# Plug Explanation



## TORQUIN (Oct 11, 2020)

I just bought a surface grinder and it had a 20A, 3 phase (4 conductor) plug. I use a 3 phase, 20A, 4 conductor plug for my old surface grinder. When I compared the two plugs I found they are different. The one I use has a smaller spade circle then the one that came with the new grinder and new grinder's plug has the lock lug on a different spade then the plugs I use. I purchased the plugs and sockets I use from Lowes/Home Depot. I have not been able to find info on the internet to tell me what's the reasoning behind this different size/configuration between these 20A, 3 phase, 4 conductor plugs.
Can someone explain to me what's going on here?

The two plugs, a new one I just bought on the left, identical to the ones I already use in the shop, and on the right, the plug that came with the new grinder.



Thanks,
Chris


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## hman (Oct 11, 2020)

I'm no electrician, but here's an attempt to explain.

The L14-20 (orange) plug on the left is actually designed for "center tapped" 230V AC single phase.  The center tap allows you to take 115 volts as needed.  See http://www.generatorsforhomeuse.us/l14-20/

The grey-faced plug on the right is an "official" 3 phase connector.  Note that is it's labeled "3øY" on the face.  Both of these connectors are rated for 20 amps.

The fact that the L14-20 has 4 lugs makes it usable on 3 phase power, and I'm sure it's commonly used for such.  But they're designed to be different for safety reasons - so that you don't try plugging a 3 phase machine into a single phase 230V outlet (or vice versa) 

Notice that the next thread in this section, https://www.hobby-machinist.com/threads/wiring-my-3-phase-lathe-to-my-rpc.87560/ that the discussion is about L15-30 and L15-20 connectors.  Both of these are are marked for 3 phase, 15 and 30 amps, respectively.

I'm sure that if  Bi11Hudson joins the discussion, he can correct any mistakes I've made.  Not only is he knowledgable, but also writes good, clear explanations.


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## RJSakowski (Oct 11, 2020)

Here is a chart with the various NEMA configurations.




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						NEMA Plug and Receptacle Configurations
					





					www.nooutage.com


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## Papa Charlie (Oct 11, 2020)

Since the are both 3 phase and both 20amps. You could switch the grinder to the L14-20 style.


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## hman (Oct 11, 2020)

Thanks, RJ!  The Nomenclature chart does a good job distinguishing between 230V single phase and 3 phase.  
...but...
Can't find the "stove/clothes dryer(?)" outlet that my shop was wired with ... NEMA 10-50


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## TORQUIN (Oct 11, 2020)

Papa Charlie said:


> Since the are both 3 phase and both 20amps. You could switch the grinder to the L14-20 style.


 Already did. Changed the plug on the new grinder to use what I already use in the shop. I hate mismatched plugs and wiring.

Chris


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## TORQUIN (Oct 11, 2020)

RJSakowski said:


> Here is a chart with the various NEMA configurations.
> 
> 
> 
> ...


Thanks, but it only shows the L14-20, not the other one I have, so not much information there.


Chris


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## hman (Oct 11, 2020)

Just figured out why my NEMA 10-50 isn't on the chart that RJ referenced - it's no longer "legal," just grandfathered.  Wikipedia has a good discussion of current and old NEMA plugs/receptacles:








						NEMA connector - Wikipedia
					






					en.wikipedia.org
				



... and here's another chart, which includes the 10-50, also does a good job of distinguishing between single and 3 phase connectors:




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						NEMA Straight Blade Reference Chart
					

StayOnline has created the NEMA Straight Blade Reference Chart so you can learn about NEMA configurations, such as NEMA 14 30 and NEMA 5 15. Contact us today!




					www.stayonline.com
				




Enjoy!


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## RJSakowski (Oct 11, 2020)

TORQUIN said:


> Thanks, but it only shows the L14-20, not the other one I have, so not much information there.
> 
> 
> Chris


Below the first chart is a chart that explains the naming practice for the NEMA plugs.  Here is another chart. https://www.conwire.com/wp-content/uploads/NEMA-Configurations-Plugs-Receptacles.pdf


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## TORQUIN (Oct 11, 2020)

Seems like the grey plug has no NEMA info on it. I don't know what the significance, if any, the "30Y" has.

Chris


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## Superburban (Oct 11, 2020)

TORQUIN said:


> Seems like the grey plug has no NEMA info on it. I don't know what the significance, if any, the "30Y" has.
> 
> Chris


3 phase with the Y connection, which has the center tap for neutral.


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## extropic (Oct 11, 2020)

I believe the Grey plug is an obsolete configuration and does not conform to the current NEMA L18-20P (3 Phase Y, 120/208 V, 20 A).

Wiring your shop 3 Phase power using single phase devices is abhorrent to me.
It's foolish and false economy. I believe, should you have a fire, from any cause, your non-compliant electricals could void any insurance claim.

Hubbell NEMA Chart


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## tq60 (Oct 12, 2020)

Did not know the 10-50 dryer outlet was "not legal", I suppose when used as described to cheat the neutral it makes sense but we still see a similar 3 prong plug in use.

We have the dryer outlets all over the shop as we pick them up at yard sales for cheap.

But we wire for 240 VAC and safety ground with no neutral so ours should be good.

The not so old welder came with similar style, L1,L2 safety ground.

The twist locks have many formats, not only do they have different size terminals, but different circle sizes and the safety ground has different configurations.

We have a collection of units that are oddball, assuming the differences are so they can be wired to insure compatibility between equipment and source, never took the time to determine exactly the reason.

Not in use...

Sent from my SAMSUNG-SM-G930A using Tapatalk


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## Flyinfool (Oct 12, 2020)

If it were me I would change all the plugs in the shop to the correct ones for the application. The electricity does not know the difference between the plugs and either will function just fine. As long as you never have an appliance that needs the single phase center tap plug that might get get plugged into the 3PH, where it will let out all the magic blue smoke, that all electronics run on, out of that appliance. I don't know how many 3PH machines we are talking about, but you can always change them over one at a time to spread the cost.

But at some point it will become a legal or technical issue. If you ever try to sell the house an inspector will point it out and you will have to change them all or tear them all out. Same goes if you ever have a fire and you have incorrect wiring, insurance companies love to find excuses to not pay.


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## jmkasunich (Oct 12, 2020)

Just to emphasize what others have said - L14-20 is NOT a three-phase plug.  It is a single phase plug, for use on regular household 120/240V split SINGLE PHASE power.  Just cause it has four prongs doesn't make it a three phase plug.  Those prongs are Line 1, Line 2, Neutral, and Ground.

An L15-20 is a legit three-phase plug.  The four prongs are Line 1, Line 2, Line 3, and ground.  Note that there is no neutral.  This is for three-phase delta loads.  There are other NEMA outlets for three-phase with neutral (L18), and for 480V three phase (L16).

You should always use the right plug for the power.  The Hubble chart that Extropic posted is a great resource.

If you have three-phase machines you'll be wanting L15, but you probably won't find them at Home Depot.  I usually get them from Flea-bay.  Both new and used are available at reasonable prices.

Using L14 for three-phase is tempting because you can by them at the big box.  But it damn well better be a one-man shop with no 120/240V single phase loads (which would also use L14).  And don't let an inspector anywhere near it.  I wouldn't do it.  Three-phase plugs for three-phase power.


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## hman (Oct 12, 2020)

@Flyinfool - I definitely agree with you regarding insurance.  But, unfortunately, home inspections (at least in Oregon and Arizona) specifically exclude detached buildings (like my shop).  As the sergeant used to say in Hill Street Blues, "Be careful out there!"


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## TORQUIN (Oct 12, 2020)

Thanks for setting me straight on this. I was totally unaware of these differences, and this is why I asked.
So, before I start hunting all new plugs and sockets, what is the difference between the L15 and L18? Why do you suppose my grinder had an L18 plug on it?

Here is my motor tag.



Thanks,
Chris


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## jmkasunich (Oct 12, 2020)

TORQUIN said:


> Thanks for setting me straight on this. I was totally unaware of these differences, and this is why I asked.
> So, before I start hunting all new plugs and sockets, what is the difference between the L15 and L18? Why do you suppose my grinder had an L18 plug on it?
> 
> Here is my motor tag.
> ...



L15 is for 240V three-phase without a neutral.  That is three hots, with 240V between any two hot wires.  Great for running motors and machinery, but inconvenient for lights other 120V stuff.
L18 is for 208Y/120, which is three phase with a neutral.  There is 208V between any two hot wires, and 120V between each hot and neutral.  Popular in small commercial buildings where there is some need for three phase motors (air conditioning, etc) but also a lot of regular 120V loads.

Your grinder motor doesn't need a neutral, so it's not clear why it was equipped with an L18 plug.  One possibility is that there is a 120V work light or something on the machine, connected between neutral and one of the hots.  Another possibility is that it was used someplace where they had 208Y/120 power and they put L18 on everything whether it needed the neutral or not.

Here is a page with a decent explanation of the various services:  https://pveducation.com/solar-concepts/common-electrical-services/

There is also the whole 220/230/240V thing.  A long time ago, the standard was 110V for lights and such, 220V and 440V for bigger loads.  For a variety of reasons (efficiency and such), they gradually increased the nominal voltage.  You'll find products marked 110V, 115V, 117V, and 120V.  All are compatible, but 120V is the current standard.  Likewise at the higher voltage end you'll see 220V, 230V, and 240V, along with 440V, 460V, and 480V.  The current standard is that 240V and 480V are "supply" voltages - what the utility delivers.  Modern motor nameplates are marked with 230 or 460V, which is considered "utilization" voltage - what the load needs.  The difference is an allowance for voltage drop in the system.  

Motors marked for 220-230V will run on 208V but can't quite deliver full horsepower.  Motors marked 208/230 will deliver full power on both voltages (but typically draw less current and are more efficient on 230 or 240V).  Motors marked with just 208V would probably run OK on 230/240 but might run hot.

If you are choosing outlets to wire your shop, the first question is where are you getting your three-phase power from?

If you have a rotary (or static) phase converter, you will NOT have a valid neutral for the three-phase side of the system and should NOT use an L18.  Stick with L15.

If your power company is giving you 240V three phase, again, you won't have a neutral and should not use an L18.  Stick with L15.

If your power company is giving you 208Y/120 then you can use L18 or optionally L15.

I use L15 on all my three phase stuff.  I'm currently in a building that has 240V three phase, but if I move, I can use the same plugs and receptacles with 208Y/120 or with a phase converter.


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## jmkasunich (Oct 12, 2020)

Oops, I need to correct myself.

I got L18 mixed up with L21.

L21 is the "proper" way to do 208Y/120 stuff.  It has five prongs, not four.  There is a round pin in the center that gives you a safety ground, and the other four are three hots and a neutral for powering machinery that has both three-phase 208V loads and one or more single-phase 120V loads.

L18 has only four prongs.  It gives you three hots and a neutral, but no safety ground.  I'm sure there are plenty of legit circumstances for using that connection, but I personally have not and would not.  I like having a separate safety ground conductor.

L15 also has four prongs, but they are three hots and a safety ground, no neutral.  Standard for three-phase 240V (and what I use in my shop for three-phase stuff).  Most machines don't need a neutral.  For example my bridgeport has a small transformer hanging on the side that is connected to two of the three hots and makes 120V for the DRO, power feed, and work light.

Based on the 208/120V marking I think your gray plug is an L18 but I'm not 100% sure.  I like it better when they print the actual number on the plug (like the orange L14).

As far as your grinder goes, if there is no load connected to the neutral prong, then you should be using L15.  You may have to dive into the electrical box on the grinder.

If you are currently using L14 for true three-phase loads in your shop (powered from a rotary or from power company three-phase), you should probably switch them to L15.  L14 is perfectly fine for 120/240V single phase loads, such as a welder.  

L15-20 on ebay - looks like both plugs and receptacles are available for under $10.
https://www.ebay.com/itm/264714630445
https://www.ebay.com/itm/113689556856
(no affiliation with these sellers, and I didn't check feedback or anything - just grabbed the first couple listings.  There are plenty to choose from.)



jmkasunich said:


> L15 is for 240V three-phase without a neutral.  That is three hots, with 240V between any two hot wires.  Great for running motors and machinery, but inconvenient for lights other 120V stuff.
> L18 is for 208Y/120, which is three phase with a neutral.  There is 208V between any two hot wires, and 120V between each hot and neutral.  Popular in small commercial buildings where there is some need for three phase motors (air conditioning, etc) but also a lot of regular 120V loads.
> 
> Your grinder motor doesn't need a neutral, so it's not clear why it was equipped with an L18 plug.  One possibility is that there is a 120V work light or something on the machine, connected between neutral and one of the hots.  Another possibility is that it was used someplace where they had 208Y/120 power and they put L18 on everything whether it needed the neutral or not.
> ...


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## TORQUIN (Oct 13, 2020)

My grinder has no work light and a separate 120V plug for the mag chuck. I do not know if it ever had a work light, or of the mag chuck was wired into the 3 phase. I do know that the mag chuck controller is not very old, so it is entirely possible that it used to be involved with the 3 phase before they replaced it.
Thank you for the extensive explanation. This is the info I have been looking for. Perhaps this should be a sticky for others encountering such plugs on equipment they purchase.

I will begin hunting L15 connectors.

Thanks,
Chris


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## TORQUIN (Oct 13, 2020)

Thanks for the Ebay tip, to all who suggested it. I just ordered 3 wall sockets, 1 extension cord socket, and 5 plugs for 71 bucks. That is a much better price than I expected to pay.

Thanks,
Chris


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## Tozguy (Oct 13, 2020)

This thread has been very helpful to me. I too vote for it to become a sticky.

Our house, shop included, is a mixture of old (circa 1947) and newer wiring. All typical single phase 120/240 residential service.
I have done most of my own electrical work with the guidance of an electrician when necessary. I follow instructions carefully but I don't always understand why things have to be done a certain way.

For example our power comes in on two insulated wires carried by a bare cable that is structural. Does this bare cable become the white wire or the ground?

Also, why are 120v receptacles and plugs polarized? With alternating current does the polarization not change at the rate of 60 cycles per second?

Sorry for the silly questions but I am very challenged when it comes to understanding electricity.


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## jmkasunich (Oct 13, 2020)

Tozguy said:


> This thread has been very helpful to me. I too vote for it to become a sticky.
> 
> Our house, shop included, is a mixture of old (circa 1947) and newer wiring. All typical single phase 120/240 residential service.
> I have done most of my own electrical work with the guidance of an electrician when necessary. I follow instructions carefully but I don't always understand why things have to be done a certain way.
> ...



The bare cable in your overheat power company service drop is the neutral.  It is connected to ground at your main breaker panel.  Ground for your house is provided by either a ground rod or by buried metal water piping.

The distinction between neutral and ground is tricky.  They are both connected to ground, but they serve different purposes.  The National Electric Code (NEC) refers to neutral as the "grounded" conductor, and safety ground as the "grounding" conductor.  Not a good choice of words for the casual reader!  The way I think of it is that the white neutral aka "grounded" conductor is a load carrying wire that just happens to be grounded.  The green "grounding" conductor does NOT carry load, its only purpose is to provide safety grounding for the exposed metal parts of equipment.

So going back to your service drop - the load current returns to the utility pole thorough that bare neutral wire.  But load current does NOT flow in the wire going to the ground rod or water pipe.

The reason for polarized plugs is not about the polarity of the voltage - as you say, AC switches back and forth all the time.  It is a safety thing.  The hot side is swinging positive and negative, but except for the brief instants when it crosses zero, it is always at a dangerous voltage.  The neutral side of the circuit is grounded at the main panel and usually within a volt or so of ground everywhere else, so it is much safer.  Now think about a lamp with a switch.  If the switch is in the hot side, when you turn the lamp off, both sides of the socket are safe to touch (for example when changing the bulb). But if you plug it in backwards, the switch is in the neutral and turning it off doesn't actually remove power from the socket.  If you touch the inside of the socket and ground at the same time you get zapped.  Polarized plugs are an attempt to keep the switch on the hot side of the circuit.


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## Papa Charlie (Oct 13, 2020)

I thought I was pretty well versed on plug types. Learned some new things, thanks very much for sharing this thread.


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## Tozguy (Oct 13, 2020)

JMK, a big thank-you!
Now I get it and it feels GOOD to finally see the light, no pun intended.
This place is GREAT!


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