Which breaker should trip with a ground fault with a subpanel?

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Looking at a diagram, is often a good way to understand a problem. Below is a simple diagram that shows the fault current path.

You can see that the fault current will flow through all the breakers, and return to the source (the transformer) along the grounding conductor.

You might be inclined to think that the fault current will be extremely high (tens of thousands of amperes). However, due to the fact that wires have resistance, the current might be surprisingly low.

Without knowing the exact length and size of each wire, it's not possible to approximate the resistance. If you did have that information, you could calculate the resistance. With that, the voltage, and Ohm's law, you could calculate the fault current.

For this example, we'll assume the fault current is less than 100 amperes.

Because the current is not above the instantaneous trip level of any of the breakers, the short-circuit protection of the breakers will not trip....

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If you want to do this correctly you need to run 10-3 UF-b not NM-b to the subpanel.

At the sub panel you need to isolate the neutral bar (remove the bonding screw or strap). Buy and install a ground bar. The black and red of the 10-3 go to the "hot" lugs. The white to the neutral bar. The bare ground to the ground bar. You need at least one 8'xЅ" ground rod at the subpanel connected by #6 wire to the ground bar of the sub panel.

If #10 is smaller than the minimum size for your sub panel's "hot" lugs you will need to add pigtails of a larger size to the black and red.

Use a non GFCI breaker at the main panel and a GFCI breaker at the subpanel. Cheaper usually and you don't have to go all the way to the house to reset a tripped...

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Hi all - I have an outdoor subpanel with switches for my well pump and for the receptacles and light fixture in the well pump house. As you can tell from the pics it is old (1950s), and also, it does not have an off switch either at the subpanel, or connected to my main panel inside my house. The power company will have to come out to power it off in order to replace the breaker switches.

One of the switches has gone bad, ie, I turned it off and when I flip it back to the on position that circuit does not come back on. In the picture, it is the two switches on the right. The two on the left, which control the well pump, fortunately still work. But i figured I might want to replace those as well, as they all feel "soft" when flipped.

Based on the pictures, I was hoping someone might be able to tell me what I need to get as far switches - ie, brand, type, etc. I obviously can't take them out until the power company comes out, and I wanted to have the new switches...

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This invention relates to ground fault protective equipment in general and more particularly relates to equipment of this type having means to indicate whether automatic tripping has resulted from an overload current condition or a ground fault condition.

Recently there has been a significant increase in the utilization of ground fault protective equipment coordinated with automatic circuit breakers in a manner such that the circuit breaker is operative to open the circuit when a ground fault is detected therein. However, tripping of the circuit breaker does not indicate whether it has been caused by an overload or by a ground fault, so that corrective measures must be delayed until the nature of the fault condition is determined.

In accordance with the instant invention, means are provided to indicate whether automatic tripping of the circuit breaker has resulted from a ground fault or an overload current condition. This is accomplished by providing an indicator that...

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Troubleshooting a ground fault circuit interrupt, or GFI, breaker is pretty straightforward. Troubleshooting the circuit itself can be quite time-consuming. The GFI breaker is designed with a test button incorporated into the breaker itself. Pushing the test button should trip the breaker. On GFI-style breakers the neutral wire going into the house's outlets is connected to the breaker's neutral connector, the white neutral that comes out of the breaker is connected to the neutral bus in the panel, isolating the neutral bus from the neutral wire going into the house. The test button actually shorts the neutral wire feeding the circuit to the neutral bus in the electrical panel creating a ground fault that should trip the breaker. It is considered a ground fault because the neutral bus in the main electrical panel is actually connected to the ground bus through the panel's metal...

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Before you do anything to check the problem, TURN POWER OFF.I have to say, without seeing it, that you need to use your fluke meter to check ohms. Turn off the main breaker. Next you need to check each breaker to ground (the green wire). The ohm setting on the meter should show a large number. If the meter shows 0 or a single digit, you have a grounded wire connected to that breaker. If 0 or low number, make sure all connections to outlets connected to that breaker are not loose or touching each other in the outlet box. If this is all too confusing or you really don't know how dangerous the power is coming to the main breaker on top(even though the main breaker is off), you need to call an electrician. I don't want you to get hurt.

Source(s): I'm an electrician.

Asker's...
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Hi Jorge,

Thank you for your question regarding the Neutral-to-Ground Bond in the main panel. The short answer to your question is that the neutral-to-ground bond is needed to properly operate the circuit breakers. Over Current Protection Devices (OCPD) such as circuit breakers and fuses actually require a short and intense INCREASE in electrical current (a short) in order to detect the fault and cut the circuit off. Without a sharp and drastic increase in electrical flow, a fault could go on without triggering a circuit breaker to stop the flow. This actually occurs quite often and can be measured easily by checking the amount of current flowing on your ground conductor. It should be less than 1-amp in most cases. If the current flowing on a grounded conductor is higher than an amp, and you are not in a high-voltage (600V+) environment, it typically indicates a erroneous neutral-to-ground bond somewhere in the system.

To visualize the reason why the...

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None of the information you will read is intended as legal advice. None of it is sufficient in itself to serve as a "how-to" to let you do your own wiring. None of it is intended to substitute for the Terms of Engagement under which I work as a contractor or consultant.

I start with an explanation of the plans that are needed before starting a sizable electrical project.
I then talk about the legal requirements for various aspects of your electrical system, including rationales and options.
From that, I lead into a discussion of lighting and controls.
Next, I introduce some information on smoke alarms that you may not know.
Finally, there are four appendices:

Children.

Plans

On any sizeable job, involving the installation of more than ten electrical devices, the first thing you need is a set of plans. You can prepare them, provided you understand the basic symbols used for electrical equipment. Sometimes, I am paid to sketch...

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Hi Max,

Thank you for your question regarding Ground-Fault Circuit-Interrupter (GFCI) protection for personnel and refrigerator receptacle outlets, it is our pleasure to help.

Refrigerators, surge-protection devices, electric motors, and many other devices have reported problems of excessively tripping the GFCI breaker when these items are in normal use. It doesn’t take much of an imagination to understand the frustration of a home owner when they come home to find all the food in their refrigerator spoiled because of a GFCI tripping during normal operation. However, modern refrigerators do not typically have this problem. In fact, if you have a refrigerator built in the last 20 years or so, if it does trip the GFCI, it is probably more likely that there is a problem with the refrigerator than with the GFCI circuit.

A GFCI (Ground-Fault Circuit-Interrupter) uses technology to sense the difference in amperage between the hot and the neutral lines, typically...

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