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This page provides an index to building electrical system installation, troubleshooting & repairs & electrical hazard at residential & light commercial building electrical systems.

Examples of information we provide in these articles include how to install, inspect, & troubleshoot building electrical systems, appliances, components.

Here we also provide articles on 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.

We also discuss 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.

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Electrical Hazards in Buildings, Electrical Inspection Procedures, Electrical Repair Procedures

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On 2023-10-11 by Dan - I did my own knob and tube re-wiring: did I make mistakes?

My house was originally wired with the old rubber-insulated wire and knobs and tubes.

Sometime before we moved in (1997) the old wire was replaced with #12 THHN that was run along the old circuit paths through the tubes and fixed in place with the old knobs.

The original splices were soldered 'Western Union' splices made outside boxes.

I have remade most of these splices with wire nuts inside metal junction boxes and added lots of tape to the others. It seems to me that it would be safe to blow in cellulose insulation.

Am I wrong? I have marked all the boxes with upright sticks.

On 2023-10-11 by InspectApedia DF (mod) - knob and tube wiring replacement/update

@Dan,

Going from #14 old knob and tube wire to new #12 wire is a nice safety improvement in that the larger wire size, provided you're still using it on 15A circuits, will run cooler.

But I'm not clear on exactly what wire you're running: did you really run single strand #12 THHN wire along knob and tube connectors?

Normally modern house wiring is run - as doubtless you know - in a combination of hot+neutral+ground conductors that not only have each its own insulator, but there is additional insulation provided by an external jacket that wraps all 3 wires.

Southwire 22968201 Stranded THHN 12 Gauge Building WireIndividual strand THHN wire like this Southwire 22968201 Stranded THHN 12 Gauge Building Wire, 500-Feet, Green (shown below) is NOT intended to run free through a building, not in air, and not in insulation.

Rather it expects to be run through conduit:

Here is Southwire's product description:

This THHN 12 gauge building wire is primarily used in conduit and cable trays for services, feeders and branch circuits in commercial or industrial applications. It is designed for use in dry locations at temperatures not to exceed 90 degree Celsius or wet locations at 75 degree Celsius. It is stranded uncoated copper wire insulated with flame retardant PVC compound. Can be used for residential and industrial lighting and power. Meets or exceeds all applicable ASTM specifications. Features conduits and raceways installed underground.

So I'm left both admiring your creativity but very nervous about your electrical wiring and I'm doubtful that your local electrical inspector is going to approve it.

In GENERAL, knob and tube wiring is legal and can continue in use in most jurisdictions as long as it's not damaged AND PROVIDED that it is not modified or extended.

You've done a remarkable modification - using ceramic knobs and tubes to route modern THHN #12 wire that was not designed for that application - with, or without being buried by insulation.

In my OPINION you should STOP WORK immediately on this design and get your local electrical inspector out to the job to see what she or he has to say - lest you spend more time and money that have to be undone.

Thank you so much for an important and helpful "knob and tube wiring replacement/update" question.

On 2023-08-06 by Anonymous - do I have to pay to replace my Federal Pacific Electric Panel?

Will the company that produced the FPE breaker box replace them at no cost?

On 2023-08-06 by InspectApedia Publisher - Will the company that produced the FPE breaker box replace them at no cost?

@Anonymous,

No, although there was at one point money budgeted for a recall with one small exception there has been none and you're on your own.

On 2023-06-01 by Stan C - can I use multiple small gauge wires to build up to a higher-amp circuit?

BELDEN (brand name) 600V Type TC-ER (UL) THHN or THWN 90C dry 75C wet OR 150V type NPLF 90C SUN RES DIR BUR --- FT4 IEEE 1202 (C) InspectApedia.com StanCHi! I have a brand new cable qualified for NPLF (non power limited fire systems). Text on the cable says (see photo):
"BELDEN (brand name) 600V Type TC-ER (UL) THHN or THWN 90C dry 75C wet OR 150V type NPLF 90C SUN RES DIR BUR --- FT4 IEEE 1202".

Does the above mean that it's legal to use either as THWN or NPLF? (NPLF = non power limited fire systems and has to withstand fire while conducting)? Thus is it allowed for regular power wiring?

My goal is to upgrade the 8 ga THHN copper cables (40A max) power cables coming from building's electric meter to my townhouse unit's panel (~90 feet). I want to upgrade to 60A per hot.

This Belden cable I have is multi-conductor: 3 insulated 12 ga conductors per cable then another outer jacket (for direct burial or conduit). Twisting three 12 ga together will give me 20A max * 3 = 60A max. Is this summation allowed per NEC code or in practice? I know that it's electrically correct since current splits and flows equally; but is it legal. I'd use one set for each of 2 hots and 1 neutral.

Buying a standard THHN 6 ga wire here is ~$400. I got this cable for free and trust its quality and its source. I bought a standard THHN 8ga for ground, so this BELDEN cable is for hots and neutral only.

Thank you!

On 2023-06-02 by InspectApedia Publisher - combining smaller gauge wires (higher gauge number - #12) to build up a larger sized (smaller gauge number - #10) wire

@Stan C,

Even though what you ask: combining smaller gauge wires (higher gauge number - #12) to build up a larger sized (smaller gauge number - #10) wire is common in some instances, I'm not sure that for your SEC twisting the ends of three #12 wires together (#12 wire supports 20A load) is sufficient nor "code legal" to build up a 60A load.

Simplifying, it's true that two #12 wires will have about 20% less resistance (typically about 0.994 ohms/ft) than a single #10 wire (typically 0.999 ohms./ft.) so resistance isn't the issue.

There are, however issues of both code and more subtle worries about overloading individual conductors that I'll elaborate here.

1. Equivalent AWGs when doubling wires:

A general rule of thumb we found in researching this question is that doubling the conductors reduces the rated AWG by a factor of 3.

So where if you double #12 wire the equivalent gauge is not a straightforward calculation - that is, if 1 #12 wire is 20A, two twisted together is not a 40A circuit - it's a bit less in capacity.

2. Code violation: NEC 310.10(h) - Conductors in Parallel

I think, however, that even if you used 4 wires twisted together to be confident of the gauge and current capacity of each "built-up" twisted wire between your meter and panel,

your wiring violates the US NEC that permits paralleling conductors only when the wire sizes are larger than 1/0. In that case, for larger than 1/0 wires, the wires must be of equal length, same gauge, and their combined size must meet the cross-sectional wire diameter guides in the NEC.

Here's the code excerpt:

NEC 310.10(h) - Conductors in Parallel
...
(H)Conductors in Parallel.

(1)General. Aluminum, copper-clad aluminum, or copper conductors, for each phase, polarity, neutral, or grounded circuit shall be permitted to be connected in parallel (electrically joined at both ends) only in sizes 1/0 AWG and larger where installed in accordance with 310.10(H)(2) through (H)(6).

Exception No. 1: Conductors in sizes smaller than 1/0 AWG shall be permitted to be run in parallel to supply control power to indicating instruments, contactors, relays, solenoids, and similar control devices, or for frequencies of 360 Hz and higher, provided all of the following apply:

(a)They are contained within the same raceway or cable.

(b)The ampacity of each individual conductor is sufficient to carry the entire load current shared by the parallel conductors.

(c)The overcurrent protection is such that the ampacity of each individual conductor will not be exceeded if one or more of the parallel conductors become inadvertently disconnected.

Exception No. 2: Under engineering supervision, 2 AWG and 1 AWG grounded neutral conductors shall be permitted to be installed in parallel for existing installations.

Informational Note to Exception No. 2: Exception No. 2 can be used to alleviate overheating of neutral conductors in existing installations due to high content of triplen harmonic currents.

3. Possible interference problems for some applications:

for some applications is that running multiple conductors of different electrical phases together bonded at each end to get a high amp circuit can cause interfering magnetic fields that can make for trouble with some electrical equipment or motors.

4. Risk of overloading individual conductors:

Another more-technical concern is that though it may not be obvious, you cannot assume that the electrical load will be divided equally among each of the individual wires whose ends you are twisting together.

5. Terminal connector concerns:

Will your built-up cable wire ends fit properly and be connected equally under their terminal lugs?


Thank you for a helpful question and one often debated among people doing wiring. We'll welcome further comment.

On 2023-06-02 by Stan C - more discussion of total current limits on electrical wires?

@InspectApedia Publisher, thank you for this clear answer!

Any idea why the total wire current limit isn't a summation of each individual one? In microelectronics work that I do, the current is considered to split equally, unless a particular metal run is much longer than others or has something that impedes the current flow.

I can only guess here that this has to do with parallel wires sharing and adding up their heat, which increases the conductor temperature and derates their performance.

The utility company where I got this cable was indeed using it to power electronic control systems in a substation, and they were twisting the wires together to increase the total current capacity.

So this fits the article you have pointed out and was also the start of my confusion.

On 2023-06-02 by InspectApedia Publisher (mod)

@Stan C,

Yes, intuitively too I thought the current limit would just be additive when we twist wires together. The engineering comments and articles I read in researching the answer to your question said

1. the math isn't straightforward

2. there can be subtle effects and interferences - an example, I think, would be a lack of uniformity of contact area could send more current through one of the member wires than another so you could end up overloading one of the members. This seems to be the principal concern, stated as "current sharing will not be equal among all of the member wires".

Some sources suggest combining multiple smaller wires to deliver higher amps while others point out that if one wire trips a breaker the others then carry more current than rated and those breakers ought to trip too.

You'll also read that there can be a question of the adequacy of the grounding conductor in a circuit like the one you describe.

It will be informative to understand why the NEC writers permit this multiple wire approach but only on larger wires.

A clue is in the specification that where allowed, using doubled smaller wires to carry a larger current requires that their wire lengths must be exactly the same. That's an example of what can happen: a member wire with more resistance, different contact area, tightness, etc. means the member wires are not all carrying the same load.

Current doesn't flow through wires and connectors exactly as we imagine. As a somewhat off-topic example look at why the Ideal 65 Twister connector for aluminum wiring melts and catches fire under independent testing (Aronstein).

What we see is that where aluminum wires are twisted together under that connector the current was flowing less directly wire-to-wire along their contact surfaces (oxidation) and more through the spring of the connector itself - I'll skip why that happened - it does. The result was an over heat and meltdown failure.

See https://inspectapedia.com/aluminum/Ideal_65_Twister_Aluminum_Wire_Connector.php IDEAL-65™ TWISTER

Some other objections such as crosstalk and interference probably don't pertain in your example.

--

Paul, Clayton R., and Jack W. McKnight. "Prediction of crosstalk involving twisted pairs of wires-Part I: A transmission-line model for twisted-wire pairs." IEEE Transactions on Electromagnetic Compatibility 2 (1979): 92-105.

aluminum wire meltdown (C) InspectApedia.com DJF

 

On 2023-06-01 by Kate - kitchen GFCI's trip too often

Question about breakers & outlets: the six outlets spaced along my kitchen counters ( 2 are GFCI) tend to trip the breakers at the service panel more often than seems right.

After removing the faceplates, I see the individual outlets are rated at 15 although the breakers at the panel for the two circuits for the anrea are for 20.

Could that be causing them to trip? Should I have them replaced w/ ones rated higher ? Thanks so much!

On 2023-06-02 by InspectApedia Publisher - would using a 15A rated receptacle on a 20A circuit cause circuit breakers to trip

@Kate,

No, using a 15A rated receptacle on a 20A circuit would not, itself, cause circuit breakers to trip.

Tripping is caused when the current load on those 20A circuits exceeds 20 amps - when too many things are plugged in and running at once on the same circuit.

You describe two 20A circuits serving six duplex receptacles ("outlets") in your kitchen - good design. But we don't know how those were wired.

First: ALL of the receptacles along the countertop in a kitchen, unless nowhere near or reachable from a sink, would be expected to be GFCI-protected. If the panel breakers aren't GFCI often we wire the first receptacle in the series using a GFCI receptacle - that can protect the receptacles downstream on the same circuit.

Now: if it's a GFCI that's tripping I'd look for a moisture problem where you're plugging in appliances.

If it's the breaker in the panel that trips and that breaker isn't a GFCI then there is a true overcurrent.

Your choices are to plug in some items into different receptacles - the ones that aren't tripping - (and let me know the result)
or
to re-wire the 6 receptacles such that in the area where you most want to plug in appliances those receptacles are served by two different circuits.

That's a bit of work to do so it's not our first choice.

When I wire a kitchen with a series of duplex receptacles, I like to split the receptacles such that the upper and lower plug receptacles are each powered on a separate 20A circuit - more work but more convenient for the user.

On 2023-02-19 by Jared Brose - lost power in half of a doublewide mobile home

Hey so I’m helping work on my neighbors double wide mobile home and I believe it’s an 80’s or 70’s model.

We are trying to get the power back on in half of the trailer and came across these wires underneath but can’t seem to find out what they are for… if anyone could maybe help me out please and thank you !

mobile home electrical crossover connector (C) InspectApedia.com JaredB

On 2023-02-19 by InspectApedia Editor - power lost on half of mobile home

@Jared Brose,

Sure. Take a look at this article

ELECTRICAL CROSSOVER CONNECTORS IN MANUFACTURED HOMES

which is almost certainly what you're describing.

On 2022-12-07 by hillpc - Why is the internal wiring in a 10 kW electric heat kit for an air handler so flimsy?

Why is the internal wiring in a 10 kW electric heat kit for an air handler so flimsy? The specs and instructions are quite clear - 40A heater current draw at 240 VAC, minimum circuit ampacity is 57 A (leading to either 6 ga or 4 ga branch wiring, depending on the type of wire and wire insulation), and maximum overcurrent protection 60 A (breaker).

But the wiring installed by the factory (Goodman) inside the heat kit itself looks positively flimsy - each hot lead (red and black) feeding the contactor looks to be no larger than about 14 ga.

How could this be, especially when compared to the branch circuit wiring? Those wires are not in the air stream, and therefore not cooled. Maybe because they're only less than a foot long?

@InspectApedia (Editor), The small input wires (L1 and L2) supplying the contactor input have no markings on them. The contactor outputs (T1 and T2) go to 2 parallel 5 kW heaters. Each heater has 12 AWG wires (with its insulation good for as high 105 degrees C) feeding its roughly 20A; this is in line with what I expect from ampacity tables.

But the small wires feeding both heaters at once (through the contactor) is what's puzzling.

Goodman Electric Heat Kit wiring (C) InspectAepedia.com Hillpc

[Ed. note] Image above is the corrected version as submitted by reader.

On 2022-12-08 by InspectApedia (Editor) - Why is the internal wiring in Goodman electric heat kit so flimsy?

@hillpc,

Thank you for the interesting question/opinion. You would expect Goodman manufacturing, producing an electric heat for an air handler, to comply with the requirements for UL listing of their product as well as building codes as well as having every intention of making their product safe. We can't see the wiring that you describe - do post some photos (one per comment) if you can.

We often see electrical wires routed through open space in an appliance, such as the wires to the heating elements in an electric clothes dryer. Properly-chosen the wire is rated to carry the current (amps) demanded by its application and to be safe, including using a wire with insulation rated to withstand the temperatures of its environment.

Take a very close look at those wires - if you can do so safely with power off - and note the letters, numbers and words describing the wire and its intended use.

On 2022-12-08 by InspectApedia (Editor) - smaller diameter wires are carrying just current to operate a control relay

@hillpc,

Thank you for the photo, question, discussion.

I SPECULATE that the smaller-diameter wires are carrying just current to operate a control relay that is switching the higher-current (larger) wires.

Check the wiring diagram.

On 2022-12-10 by hillpc

@InspectApedia (Editor), Thank you, you are correct. The actual high inlet current supply to the contactor is supplied by the inlet lugs on it (see pic; they are just like the outlet lugs for Romex on circuit breakers).

The flimsy black and red wires that I thought were inlets to the contactor are actually there to supply 240 VAC back to the control transformer and the blower. I'm so embarrassed. Please feel free to delete the post.

Goodman Electric Heat Kit wiring (C) InspectAepedia.com Hillpc

@InspectApedia (Editor), Here's an interesting observation on ampacity for this job. The Goodman instructions say to install supply wiring (to the input lugs) that's good for 57A (Minimum Circuit Ampacity). If Romex (NM-B), that's 4 AWG conductors (4-2 w/ground). I've found it difficult to find 4-2 w/ground Romex. 6-2 w/ground Romex is only good for 55A.

If I use THHN individual conductors (inside conduit) instead of Romex, I can get away with 6 AWG conductors. So I'm planning to put in a couple of 6 AWG THHN conductors (and a AWG 10 ground; appropriate size ground depends on the rating of the breaker) inside some conduit from the breaker panel.

On 2022-12-10 by InspectApedia (Editor)

@hillpc,

Actually your post and photos will be helpful to lots of other folks. Please try to talk yourself out of being embarrassed. I'm way ahead of most people in making mistakes. We learn more from mistakes than from getting it right the first time - as long as we survive the error.

 

On 2022-09-13 by Cindy - lost power in half of a Belmont mobile home doublewide - where are the wires & connectors?

1997 Belmont Home - 32x60 - lost 1/2 phase of power on one side. Where is the electrical run? In the walls or underneath? How to we find them to replace?

On 2022-09-13 by InspectApedia-911 (mod) - lost 1/2 phase of power on one side 1997 Belmont home

@Cindy,

At this article MANUFACTURED HOME CROSSOVER CONNECTORS  you'll find the typical locations of mobile home crossover connectors. A loose plug or connection at one of those is a common cause of losing power in half the home.

On 2022-08-03 by Josephine Kerns - phone connection inside doesn't work but outside the house it does

When I use the phone jack on the box outside the house, it works. But the two phone jacks I found in the house are "not connected to service". I have cleaned them out of dust and debris and the wiring appears sound. Can I solve this problem on my own?

On 2022-08-03 by InspectApedia-911 (mod) - find and fix a problem with phone wiring and jacks

@Josephine Kerns,

How to find and fix a problem with phone wiring and jacks.

Since we don't know anything about you I can't say if you can solve this phone wiring problem on your own, but in general, troubleshooting phone jack wiring problems is within the scope of many people - homeowners, renters, and users.

As long as you're ONLY fooling with phone wiring it's safe in that no high or dangerous voltages are involved.

The most likely reason that your phone would work when plugged into an outdoor phone jack but not work indoors is that one or more of those phone wires has become disconnected.

So start outside and note where the phone wires enter your building.

Find that location inside and find the entering phone wire.

Follow the wiring to each wiring splice or junction and inspect those carefully to see which wire connection has come apart.

Consider that while it's possible that phone wires running through hidden places like walls or ceilings might be damaged or cut (someone drives a nail to hang a picture and a nearby phone wire was not protected), that's not likely.

Instead, most phone wire problems occur at the connections. Those are usually visible in phone jacks or phone wiring junction boxes.

Since you have two indoor phone jacks and both are dead I suspect that the broken phone wire connection is at or ahead of the first phone jack.

Take a look, follow the wires, and let me know what you find.

On 2022-08-03 by Josephine

@InspectApedia-911, This is what I found:

Exterior phone jacks (C) InspectApedia.com Josephine

On 2022-08-03 by InspectApedia (Editor)

@Josephine,

Okay that's a good start, so it looks like that beige colored wire is the telephone line going into your building.

Now see if you can find that on the inside of the wall at about that same location in the structure.

Then follow it.

On 2022-06-01 by Mike - wiring for surge protector at electrical panel?

I replaced my whole house surge protector on my 2 wire outside Main lug panel. I put the 2 black wires on the 20a double pole/throw breaker. I put the green & white in the common/ground bar. Is this a correct install?

On 2022-06-01 by InspectApedia-911 (mod)

@Mike,

I'm sorry because I really wish I could be helpful but I can't be confident that I can tell you what the correct wiring connections are with any assurance of safety for a system about which we have no information whatsoever.

On 2022-04-02 by Michael - missing strain reliefs on main panel - unsafe?

Electrical short-cuts, doing the right thing here would have cost less than $5. The feeds are floating in the hole at the top of the panel. I get amazed every day....

Missing strain reliefs (C) InspectApedia.com Michael

On 2022-04-02 by Inspectapedia Com Moderator - missing strain reliefs are signs of poor electrical work

@Michael,

Thanks for the excellent photo and the post of missing strain reliefs at an electrical panel.

I agree completely that it shows a shortcut indicating work by someone who's not a trained electrician and whose other wiring details might also be unsafe.

On 2022-03-09 by Anonymous - who can tell me the age of my electrical panel?

Who would i need to contact to find out when my electric panel was installed in my home?

On 2022-03-09 by Inspectapedia Com Moderator

@Anonymous,

Please see

ELECTRICAL PANEL AGE

where you'll see what clues people typically seek to make a reasonable guess at the age of an electrical panel.

We look at panel type, size, labels, building age, wiring type, age etc.

On 2022-02-05 by Glenn - I need to verify the bus rating in a GTE Sylvania Zinsco electrical panel

My question is I have a Sylvania zinsco type 100a main breaker panel 390-205-09 ( this is all the panel has for identification ) plus a label for the enclosure (BW-819527 )

I need to verify the bus rating of this panel, I am looking to get a solar system and want to use the existing panel, hopefully you can help with some info that I can print out for verification. My home is located in Rohnert Park CA 94928 and was built in 1980
Thanks

On 2022-02-05 by Inspectapedia Com Moderator - replace the Sylvania Zinsco panel

@Glenn,

I'm sorry to have to tell you that the AMPS rating for your electrical panel is IMO zero because it's a known fire hazard.

I don't mean to sound glib but the fact is you should replace the panel. Please read details at

ZINSCO SYLVANIA ELECTRICAL PANEL HAZARDS

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