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Home > Archive > Electrical Engineering > June 2006 > 240V 2 wire breaker 120V to Ground 0 across wires
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240V 2 wire breaker 120V to Ground 0 across wires
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| alexbarham@yahoo.ca 2006-06-16, 1:25 pm |
| I am installing a water heater. I know that the supply requires 240V
across both wires (red & black). However, when I measure one wire (red
or black) to ground, I get 120V. But, if I measure across the 2 wires,
I get 0 volts. What would cause this?
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| furles@mail.croydon.ac.uk 2006-06-16, 1:25 pm |
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alexbarham@yahoo.ca wrote:
> I am installing a water heater. I know that the supply requires 240V
> across both wires (red & black). However, when I measure one wire (red
> or black) to ground, I get 120V. But, if I measure across the 2 wires,
> I get 0 volts. What would cause this?
It sounds like you're talking about an American centre tapped 120/240V
single phase system, but it would be a good idea to say if that's what
it is; electrical systems vary considerably in different places. If
this is the case, since both red and black wires are reading 120V to
ground they must be connected to something. Since there is no Voltage
between them it sounds like they have both been connected to the same
side of the supply, i.e. both wires are going to where the black one
should be connected, or where the red wire should be connected, rather
than one wire to each side.
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| gfretwell@aol.com 2006-06-16, 1:25 pm |
| On 16 Jun 2006 09:06:40 -0700, alexbarham@yahoo.ca wrote:
>I am installing a water heater. I know that the supply requires 240V
>across both wires (red & black). However, when I measure one wire (red
>or black) to ground, I get 120V. But, if I measure across the 2 wires,
>I get 0 volts. What would cause this?
Two possibilities. You either have both hots on the same phase or one
of them is open, effectively the same thing since the other phase is
reflected through the load. Check it at the breaker, If that is bad
too, be sure the breaker is hitting both phase rails. Some small GE
(and perhaps others) panels will let you plug in a double breaker on
the same bus
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| Bob Weiss 2006-06-16, 5:25 pm |
| alexbarham@yahoo.ca wrote:
> I am installing a water heater. I know that the supply requires 240V
> across both wires (red & black). However, when I measure one wire (red
> or black) to ground, I get 120V. But, if I measure across the 2 wires,
> I get 0 volts. What would cause this?
>
It sounds like you installed a "twin" breaker, rather than the required
2-pole breaker, so both terminals on the breaker are being fed from the
same phase.
What you need is a real 2-pole breaker, which will clip onto 2 adjacent
busbars, providing 240V between the 2 phases.
Bob Weiss N2IXK
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| alexbarham@yahoo.ca 2006-06-16, 5:25 pm |
| Thank you all. This greatly helps my understanding of what is going on.
I'll have someone look at the breaker panel and fix the issue.
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