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240v GFCI – how does it work?

JTC1 | Posted in General Discussion on February 28, 2009 02:32am

I have a freshly wired 50A outdoor sub panel supplied by a spa manufacturer.  Sub panel supplied by 2 hots, neutral and ground wire. Panel box is grounded, neutral floats (not bonded to sub panel enclosure).

Sub panel contains two, two pole GFCI breakers, one 20A, one 30A. Breakers installed, white pigtails connected to the neutral bar.

20A breaker runs the heater only (pure 240v load).

30A breaker is split into two 120v circuits once connected to the spa unit.  The two 120v circuits drive the pumps, aerators, lights, timers, etc.

Manufacturer’s wiring diagram shows:

from the 30A breaker – 2 hot wires plus a white neutral wire (OK, OK, grounded conductor)

from the 20A breaker – 2 hot wires only

in addition there is a green grounding conductor from the ground bar of the sub panel to a ground lug in the spa – sized at #10 awg 

My rudimentary understanding of GFCI devices says that GFCIs work by comparing the current going out in the hot to the current returning in the neutral — difference? = ground fault = trip.

There are warnings everywhere to connect the neutral wire to the 30A GFCI breaker rather than the neutral bar in the sub panel. Pigtail from breaker then connects to the neutral bar – I understand how this works.

My question:

How does the 20A GFCI work with no neutral from the unit for comparison purposes?  I understand how the 240v heater works with no neutral.

Does the GFCI compare outgoing and returning current in the two “hot” wires?

Edumicate me please – or at least reassure me that it will work reliably by magic, voodoo or extraterrestrial oversight.   

Jim 

Never underestimate the value of a sharp pencil or good light.
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Replies

  1. USAnigel | Feb 28, 2009 03:30am | #1

    Hey Jim

    Thing to remember is the 240volt comes from two 120volt lines. These lines are 180 degrees out of sync. Think +120 and -120volt at the same time, this equals 240volt.

    GFCI is a device looking for imbalance. So any stray voltage will trip the breaker.

    Just like a balance scale as long as you add or subtract the same from each side it stays in balance but spill a little and it goes out of balance. This is why the neutral needs to be connected to the GFCI breaker. So all things can be monitored.

    A shared neutral will trip the breaker. ie if a neutral return is added before the GFCI when somebody adds a socket.

    1. JTC1 | Feb 28, 2009 04:03am | #2

      >>Thing to remember is the 240volt comes from two 120volt lines. These lines are 180 degrees out of sync. Think +120 and -120volt at the same time, this equals 240volt.<<

      I understand this is the way a 240v line works.

      >>GFCI is a device looking for imbalance. So any stray voltage will trip the breaker.<<

      So the GFCI looks at the two hot wires in this application and seeks to find any imbalance between the two.

      That is what I theorized.

      Thanks,

      JimNever underestimate the value of a sharp pencil or good light.

      1. mike585 | Feb 28, 2009 04:18am | #3

        The system doesn't care about the voltage. If your circuit is designed for the current to feed from the red +120 v line and return on the black -120v line then any difference is due to fault and the breaker opens.  I only used the + and - to show that the difference is 240v.

  2. DanH | Feb 28, 2009 04:56am | #4

    In a GFCI the two (or three) current-carrying conductors are run through a "current transformer" (a toriod transformer, if you're interested). This transformer converts the net current in the conductors to a voltage that is sensed by the GFCI electronics.

    With a single-pole GFCI the hot and neutral run through the transformer, and if ever there is an imbalance between the two a voltage is sensed and the GFCI trips.

    With a two-pole GFCI two hots and the neutral run through the transformer. Current can be split in any fashion between the three conductors and it will still cancel out if there's no leakage to ground. If the neutral isn't connected then that wire is effectively ignored by the transformer (since there'll never be current in it) and the current between the two hots is compared.

    In theory you could have a dozen separate conductors, or 100, and it would still work the same -- just run all the wires you want compared through the hole in the transformer.

    The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

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