BASICS
ELECTRICAL GROUNDING BASICS
ELECTRICAL OUTLET, HOW TO ADD & WIRE
ELECTRICAL SPLICES, HOW TO MAKE
ELECTRICAL TOOLS & TESTS
ELECTRICAL WIRE STRIPPING TIPS
ELECTRICAL WIRING BOOKS & GUIDES
OLD HOUSE ELECTRICAL WIRING
OLD HOUSE ELECTRICAL GROUNDING
ELECTRICAL DISTRIBUTION PANELS
ELECTRICAL GROUND SYSTEM INSPECTION
ELECTRICAL SERVICE DROP
ELECTRICAL SERVICE ENTRY WIRING
ELECTRICAL SPLICES, HOW TO MAKE
EMF RF FIELD & FREQUENCY DEFINITIONS
ELECTRICAL GROUND SYSTEM INSPECTION
FEDERAL PACIFIC FPE HAZARDS
FIRE SAFETY Checklist, CPSC
GFCI PROTECTION,Testing GFCIs AFCIs
HEAT TAPE USAGE GUIDE
Hertz - Definitions of KHz MHz GHz THz
KNOB & TUBE WIRING
LIGHTING, EXTERIOR GUIDE
LIGHTING, INTERIOR GUIDE
LIGHTNING PROTECTION SYSTEMS
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
MANUALS & PARTS GUIDES - HVAC
MAIN ELECTRICAL DISCONNECT
MAIN DISCONNECT AMPACITY
MOISTURE SOURCES in PANELS
MURRAY SIEMENS Recall
PHOTOVOLTAIC POWER SYSTEMS
PUSHMATIC - BULLDOG PANELS
REMOTE ELECTRIC POWER, PHOTOVOLTAIC
RUST in ELECTRICAL PANELS
SAFETY for ELECTRICAL INSPECTORS
SE CABLE SIZES vs AMPS
SIEMENS MURRAY Recall
THERMAL EXPANSION of HOT WATER
THERMAL EXPANSION of MATERIALS
UNDERGROUND SERVICE LATERALS
VOLTS / AMPS MEASUREMENT EQUIP
VOLTAGE MEASUREMENT METHODS
WIND ENERGY SYSTEMS
WIND TURBINES & LIGHTNING
ZINSCO SYLVANIA ELECTRICAL PANELS
Building electrical system installation, troubleshooting & repairs & electrical hazards. How to install, inspect, & troubleshoot building electrical systems, appliances, components. How to detect & report electrical hazards, defects in residential and commercial electrical panels, switches, fixtures, electrical wiring & grounding systems. Proper electrical repair methods for unsafe electrical conditions.
Safety for the electrical inspector, aluminum electrical wiring hazards, how to determine service voltage and ampacity, how to inspect electrical panels, and significant electrical hazards of Federal Pacific Electric Stab-Lok breakers and panels, Federal Pioneer product warnings, certain Square-D product concerns, Zinsco and Sylvania circuit breakers and panels, multi-wire branch circuit protection, inspection and repair of low voltage wiring systems, and proper installation of lightning protection systems on buildings are addressed.
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Is it possible to connect two sources of Electricity to one Sub-Panel? - Muhammad K., Jordan
Reply: Use an isolation switch to avoid simultaneous (and unsafe) electrical power feeds to an electrical panel or sub panel - use backup electrical generator hookups as an example
By "possible to connect two sources of Electricity to one Sub-Panel" I presume you don't mean is it physically possible to connect two power sources to a single electrical panel since that would be trivial to accomplish; Rather I presume you mean is it acceptable practice or is it "safe" to connect multiple power sources into a single electrical panel or sub-panel.
The basic answer is no. In general it is very dangerous to have multiple sources of power into a single panel or subpanel because of the possibility of backfeeding and shocking someone who thinks power is off from a given source. So we don't hookup simultaneous live electrical power sources to a single electrical sub panel or main panel.
We do not hook up multiple electrical power sources to a single electrical sub panel without using an isolation switch.
In our photograph of an isolation switch hook-up (above left), the main electrical panel (1) is at left. In the photo center is an isolation switch (2) that allows the homeowner to switch individual circuits from being powered either by the main electrical panel's service entry mains or by an electrical generator (located outdoors) that is connected to the isolation switch by a removable plug shown hanging on the wall (3). You can see the receiving plug receptacle at the bottom center of the isolation switch (2).
The sub panel shown at right (next to our client) was an addition to the original electrical system to support central air conditioning and is not part of this discussion. See BACKUP ELECTRICAL GENERATORS for details.
Reader Question: History of Electrical Wiring in the U.S. - Split Receptacles or Electrical Outlets?
When were houses commonly built with split outlets/receptacles? I am doing research to find out how many houses in the US may not have them. Thanks! - Maria S.
Reply: A Nano-History of Electrical Wiring Devices in North America - Guessing: 1950 - 1965
You are referring to the practice of providing separate power from separate electrical circuits individually to the upper and lower receptacle openings of a duplex electrical receptacle.
That feature has been technically possible and therefore almost surely was done in some homes from around the time that duplex receptacles were first installed. (See Split Wired Receptacles under MULTI-WIRE CIRCUITS for details.)
The two-pin electrical receptacle was invented by Hubbell in 1904 as a device that screwed into light bulb sockets (electrical power for lighting was sold at a lower rate! - Wiki.)
Grounded electrical receptacles date to around 1915, though they were by no means in widespread use until much later.
Because sources (Wikipedia and others) note that the dominant way to plug in electrical appliances was by connecting them (using the screw-in adapter) to light bulb sockets into the 1920's (in North America) it is reasonable to argue that it was not until the mid or later 1920's that duplex electrical receptacles began appearing in homes.
Labre patented the grounded plug in 1928. Ten years later, twist-on locking electrical receptacle connectors date to Harvey Hubbell way back in 1938.
Polarized plugs (one blade wider than the other) were not introduced until 1948 and were not widespread before the 1950's. (The neutral wire is connected to the larger slot on the electrical receptacle)
Ring circuits (adopted only in the U.K.) first saw use in the U.K. around 1947 - provided the functional equivalent of our split wired receptacle approach and suggest the history of origin of the idea.
The sum of this history to date (subject to revision as our research continues) is that I'd place duplex receptacles in widespread use by 1935-1940 and it would be fair to assume (barring a code restriction yet to be found) that split wired receptacles, being physically possible, appeared in some uses as early as 1940.
IN sum it is more likely that split wired electrical receptacles were in use by the late 1940's, expanded during the post-Korean ware boom, and began seeing widespread use in North America after 1965-1970. That last OPINION derives from guessing at the onset of surge in use of multiple electrical appliances at once in home kitchens.
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Frequently Asked Questions (FAQs)
(May 29, 2011) paul macgregor said:
hi , i have a install of a/c its a LG single phase ducted system ,32 amps but the cable run is 55 / 65 meters i think this should be 10mm cable ??? or can 6mm be used
(May 31, 2011) electricaldave said:
Canadian Electrical Code requies that the prober size wire be use for the rated load. By the looks of your unit, I would suggest using a #8 cable. This is rated for 40amps. Wire is rated with 80% load rating and with the lenght of the run, this cable would work best.
Question: main breaker is not switching off power to the electrical panel
(Sept 1, 2011) Tim Schwartz said:
I have a zebco panel and when i turn off the 100amp main i still have power, any advice?
Watch out: in any electrical panel, if you still have power when the main disconnect is in the "OFF" position then this is a VERY dangerous condition as you cannot, using normal homeowner controls, turn off electrical power in an emergency, and more, it is likely that the main disconnect is not going to trip on a large overcurrent and so is not protecting the equipment and building from an electrical fire.
The brand "Zebco" is not one I know, and I'd like to see photos of the equipment and all identifying labels and markings so that we can research the brand further. Use the CONTACT link found at page top, left, or bottom to send us photographs. (Zebco is a brand of fishing tackle).
On a few occasions I've encountered electrical wiring connections at which power was brought into an electrical panel from more than one feeder, wired or I should say "back-wired" to the panel bus through a subordinate circuit breaker. For example if someone installed a local or backup electrical generator and connected it to back feed the electrical panel through another breaker (without a proper isolation switch) you could find that the panel is energized even though the main disconnect was off.
Panel backfeeding without an isolation switch to prevent both main disconnect and backefeed source from powering the panel at the same time is improper and unsafe and would need to be connected.
Presuming that you don't have a back-fed panel from a second electrical source then you immediately need a licensed electrician to replace the faulty circuit breaker or if necessary the panel.
(Oct 12, 2011) Maria S said:
When were houses commonly built with split outlets/receptacles? I am doing research to find out how many houses in the US may not have them.
Interesting Question, Maria.
I've posted my preliminary research and opinion above in this article
Question: how to identify the electric panel ampacity and voltage
(Feb 7, 2012) Bill R said:
in an apt bldg, each meter for each of the 50 apts contains an individual electric meter with a disconnect below it that appears to 'span' over 2 breakers but the 'bar' that spans over the 2 breakers reads 60 - does this mean that this is a 60 amp electric service?
Bill, it sounds as if you're describing a 2-pole 60-A main breaker - that's normally giving 240V service and 60 Amps as marked on the breaker.
Question: using a grid inverter
(Aug 21, 2012) inetdog said:
One notable exception to the limitation of one power source at a time is the use of a grid-tied inverter for solar PV generation. The grid-tied inverter is designed to synchronize to the utility voltage, and will only output power when the utility grid is present.
Question: open neutral wire at receptacle
(Nov 30, 2012) gary said:
i have a open nuetral at home showing at the recepacle i used a plug in tester and it shows that,the basement is finished no drop ceiling and upstairs as well where can i go from this point.
You need to trace the circuit wiring, Gary. There are quite a few devices that can help you do that - check online or at your local building supplier.
Question: apartment fire, aluminum electrical wire: does it need to be replaced?
(Mar 7, 2013) firstname.lastname@example.org said:
A fire occured in a unit of an apartment building. all interior finished on the walls had to be replaced, the electrical wiring is aluminum. Does the wiring needs to be replaced with copper to meet the building code?
Charles, building codes don't explicitly require replacement of aluminum wiring though there are code sections that allow that interpretation.
Watch out: I'm not an expert on repairing fire-damaged wiring but it seems to me the concern would be that the wiring insulation has probably been compromised - made unsafe - by exposure to the heat of the fire. Therefore it's likely that an inspector would want it replaced regardless of whether it had been originally copper or aluminum conductor wiring.
Question: using an electrical tester to find hot wires
(Apr 3, 2014) gm1761 said:
I'm using the usual electrical tester to confirm if some areas where there once light fixtures are "hot." Most of my readings are correct as the tester works on installed fixtures and outlets. My problem arises when the tester does not show a hot area when the switch and light are both on. Any reasons for this?
GM I can't quite figure this one out. The only thought that occurs to me is a wiring snafu; if we connect a neon tester between two hot legs on the same phase (rather than a hot leg and a ground or neutral) it will now show current flow.
Question: power off in one room
(May 4, 2014) Anonymous said:
One whole room does not work
Anon if power is on for all circuits at the electrical panel then an electrician will trace the circuit to find the open connection. Meanwhile turn off that circuit to be safe.
Question: two wire circuit, no ground at switch boxes
(June 3, 2014) DanW said:
I have a two switch box with two wire no ground that controls the lights and ceiling fan. The light works but the ceiling fan does not. I have just installed a new fan and still have the same issue. I changed the switch for the fan to a new one. I am at a loss as to why the fan will not work. I have checked all connections and they are tight and secure. Help!
A DMM or VOM can confirm which are hot and neutral wires.mbe sure to reasd our article on using DMMs VOMs safely
By the way, the switch itself probably includes a grounding screw and should be grounded - so the wiring may be incomplete and not safe if you're missing proper grounding.
Question: trouble tracing wires in 24 pin cable
(June 24, 2014) Michael said:
Have a cable with 24 pin plugs on each end. Have good continuity from each pin from plug A to plug B; however, pin x and M are shorted together on Plug A but on Plug B pin x and M read open. How is that possible?
Perhaps an internal short between two wires in the cable or between 2 pins in the plug. On some cable assemblies individual wires may be deliberately joined as well, depending on the application.
(June 24, 2014) Michael said:
Shouldn't the short be able to be seen from either plug A or plug B?
Michael I surely don't have a clear idea of what you are testing.
In general if two wires are shorted together you will see continuity on both, but if you put a test current on A and A is also shorted to B you will probably see the current or signal on BOTH A and B at the other end of the cable.
Question: electrical problems in mobile home
(June 24, 2014) Danny said:
we have an older mobile home that has had no probs for 14 yrs,a few weeks ago all the lights and plugs went from 110v to 220v burned tvs cd players rite up so my ? is dose anyone know the reasons this happend and whats the repair that has to be done...thank you
Danny I'm not clear just what happened. You are describing what I'd expect to be found to be a voltage surge coming in from the power transformer. You'll want to check with your electrical company who may need to check the step-down transformer.
Keep in mind that other failures such as a lightning strike or a short circuit also cause power surges that damage equipment.
The repair will probably include replacing damaged equipment, diagnosing the cause, and if the cause is found to be fluctuating power you'll want to be sure your home has adequate surge protectors installed
(June 26, 2014) sandeep said:
We are using a condensing fan in a unit input voltage is 380v but wn we are running it on the site its giving 600rpm as per fan specification at delta connection it will give 900 rpm we check the connection also its was delta. When we check this fan in our factory again we 900 rpm as per our desired.
Hello again sorry i didnt give enough info,there was a water leak under the kitchen sink rite under the sink is a 110v recptl it did not have a gfi present,thats when the trailer surged to 220v but upon inspection of the plug i didnt see any shorts they do however have the plug backwired
Backwiring is not reliable
Question: single wide mobile home electrical problems
(Aug 2, 2014) Anonymous said:
hello we have an older single wide,we have replaced the gas stove and water heater with electric units and they are working fine but although all the outlets read 117v there dose seem to be enough amps being pushed thru them when a load is put on any plugs the lights go very dim or out. thank you...Danny
Danny, we may be confusing voltage (nominally 120V so your 117V is normal) and amps or current. If lights dim I suspect either bad wiring connections or an overloaded circuit - too many things ripunning on one circuit. Check too that the ampacaty of the circuit breaker or fuse is properly matched to wire size so as to avoid a fire. Check too for problem brand breakers that may not be tripping earn they should in rennes to overload - aLso a fire risk.
Question: can I convert radiation into current?
(Aug 4, 2014) prasanna said:
Is there is any possibility of converting radiation into current
Solar radiation is used to heat water to drive a turbine or directly to produce electricity using photovoltaics.
Nuclear radiation is used (dangerously) in nuclear reactors to drive turbines to produce electricity.
Magnetic "radiation" or fields are key in the operation of electric motors.
Question: floors are vibrating
(Aug 14, 2014) email@example.com said:
Hello. Me floors are vibrating like I have grounding issues. I don't know what it is and electricians are expensive. It is a constant vibration and it is strong, medium and light modes. Any ideas? Thanks!
Sounds like we're confusing building vibration with electrical grounding.
Question: defective light pole sensor dusk to dawn light
(Aug 23, 2014) Paul Catsburg said:
My exterior light pole had a defective sensor (automatic dusk to dawn type); I have since replaced the sensor, but the power to the light pole is non existent. I have checked all breakers at thE main panel, and still no power ? This is in an Onrraio Canada house built in 2003; is there something else that SHOULD BE CHECKED ?
Watch out for a dangerous buried broken electrical wire. I'd shut power off to the pole until an electrician has debugged the circuit.
Question: Eagle brand AC DC receptacle tester operating range
(Sept 16, 2014) al said:
I have an Eagle brand AC-DC 90 to 660 V neon receptacle tester. It works fine on 120 v, but I am wondering if I can stick this into a 240v outlet? It has no other markings on it.
Since your tester is marked as 90 to 660V and also as AC/DC, it is designed to handle voltages up to 660V which is more than the 240V you ask about - so yes.
WATCH OUT: making a mistake can still get you killed - such as touching live contacts or wires.
Question: aluminum ground wire has been cut, breakers are tripping
26 Sept 2014 George Nochta Santee CA said:
I have a rental built in the late '50s. The electric panel is an older 50 amp service with an aluminum wire ground that is routed to the water heater closet where it is grounded to the cold water supply line. My tenant says when she is using her clothes dryer and runs her vacuum at the same time. The 115 circuit breaker blows for the outlet the vacuum is plugged into. The dryer is on a dedicated 220 30 amp circuit. When I checked on my own, I found that the aluminum conductor had been cut apparently when someone had replaced the water heater at some point. Could this severed ground line be causing the interaction between the 200 30 amp dedicated circuit and the 115 15 amp breaker that is tripping?
Watch out: A missing electrical ground connection is DANGEROUS as the electrical system is then lacking a critical safety feature. That shoudl be repaired.
But a missing ground won't explain an overloaded circuit that trips the breaker; more likely the circuit is simply overloaded when both devices are running at once OR one of the devices is shorting to ground somewhere else.
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