Generator Head Wiring, a verbose page on the difference between a center tapped head, and a four wire head.

This page shares information that is typical of many gen heads, not just ST types. This is educational only... never play with electricity, in today's world it's never your fault.  Attorney's love left thinking.... opps.. I forgot, we were talking gen heads...

I've seen some generator heads being sold that are 3KW and center taped.  A single phase generator head sold as a 120/240 volt head often has only three wires.  If the head is sold as a (120 or 120/240) volt head, it should have four wires. This provides more options and flexibility.

So what's the big deal?

In my humble opinion, it's no big deal as long as the head provides about twice the KW as you might use for a  >SINGLE< 120 volt load.  Let's take a 10KW head, half of that would be 5KW. and that's plenty for a skill saw or most of the single 120 volt loads we might wish to run.

A center tapped 3KW head will deliver only 1500 watts to a single 120 volt load. This would be pretty wimpy power for a direct drive skill saw, and there's bound to be at least one guy who bought a small head thinking he was going to run one. If the head isn't set up properly (fused or breakered at the proper amperage), you could smoke the gen set, or possibly damage the skill saw. Some folks might even think they can pull the full 3000 watts of one winding.... they'll be plenty unhappy when the winding goes, and it will if they didn't put a proper fuse or breaker in the circuit.

So what do you do if you bought a center tapped head and it's not going to meet your needs? If you have average skills, and a little patience, you can often disassemble the gen head and convert it to deliver full power into a single 120 volt load. I'll also show you how you can flip a switch and get the 240 volts when you need it.

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The above schematic shows the stator windings with four leads. If U3 and U4 are tied together, you will provide 240 volts across U1 and U2, and you'll see 120 volts between U1 and U4/U3, and 120 volts between U2 and U4/U3.  If you wire the head where U2/U3 are strapped together and U1/U4 are also strapped together, you will have 120 volts across the connections, and you can provide the full rated output of the head into a SINGLE 120 volt load. 240 volts is not available in this set up, and some folks don't need it or want it.

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The above schematic shows a center taped head, note there's just three wires. the center tap is often labeled N for neutral. What should be obvious here is you have no choice as to whether you're going to have 240 volts or not. It's going to be here whether you want it or not. This is fine if you have a larger capacity head. At three KW or less, it's a poor choice (IMHO).

So now you know the difference. If you are one of those people that bought a 3KW or 5KW head and discovered the limitations only after you bought it.... and you think the limitation is far too severe, read on at your own risk; I'll show you how to modify it.   I have heard that the typical charge for this mod is about $125; if you can find someone to do it.  Of course, we're going to teach you to do it for free, and we're going to tell you how to properly fuse or breaker your head as well.

But first, you should  know how to solder, you need a soldering iron, you need to know what heat shrink is and you need some patience. It's not hard, it's just a tiny bit tedious. 

Of course we'll be talking about the ST head, but  the concept is the same for heads of different makes.

The first step is to get into the head to make the changes, it is best to pull the rotor and get the brushes out of the way.  these are all simple things on an ST head, don't be intimidated, my daughter could have taken this apart at six years old, and put it back together too.

Take off the vent covers, and the slip ring cover.

  1. remove the four bolts with 10mm heads on the end bell, this releases the bearing retainer/brush holder post.
  2. remove the four bolts that hold on each bell end..... '8 bolts total'. 14mm
  3. pull the brushes up in their holders so they clear the slip ring.
  4. Use a rubber hammer to loosen the slip ring bell housing.
  5. Remove the slip ring assembly
  6. Grab the rotor shaft and carefully guide the other end of the shaft out through the stator housing. Find a safe clean place to set your loose parts!
  7. Turn the wiring end, (slip ring end) towards you on a sturdy work bench. Get some decent light to work under. 

Now for the fun!

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Here's what you should be facing. You'll have to carefully trace the (N) lead down through the lacing. Use an exacto blade or something that will carefully cut lace without nicking enameled wire, it's pretty easy if you go slow. Don't be afraid to move other wires out of the way and to prod and pry a little as long as you're careful.

If you look at the wiring below, you'll see a pig tail right in front of the needle nose pliers, this is where stator windings are joined, and sheathing is placed over them, this is not something we'll need to mess with. To the right of that is the CENTER TAP we are going to split. You can see two groups of wire coming out of the stator and being terminated to the (N) lead forming the center tap.

Use your exacto blade to carefully remove the sheath and expose the solder connection.

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As you can see above, I've carefully removed the splice, cleaned the varnish off the wires with 300 grit sand paper and re terminated the Neutral lead to one side of the splice.  Although it's not clear, I've divided the stranded wire and soldered half to each of the varnished wires coming out of the stator.  Make sure you use a big enough soldering iron and that you do your bending on the stranded side versus bend and kink the varnished wires. Always make a good mechanical connection before you solder wiring. check the color of your solder joints, they should be shiny, not dull. At this point,  I made another lead up of similar stranded wire and terminated it to the remaining group of wires coming out of the stator (on the left above) using the same methods.

Now it's time to slip heat shrink of the proper size over your solder joints, be generous and overlap on each side a good amount. shrink it with a BIC lighter or other heat source, and lay the wiring back into the stator the way you found it. look around and see if you can find some lacing similar to what you've cut out. when it's all in place, you can paint on some electrical  varnish and let it dry. Maybe finger nail polish would work? cut off the excess lacing.

Now it's time to 'ohm out' your work.  Make sure you have continuity between U1 and U3.. and U4 and U2. Check that you are on the meg ohm scale between U3, and U4, it should be open.   Re label your leads from U1, N, U2 to ......  

U1------------------U3     U4-----------------U2

And finally, we should talk about adding  breakers or fuses. It would be a shame to smoke your head when something gets shorted out; but first, let's take an imaginary trip to your household fuse (breaker) box,


A typical fuse box is set up where every other breaker is feed from a different phase. If you have a typical box, you may see two breakers in the same housing. When the electrician plugs in a double breaker, each side is automatically feed from a different 120 volt leg (phase). Put your volt meter across the red and blue and you'll get 230 volts maybe higher or lower in your area.

Now for the fun. Lets say each breaker is 15 Amps, And both red and blue breakers feed (separate) 115 volt loads, they'd need to be around 1725 watts (15 amps) to blow.

If you use these two breakers to feed a 230 volt circuit, you'd have twice the voltage and you would need to trip out at the same amperage to protect your equipment. In this case, you can see that the voltage travels through both breakers and one or the other will trip first. the breakers don't care if you are going to use 115 or 230 volts, they will perform their function at the specified amperage.

Best wishes,

George B.