Can you put a GFCI into a circuit with an existing GFCI and what will happen if the two GFCIs are in a series circuit?

Answer 1

Note: The latest types of Ground Fault Circuit Interruptor (GFCI) are also called Residual Current Devices (RCDs).

"Can you put a GFCI into a circuit with a GFCI?" is probably the same question as "What if two GFCI are in series in a circuit?" and the answer is that one GFCI will most likely trip earlier than the other to break the circuit. Which one will trip first depends on the actual difference in sensitivity between the two GFCIs. Leakage currents are measured in mA (1 mA = 1 thousandth of an Ampere) and GFCIs are designed to trip at a certain leakage current with a tolerance of say + or - 3%. So if the basic trip current was 20 milliamps one GFCI might trip at 19.4 mA and the other at 20.6 mA and they would both be within the design specification. The one that trips at 19.4 mA would be expected to trip first. <><><> This question requires answering by a VERY knowledgeable electrician, or an electrical engineer. I am neither, but until the right answer comes along, I will attempt to provide some information which hopefully will allow a better understanding of the issues involved. A GFCI [ground fault circuit interrupter] is much different from, although similar to, a circuit breaker. The DIFFERENCE is WHAT they are designed to detect and react to. The purpose of a fuse or circuit breaker is to detect excess or MASSIVE current flow [ie. a SHORT CIRCUIT, a direct short to ground], and then very quickly turn off the electrical current to the circuit, in order TO PREVENT a FIRE. This is the only thing the breaker is designed and intended to do. This limited capability will not prevent a person from being electrocuted when a fault occurs within an electrical device, if the current flow happens to be low. Voltage and current faults which can kill a person usually will not be detected by a circuit breaker. A ground fault circuit interrupter [GFCI], on the other hand, is designed to, and will, detect those low level currents which can kill a person, BUT which are not great enough to trip a breaker. When a GFCI detects potentially dangerous current it, like a breaker, trips in order to open the circuit. Now as to WHY, when you placed two GFCIs in series [one in the breaker panel feeding one in an electrical receptacle], the first GFCI tripped when the second GFCI is "tested." When you press the TEST BUTTON on a GFCI it puts a high resistance load on the circuit, which simulates a "small" ground fault, and I think both GFCIs "see" the same fault and react as designed and intended. Again, a true professional should "bless" my comments, but I think that there is no danger or hazard in placing two GFCIs in series. On the contrary, I think there is redundant capability in the pairing, with the first GFCI functioning with, and as a backup to, the second. I suggest you contact your local electrial inspection department regarding what the code(s) say about this issue, and also call the tech support people at the 800 number on the packaging for the GFCIs. Another source of information is a local electrical engineer, if you know someone who can help you make contact. Until a more informed answer comes along, I hope this helps you to understand the issues involved. <><><>

As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.

Answer 2

It is not recommended to install two GFCIs in series as they can interfere with each other's operation and lead to potential tripping issues. It is best to have only one GFCI per circuit for proper functionality and protection against electrical hazards.