[Elecraft] 240V Line

[Jim Brown]

On Tue,12/30/2014 12:01 AM, Edward R Cole wrote:
> I concur.  As result of this discussion and because I will soon add 
> another 240vac outlet to serve my 50v-50a switching PS being installed 
> to power a 1100w surplus ch.2 TV Harris amplifier (for use on 6m), I 
> looked closely at some of the twist-lock plugs I had in my parts.
>
> They are three contact plugs, so are unable to carry a safety ground 
> for splitting out 120vac. 

WRONG!  ALL power outlets MUST include safety ground (the green wire). 
To do otherwise is both unsafe and a violation of virtually all building 
codes in the civilized world. A 240V outlet with a 3-circuit plug MUST 
carry the two 240V phase conductors and GROUND, NOT NEUTRAL. If you want 
to connect equipment that includes a 120V load to that circuit, you MUST 
use a 4-circuit plug for that load.

In North America, nearly all homes are supplied by a transformer 
(outside the premises) with a center-tapped 240V secondary. The center 
tap is the neutral. 240V loads are connected end-to end, 120V loads are 
connected end to neutral. Those ends are called "phases." To connect 
dedicated 120V loads to that circuit, the 120V outlets must have one of 
the "phase" conductors, neutral, and ground. To equalize loading, it's 
good practice to split those 120V outlets between the two phase 
conductors (that is, opposing sides of the 240V transformer).

> They are legal for 240vac as the third contact is the safety ground. 

Yes.

> When wiring my shack for 240vac I bought No.8-4 conductor cable 
> (three-No. 8 and one solid copper No.12 wire in the cable.  So the 
> 60amp load box is properly connected to provide 120v break out as well 
> as 240vac with standard breakers.  But my 240v outlets are only good 
> for 240v as a result. 

That sounds fine, except that what you can connect to those 240V 
outlets  depends on how they are wired. If they are 3-circuit outlets 
with phase, phase, and ground, you can, indeed, connect only a 240V 
load. If they are 4-circuit outlets with phase, phase, neutral, and 
ground, you can connect a load that draws both 240V between the phases 
and 120V from one phase to neutral. Also, I'd be concerned about the 
size of that ground conductor. In general, the ground conductor must be 
sized at least equal to the phase conductors.

73, Jim K9YC