My GFCI always trips whenever refrigerator is connected to it maybe because of the defrosting thing and my old built in the 1930s without third grounding wires. I have to take down the house just to replace all the conduits. Can I used 1:1 isolation transformer on refrigerator (without the secondary connected to any ground)? If the refrigerator amperage is 1.2A. Can I use a 1.28A isolation transformer on it?

Is there any side effects like statics building on the refrigerators since the secondary is floating and not grounded? (If I'd ground it, it defeats the purpose of isolation transformers)

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    Have you considered rearranging your kitchen wiring so that the fridge receptacle's no longer on a GFCI? – ThreePhaseEel Dec 5 '18 at 0:32

Ground the fridge!

Grounding the fridge will not defeat the purpose of the isolation transformer, but rather complement it.

You may be operating on a misconception that GFCI devices care about ground. They do not. They are not connected to ground in any way and have no idea what is going on with ground. Actually, what grounds do to GFCIs is make ground-fault current more likely to flow, which trips them obviously. The isolation transformer will prevent that.

The reason to ground the fridge is to assure the fridge chassis and the sink, light switch cover screws and other grounded things remain at the same voltage. It will also assure a GFCI trip if the isolation transformer itself leaks, something you want for safety.

An isolation transformer is a good solution, though

In this configuration, the GFCI will only "see" the isolation transformer primary. Currents will be equal in the transformer, where else can it go?

I never believed any of that "Harmonics in the AC motors ingest some of the electrons, causing false trips" nonsense, but if that were so, the electron losses would be on the isolation transformer's secondary, and the GFCI would not see them.

Fridges shouldn't be on GFCI in the first place

We have never found anything in Code that requires refrigerators be on GFCI, except for ones in garages or basements only befause garages and basements need GFCI, not the fridge. Even there, authorities tend to be warm to the idea of dedicated non-GFCI circuits for fridges.

Look at the reasons for GFCI, they simply do not apply to a grounded refrigerator. However, fridges are life-safety devices; if their GFCI trips and someone resets it, they may not realize the fridge was off and the food has spoilt. Then the aide comes in to feed grandmother, and aides have an ethic about not eating your food, so they never check.

Old refrigerators should go anyway

Because new fridges are much more efficient. It may seem like a cost savings to run an old fridge, but that is only a lack of information on your part. You are not seeing the cost of the electricity for the fridge. If you put a Kill-a-Watt or similar meter, you would get a baseline. If you then bought a brand new fridge and did the same test, you would find the new fridge is saving you $10/month or more. Since this test is not practical for you, well, you never find out!

The upshot is a new, efficient fridge will pay for itself in a few years with the power savings, and then the savings are pure profit.

Some electricity companies are either subsidizing or simply giving away new fridges to customers with obsolete fridges. Replacing 300MW of fridges with 100MW of fridges is cheaper than building a 200MW power plant. The cost is covered by ratepayers on a per-kilowatt-hour basis, especially the ones who are still running their inefficient fridge.

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  • I never meant to use isolation transformer together with the GFCI. Just isolation transformer only. My refrigerator is latest just 10 years old. I never ground it. I just want to use isolation transformer so in case the hot line inside touches the metal enclosure, no one would get a shock. Isn't it the purpose of isolation transformer is when you touch a live, you won't get a shock even if your bare feet touches the ground. – Jtl Dec 5 '18 at 2:34
  • Your reason to ground the fridge is to assure the fridge chassis and the sink, light switch cover screws and other grounded things remain at the same voltage. But why do they have to be in the same voltage? My ref of 10 years was not grounded and no problem with it. Only concerned what would happen if the hot shorted the case, this is why I want to connected it to isolation transformer. My 50 year old house has no grounding and we have to jack hammer the compound driveway and all the neighbors won't agree to it. – Jtl Dec 6 '18 at 2:07
  • @Jtl that sounds awfully defeatist to me as there's more than one way to get a ground, but whatever. Certainly you are better off with an isolation transformer than nothing at all, and that GFCI (whole house?) is a wise thing. I would prefer to see something more like 8ma GFCI's on individual circuits, as those will be much better for life safety. – Harper - Reinstate Monica Dec 6 '18 at 3:48
  • I'd be using a toroid isolation transformer. So in case the live wire inside the ref touches the enclosure. No one would be shocked as it's an isolation transformer, isn't it? They use these IT in the work bench to prevent shock precisely because it's isolated from ground. – Jtl Dec 6 '18 at 3:56
  • @Jtl as long as nothing goes wrong with the isolation transformer, no. It would take a double failure, except that the whole reason we're here is that the fridge already has a ground fault, so the iso trans is a single point of failure. Either a GFCI or a ground wire would give us a second layer of protection. – Harper - Reinstate Monica Dec 6 '18 at 5:18

If you plug the isolation transformer primary / line side in the GFCI, and the refrigerator in the isolation transformer secondary / load side, an imbalance in current between line and neutral on the refrigerator will not trip the GFCI.

The EGCs of the line and load side of the isolation transformer (ground prong) and the isolation transformer's metal chassis are all bonded.

So this arrangement should prevent the nuisance GFCI trips but will also defeat the safety aspect.

As long as the transformer is sized for the load it will be fine, but the refrigerator's motor may draw an inrush current in excess of what the transformer can handle. You'd want an isolation transformer that can handle the brief inrush and rated to handle the refrigerator's running load. It's usually best to size this kind of thing with some breathing room. An isolation transformer rated for 2A or 240W at 120V would be a little more breathing room.

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  • for a 1.2A modern refrigerator.. what is usually the inrush current during startup? I'd be using toroid isolation transformer. What would happen if the inrush current is more than the toroid ampere capacity, can it damage the wires for just brief inrush current? – Jtl Dec 6 '18 at 3:53

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