March 19, 2013, Email of the day, 120 VS 120/240

Draw your configuration and include the load.

Draw your configuration and include the load, are you wired series or parallel?


From: Robert C.

Subject: ST Head Wiring


If you re-wire the ST Head, and eliminate the 240v.  Does the AMPS increase? If I have a 2900 watt ST head, will the amps be close to 24?

I’m new at this.




Robert, you are exactly right, it’s a trade off, the power figure stays the same, so if we choose to wire 120 VAC only, we can increase the current, but we need understand, that happens when we put two windings to work.  For others who visit this page, I’ll extend the conversation.

It’s easy to keep it straight if you think of the two separate windings in the ST as flash light batteries.

We can arrange the windings or batteries in series or parallel and the result will be that we have the same power figure, but our voltage and current are different.

Some years back, I warned that the ST 3kw head can provide good service, but owners need a clear understanding of the limitations of 120/240 operation before they make use of it.


Lets keep it simple and use batteries that have a voltage of 1.2 volts for our comparison.



In both cases we are stuck with the power figures, there’s no magic to create more power potential than what we have, BUT we can trade off E and I  (voltage and Current) in how we arrange the batteries or the stator windings.


In the case of the 3KW ST, we have 25 amps at 120 volts, and if we want to arrange the winding in series to get the 240 volts we reduce (or trade) current capacity for the higher voltage.


120volts times 25 amps = 3000 watts

240volts times 12.5 amps= 3000 watts


WHEN we are wired for 120/240 operation, our 120 volt loads are powered by a single stator winding, the other winding can NOT help supply current to that load. Yes, we can have another load on the other winding at the same time, but that doesn’t help us supply more than the 12.5 amps to the load.


I think the danger come into view when we visualize using a portable table saw, and loading it down when ripping a board. Here’s where you WILL either protect your ST with properly rated breakers, OR risk burning out a stator winding.


We need understand how the damage we cause might not be readily seen, we can heat up that winding where we damage the insulation and send it towards a premature failure, it may take some time to see the damage, but with enough heat cycling and use, the damage shows up.

In smaller gen heads, there’s a ‘trade off’ option you can wire in. It allows you to select 120 or 120/240 VAC operation at the flip of a switch.


This of course is no substitute for proper fusing or circuit breakers, and we need to consider it is the individual winding we are attempting to protect.

Stator Worst Case


I always appreciate comments, do help me keep info accurate..



  • Q: What past utterpower article covers the re-wiring of stators, and shows  the addition of a simple DPDT switch to select stator configuration?
  • Fact: If we were to cover this topic well, we’d need a lot of pages, and why do that when Bill Rogers’ book covers it so well? The analogy between ST stators and batteries helps us remember the trade offs of E and I, but in AC, we have other things going on, and we can make good use of inductive and capacitive reactance, and learn how by reading Bill Rogers’ book.











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One Response to March 19, 2013, Email of the day, 120 VS 120/240

  1. Howard says:

    Hi all. Was the last question, re. 120/240 switch, ever answered? I’ve tried joining whatever groups I could find, and never get approved, so I can post. Tried here with borrowed facebook, and that link is dead, as well. I hope this hobby/preparation isn’t completely dead. Any help appreciated by this new changfa/st owner. Howard.

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