2+ Ground or 3+ Ground for these 240v Outlets?

[bobinyelm]

This is probably best answered by those very familiar with electrical code, since it WILL be inspected by a city electrical inspector.

A friend in installing a new branch circuit from her main service panel located in her home to her attached garage where she will be placing 2 different types of 240v outlets to power kilns for her ceramics hobby.

EACH consumes 22 amps, but one uses a 3-Prong 240v plug with a "D" shaped for the "ground," and the other uses a "Y" shaped plug with 3 flat prongs. The kilns are made by different manufacturers, probably explaining the plug differences.

Her dad (owner of the house and her landlord) obtained a DIY permit and the job will be inspected by the city electrical inspector when completed.

She knows I have done work wiring my past homes and shops, so asked me to help design the wiring plan.

In my past, when wiring for appliances such as directly wired water heaters, welder outlet receptacles, my car-lift in my shop, and compressors (some direct wired, others plug-in), etc., has involved using 2-Wire NMC w/Gnd (or as appropriate) and connecting the ground wire to the 3rd connection in the receptacle ("D" shaped) since none of my "appliances" use 120v from the circuit to operate any of the features of the devices (as stoves, clothes driers and other houseold appliances might). My installations all passed governmental inspections and passed.

BUT...

All of the wiring references I have (books) as well as online guides show cases where either 2+Gnd cable or 3+Gnd cable is used. Virtually all show 2+Gnd being used to wire 240v/20a receptacles, but for 240v/30a and higher 3-Pronged outlets they often (but not always) show 3+Gnd cable being used, with the white neutral wire being connected to the receptacle's "ground" connection (whether flat or "D" shaped), and the ground wire simply being connected to the receptacle box (if metallic) or unused if a plastic box is used.

When using 3+Gnd cable, they say the appliance gets its ground back through the bonded white neutral in the service panel so that the appliance's 120v features (timers, lights, etc.) are properly connected, but in "my" case the appliances (kilns, or essentially ovens) there are NO 120v features in the devices.

So... Here is my question:
Should we use 2+Gnd wire and connect the ground wire to the plug pin that connects to the metal outer case of the kiln...
or 3+Gnd wire and connect the white neutral to the 3rd contacts that are connected to the metal case of the kilns,
or use 3-wire w/ ground for the circuit run, and wire the "D" shaped contact to the bare ground, and the Flat 3rd contact to White neutral? (The run from the panel is about 30ft, and the outlets are only a couple of feet apart)

Note-It's kind of seems logical (to me) that any "D" shaped contact is meant to be ground, but not so obvious that any flat contact should be ground. In my past wiring, ALL my 3 Pin 240v outlets had "D" shaped grounds, and interior 240v outlets were 4-Pin type, so both ground and neutral were used. In THIS case, however, both types of 3-Pin receptacles will be connected to the same circuit (both kilns will never be used at the same time), however.

Why not ask the Inspector?
I live in the county (state electrical inspectors) and they will answer questions, but this installation is in the city, and their philosophy is that they are there to INSPECT and NOT Instruct, so if you don't KNOW, then hire a licensed electrician, or be ready to re-wire as many times as it takes to get it right (additional inspections are at owner cost, of course).

Thanks,
Bob

Just to confuse things, I found THIS page showing flat blade connections shown as being connected to ground (bare wire):http://waterheatertimer.org/How-to-wire-240-volt-outlets.html

 

[wyliesdiesels]

Way too much to read right now but if theyre both 240v only then they should both have ground pins.

Sounds like one has a 120v/240v 3-wire non grounded plug on it.

Do either of these have 120v controls? If so then you need 4-wire plug.

And never swap neutral for ground or vise versa.

Thats creating potential for shock hazard..

 

[Norcal]

The one kiln w/ a 4-wire plug may use 120V controls, the 3-wire crow foot design with 3- flat prongs were for cooking equipment (50A) or the 30A having a "L" shaped prong was intended for clothes dryers. Both were used where it was then allowed to ground the frame of the appliance to the neutral.

 

[AntonLargiader]

EACH consumes 22 amps, but one uses a 3-Prong 240v plug with a "D" shaped for the "ground," and the other uses a "Y" shaped plug with 3 flat prongs.

How about some pics?

The page you linked to shows a number of configurations with their proper connection methods. Describing them is tough.

 

[bobinyelm]

To clarify,

1) NONE (neither) of the kilns have 4-wire plugs.

One has a 3-Wire (looks like a big 120v plug), and the other uses a 3-Pin of 3 flat blades in a "Y" configuration. Links to pictures of the plugs are at the bottom of this posting.

2) NEITHER kiln has any 120v control wiring in it. Both just have a bimetallic spring thermostats hooked to heating elements.

My question came up because I have several Wiring Guides for 3-Pin receptacles, and
SOME show the 3rd pin of the receptacle being the bare ground wire (2-wire+Gnd cable supplying power), and other show a 3wire+Gnd cable with the 3rd Pin connected to the white neutral of a 4 Wire + Gnd cable, with the bare ground in the cable either cut-off or terminated by a wire nut, or if a metal box is used, connected to the metal box itself.

Again (re-worded), my question is:

If one is wiring a 240v circuit terminating in 3-Pin receptacles that will supply only 240v appliances not having any 120v circiuits inside them, is it permissible to use 2Wire+Gnd NMC to feed those receptacles?

This is strictly a code question since the installation will be inspected by a City Electrical Inspector based on a DIY Permit issued by the City concerned.

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Background:

The question arose when I saw some illustrations showing a 3Wire+Gnd NMC supplying 3-Pin receptacles in wiring guides where the 3rd Pin was wired to the white neutral of the cable, with a statement saying that the appliance plugged into the receptacle would get its "ground" through the white neutral that was bonded to the ground inside the service panel.

Since some appliances that use 3 Pin plugs especially in a "Y" configuration DO HAVE 120V circuits inside them (like clothes driers) I suppose the inspector could make the point that because at some point in the future a person could plug such an appliance containing 120v control circuits into that receptacle that receptacle, a 3 Wire+ Gnd cable should be used to supply the receptacle.

As I mentioned in MY past, I have always just wired 3 Pin receptacles using 2 Wire + Gnd cable as my "appliances" were things like welders, compressors and large shop equipment, none of which were household stuff (like clothes driers).

These are the type plugs that are used by the kilns (each uses a different plug from the other though they each use 22 amps):

https://www.google.com/imgres?imgur...vdAhWDIjQIHfsaChAQMwiHAyhSMFI&iact=mrc&uact=8

https://www.google.com/imgres?imgur...L3XO5fCUe9GluDspxlx0L6U=&docid=-7uWOGrXeNB-hM

 

[FordTruckWench]

If one is wiring a 240v circuit terminating in 3-Pin receptacles that will supply only 240v appliances not having any 120v circiuits inside them, is it permissible to use 2Wire+Gnd NMC to feed those receptacles?

Yes! In fact that's the expected way to do this. If you are doing this today, you'll want to use a 3-pin receptacle type that has two hots and a ground. Note, you may find it difficult to obtain fat gauge (high current) "2Wire+Gnd NMC". You may be forced to use individual conductors in conduit, or a four conductor cable (2hot + neutral + ground) with the neutral not connected at the receptacle end.

https://www.google.com/imgres?imgur...vdAhWDIjQIHfsaChAQMwiHAyhSMFI&iact=mrc&uact=8

https://www.google.com/imgres?imgur...L3XO5fCUe9GluDspxlx0L6U=&docid=-7uWOGrXeNB-hM

The first of these is NEMA 6-50. It is the currently correct style to use with a 50 amp "appliance" that doesn't need 120V, i.e. doesn't need neutral.

The second one is NEMA 10-50. It has two hots, a neutral, but no ground. It might commonly be called an old style oven plug. It is obsolete for new construction. A diligent inspector shouldn't allow this in new construction. On the kiln with this style plug, change the plug end to NEMA 6-50.

More information here: https://en.wikipedia.org/wiki/NEMA_connector

...

You may wonder why older appliances were allowed to ground via the neutral. Think back to an era of small bungalows where all electrical service is via a small panel containing fuses. There are no subpanels. Ground and neutral are bonded in this one panel. (Actually, there probably isn't even the concept of a separate ground.) For any circuit that makes a home run back to the panel (and with no other legs), ground and neutral are essentially the same thing.

Now imagine that (copper) wire is expensive. Now consider the thick gauge needed in the ground wire already going to an oven or clothes dryer. When that appliance needed a tiny bit of 120V power, the designers saved money by "trickling" the power back through the fat ground wire instead of adding a fourth conductor that's just going to go back to the same place. Because they are now running power through that wire, they put insulation on it and renamed it to "neutral".

Look at today's McMansions - they may have multiple subpanels located around the house. In those subpanels, ground and neutral are not the same. Grounding through neutral is no longer safe. It is only recently that code has caught up to this reality - it finally necessitates a four wire connection for ovens and dryers.

My house is old enough to have three wire connections for the oven and dryer. It also has a subpanel. For curiosities sake, I just measured the voltage difference between my kitchen oven and the sink faucet: Around 9mV AC. This is due to the ohmic loss in the thick cable between the subpanel and the main. At the time of the test, there was very little power consumption in the house. If there was a higher load (dishwasher, fridge, microwave, TV, stereo, vacuum, hairdryer, etc.), that 9mV might instead be 90mV or 900mV! Uh-oh.

 

[mike93lx]

I agree, i would just change to a 6-50, and install two identical receptacles.

 

[ard]

Not sure if this was mentioned, but I'd run a 4 wire circuit for both- how you wire the actual receptacle can vary, you can wire it as 3 wire as needed. But at least the wiring in the wall will support whatever is needed later.

 

[wyliesdiesels]

Since neither have 120v controls, they should both have a 240v grounded plug.

The one with the 3 flat blades needs to be changed to a 240v grounded plug.

Yes you can use NM-b wire...

 

[bobinyelm]

Great explanation FordTruckWrench and makes perfect sense.

Drawing much current through the ground raises the potential of the ground (appliance case) noticeably above actual ground. Likewise with a bonded neutral like most houses still have. I know I had to use a floating neutral in my shop, which is on a sub panel I would imagine for the same general reason.

Because neither kiln uses ANY 120v power, the ground wire is just that- a ground wire for safety.

 

[bobinyelm]

Since neither have 120v controls, they should both have a 240v grounded plug.

The one with the 3 flat blades needs to be changed to a 240v grounded plug.

Yes you can use NM-b wire...

That was one question that was raised- Is it "legal" to wire the 3rd blade to ground, rather than to neutral. Some of the wiring diagrams showed that receptacle wired that way (3rd blade grounded via the bare wire), and it troubled me.

I called the kiln manufacturer of the one with the Y blades, and they said it left the factory with a NEMA 6-30 and suggested it be restored to that configuration as it has no 120v components.

That will allow the kilns to be interchanged position-wise as well, which is a plus beyond only needing the 12-2 w/ ground wire.

Thanks!

 

[FordTruckWench]

which is a plus beyond only needing the 12-2 w/ ground wire.

12-2 can't be used for the 22 amp load. You'll need at least 10-2.

Also, the pictures you linked are of 50 amp plugs. They both have 30 amp variants. Did you get them mixed up, or do the kilns have 50 amp plugs?

 

[bobinyelm]

Not sure if this was mentioned, but I'd run a 4 wire circuit for both- how you wire the actual receptacle can vary, you can wire it as 3 wire as needed. But at least the wiring in the wall will support whatever is needed later.

Makes sense, but the friend with the kilns is on a tight budget, so teh difference between 10-2 w/gnd and 10-3 w/gnd for her is significant.

In fact, her garage is presently wired with very old NM cable from 1945, and none of the 120v outlets have grounding wire, while most have had grounded outlets connected to them- some are even 20a 120v receptacles all w/ 14-2 wire! Most all of that is the old woven paper/fabric jacket and the individual wires have what I think is deteriorated rubber insulation you can strip away with a thumb nail. A couple were added later and are 2-wire vinyl w/o ground and someone just draped them over/under ceiling joists in the garage (hanging down a foot or two) and not secured anywhere-even to the wall where they run against ship board siding in the garage where the studs are covered.

I am trying to get her (the land lord really) to upgrade the wiring, but he pulled the permit and the City Inspector told him anything in the garage is "grandfathered" so he will turn a blind eye to it, so the land lord doesn't want to spend a penny.

I am HOPING the Inspector will ding the location if he sees obviously unsafe things that don't even meet code from when the house was built, or when the unsafe wiring was installed (the vinyl sheathed NMC is date stamped in the 1970s and obviously was never permitted or inspected).

I am not against DIY installations at all (I worked for my electrical contractor uncle who hated/resented owner/builder installations) as long as the work is safe and done to code. But like so much un-permitted DIY, this garage is an accident waiting to happen and SHOULD be gigged IMHO.

At the very minimum, he should be made to replace the grounded outlets with un-grounded outlets (still available at box stores), and the draped wire should be removed totally. There are outdoor aluminum boxes used indoors
with no cable clamps and the jacket stripped way back and the inner wires pulled tight against the box's sharp threaded openings, and some were not even inserted into the holes-just wired over the edge into the box, none of which have any covers!

I am campaigning to have all the wires replaced with NEW NMC 2+Gnd since the owners are going to sheetrock the walls and ceiling ("to increase the house's value") sealing the bad stuff inside for eternity for some poor unsuspecting future buyer who may assume all's well unless they get an astute home inspection. The house is full of reversed polarity 3-wire outlets that have no grounds (I walked around with a Outlet/GFI tester myself).

Last question-

Does the latest code require an arc-fault/GFI CB on indoor circuits in an attached garage? I know any new outlets must be TR type as well, of course, and any accessible to the outside must be WR and have an appropriate cover allowing the cover to be closed over any plugs and cords being supplied (no more mini flip up covers over each outlet).

My own house is only 2 years old (professionally built and wired to then-latest code), and it has no AF CBs for the attached garage, and GFI protection is via individual GFI outlets supplying themselves and connected outlets. My shop, which I built last year, did not require any AF CBs even in the office area, but did require every 120v circuit to be GFI protected, and all outlets under 8ft high to be TR as well.

The indoor circuits are all CAFCI protected, even one that supplies the freezer outlet in the garage, which I'm not sure is the best deal given how the CAFCI breakers popped if you sneezed for the first 3 months of use (they have behaved themselves lately).

Bob

 

[Milton Shaw]

I used a 6/3 with ground to wire up my wife's new kiln. I did put in a sub panel and 40 amp breaker in it for the kiln. Less than two weeks later she bought a 120 volt 16 amp tabletop kiln on a bargain price and I was able to put a new 20 amp circuit in the subpanel for it. That's why I bit the bullet and ran 6/3wg instead of 6/2 wg. I think it cost 6 or 8 dollars more for the extra wire. The table top kiln lets her fire and look at it often for quick small projects. The bigger kiln takes 12 hours plus to run a cycle so she likes having both.

 

[bobinyelm]

12-2 can't be used for the 22 amp load. You'll need at least 10-2.

Also, the pictures you linked are of 50 amp plugs. They both have 30 amp variants. Did you get them mixed up, or do the kilns have 50 amp plugs?

Sorry- The 12-2 was a typo (I know that's good only for 20a). The purchased cable is 10-2 w/ gnd.

And the picture was meant only to show 3rd pin configuration using a "D" shaped ground rather than a flat blade. The actual receptacles will be NEMA 6-30 to match the one kiln already so equipped. I was being sloppy-thanks for keeping me honest.

 

[bobinyelm]

I used a 6/3 with ground to wire up my wife's new kiln. I did put in a sub panel and 40 amp breaker in it for the kiln. Less than two weeks later she bought a 120 volt 16 amp tabletop kiln on a bargain price and I was able to put a new 20 amp circuit in the subpanel for it. That's why I bit the bullet and ran 6/3wg instead of 6/2 wg. I think it cost 6 or 8 dollars more for the extra wire. The table top kiln lets her fire and look at it often for quick small projects. The bigger kiln takes 12 hours plus to run a cycle so she likes having both.

Makes sense.

In this case, the young lady is trying to save money because it's not her house, and the installation is on her nickel.

Given the state of the rest of the house's wiring, any additional expenditure would be best spent making its wiring safer. The land lord will be selling the house, and if she can qualify, she may buy it then have a considerable expense making it more modern everywhere.

 

[wyliesdiesels]

Last question-

Does the latest code require an arc-fault/GFI CB on indoor circuits in an attached garage? I know any new outlets must be TR type as well, of course, and any accessible to the outside must be WR and have an appropriate cover allowing the cover to be closed over any plugs and cords being supplied (no more mini flip up covers over each outlet).



Bob

If youre asking about the 240v outlets you will be putting in, the the answer is NO. Code does not require 240v outlets to be GFCI or AFCI protected.

120v outlets in a garage ARE required to be GFCI protected however.

 

[bobinyelm]

If youre asking about the 240v outlets you will be putting in, the the answer is NO. Code does not require 240v outlets to be GFCI or AFCI protected.

120v outlets in a garage ARE required to be GFCI protected however.

No, I am aware that 240 outlets require NO protection (AFCI or TR outlets), but I was asking about 120v outlets actually.

In my own detached shop only GFCI was needed, but I was curious about attached garages possibly requiring both GFI and AFI. You imply only GFI is required-

Thanks

 

[American Locomotive]

That was one question that was raised- Is it "legal" to wire the 3rd blade to ground, rather than to neutral. Some of the wiring diagrams showed that receptacle wired that way (3rd blade grounded via the bare wire), and it troubled me.

Maybe at one point it was legal, but that prong should always be connected to neutral, since it's a current carrying conductor. It doesn't matter anyways, since that type of plug is no longer installed in new installations.

That type of plug is called a NEMA 10-50. The 30 amp version (typically used for clothes dryers) is a NEMA 10-30. The other type of plug you mentioned (the one with the round ground pin) is called a NEMA 6-50. Modern 4-wire appliances (ones that have 120v loads in addition to 240v loads) use the NEMA 14-30, 14-50 or 14-60 plugs.

I called the kiln manufacturer of the one with the Y blades, and they said it left the factory with a NEMA 6-30 and suggested it be restored to that configuration as it has no 120v components.

That will allow the kilns to be interchanged position-wise as well, which is a plus beyond only needing the 12-2 w/ ground wire.

Thanks!

Then that would be the way to go. Install NEMA 6-30 plugs, NEMA 6-30 outlets, and use 10-2 wire.