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Phil, Mechanical Engineer
Category: Electrical
Satisfied Customers: 7496
Experience:  Retired electrical contractor, 51 years experience.
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acquired an old (1940s) 3 phase Delta Bench Grinder with lights

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acquired an old (1940's) 3 phase Delta Bench Grinder with lights set-up attached. light bulbs are missing and seemed to have been wired in series judging by the wires that are there. we are replacing all the wiring and put new lights on it however.

we are in a 208 3 phase system

The motor has L1,L2,&L3 and a chassis ground. The lights for the wiring have been cut and are connected to nothing at this point.

The machine will be wired directly to a service disconnect and that service disconnect will be wired directly to the main panel.

My question is this:

can we safely use 1 of the legs as a line feed for two new 120 volt lights wired in parallel, and connect the neutral side for the lights directly back at the service disconnects ground? So basically the chassis ground and lights neutral are combined together at the service disconnect? Or could this cause a potential issue and thus we have to run a separate ground and neutral all the way from the panel, to the service disconnect, and from the disconnect to the unit?

Welcome to Just Answer!

Three phase power generally has a 'stinger' leg. That reads higher voltage to ground than the other two. Mark that one with red electrical tape, do not use it in your lighting circuit.


Two of the lines of your 208 three phase power will read 110-120 volts to the grounded neutral. You can use either of those to power your lights.




The exact voltage varies depending on how the transformer is tapped to supply the lighting circuits.

Check voltage between all 3 lines and ground, and tell me what voltage reads you get,

we can go from there until you have the complete answer you need.


Customer: replied 2 years ago.

Thank you Phil. I was aware of a high leg setup and I *think* this location does not have that but glad you reminded me to check. And that I will.


So while I check that are you saying it would be 'electrically' 'ok' (if that is a word ;-) to ground a neutral wire at the service disconnect location, whereas between the service disconnect and the main panel, there is nothing along the way? It is a dedicated 3 phase service interrupt in place just for this tool. Or do we HAVE to run a complete and separate neutral all the way back to the main panel as well?

thanks again!

Hello again,

The electrical panel should have Line1, 2, and 3.... and a neutral wire the same size, generally white.

If it does use one of the lines that measures 110v~ to the neutral for the lighting circuit, Line 1 for instance and Neutral. There MUST be a neutral coming in with line 1, 2 and 3 for this to be OK


If the neutral is connected to ground bar inside the panel, then it is a grounded neutral system... use the neutral in that case as well.

If the system neutral is not connected to the ground bar, and is isolated electrically from the metal panel, then you can use the neutral for your 110v circuit as well, but then must run a wire from the ground bar in the panel to the the frame of the grinder.

You run the ground wire from the panel to the frame of the grinder in any case.


If your 208v 3 phase is only Line 1,2 and 3 at the panel, and no neutral that goes back to the utility company transformer... then we can't have 110v lights, they must be 208 volt rated lights, that you power from the 3 phase.

Some people use Line 1 or 2 (not the stinger) to ground for their 110v power but that is NOT LEGAL, nor is it entirely safe.

Let me know what you have at the panel where the 3 phase originates. We can go from there.


Phil, Mechanical Engineer
Category: Electrical
Satisfied Customers: 7496
Experience: Retired electrical contractor, 51 years experience.
Phil and other Electrical Specialists are ready to help you
Customer: replied 2 years ago.

more details:

phase to phase in the panel we are 212 L1-L2, L2-L3, and L1-L3.


After further exploration here's what we found:


there were 4 lights total. 2 on each side of the machine. On each side, the 2 bulbs were wired in series. So the lights all together were most likely wired in parallel/series.

Does this then mean as far as restoring the unit goes, the bulbs (if we can find them) would actually be 110/120 volt bulbs because of the series wiring? And that introducing a neutral wire is not needed? Say using L1 and L2 as the wires for the lighting circuit?



Customer: replied 2 years ago.

I got an email from you/here today, I assume it was an automated one?
I am all squared away with this project. As it turns out my original confusion was verified once I found an old wiring diagram of the tool on the web (from the 1940's)
Indeed what they were doing was running 2 120 volt lights, wired in series, and using 2 legs of the 3 phase power (no stinger), essentially dropping the voltage of the lights to 120+/- each.
I have never run lights in series before and was unsure if this tool was wired properly as it had been cut up / hacked up recently.
Thanks for your assistance and guidance and I have paid you thru the site!

Hello again, you are correct that must have been an automated message.

that is absolutely amazing that you found a diagram on that thing from the 1940's!... that is very interesting about the lights in series, I'd of never thought of that.

Thanks for your positive rating, indeed, that pays me.

Customer: replied 2 years ago.

Yes indeed. Hand drawings as well!

As an electrician, you'd appreciate this I'd imagine:


I appreciate the art of it all... that was drawn a year before I was born probably.

When I was working at Boeing military air craft in Wichita KS...early 90's I had a small fill-in job to do and did the drawing by hand with the architectural type extended tails on the P's and T's etc lettering using a straight edge with some understated flourish. an art piece more or less on scratch paper... showing light perspective lines etc.


I did projects on and off for those guys for several years.


last time I was there, I noticed one of the project managers had the sketch on the side of his desk. (2 or 3 years later).




I got an 11 x17 limited edition print of the super sonic B-2 bomber signed by the crew while I was there they all signed it while I was standing there it was a mach 2 or 3 bomber, to this day the exact speed is secret.


They practiced take offs from a very short runway, and hit 45 degrees within a second of two of getting off the ground, then banked it wings vertical to the ground with a full test load of bombs on board... it shook the earth from half a mile away.. the most impressive thing I have ever seen.



.. I was retained to solve their 5 axis mill problem.. every spring and fall the scrap rate on the huge titanium wing bosses were out of tolerance... they were leaving the 8' x 3' x 5' billets out in the sun.. and they shrank during the milling process once inside and cooled with the cutting fluid.


They built air force 1 and 2 there, using green card labor, and ran the low voltage avionics in the same conduits as the 480 volt wing flap and tail section drivers.. the plane wouldnt get off the ground... but they saved $15 dollars an hour on the labor though.


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