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I would like to answer watterinja’s question, “I wonder if Thailand covers the installation & operation of these devices in some sort of building code?”, as well as address some of the basics of wiring an instantaneous water heater.

There are two Thai regulations that I am familiar with that cover instantaneous water heaters. The first, Thai Industrial Standard (TIS) 1693-2547, Industrial Standard for Electric Instantaneous Water Heaters: Safety Requirements, addresses the construction and characteristics of the appliance, but generally not the electrical installation requirements. The electrical installation requirements are governed by Engineering Institute of Thailand (EIT) Standard 2001-51, Standard for Electrical Wire Installation for Thailand 2545 (with Revision 1 2551). This is the Thai electrical code and it governs, among other things, all electrical wiring in the home. The points below are based on these two regulations.


Grounding Whether an appliance must be grounded (earthed) depends on its class, not on the type of appliance. Class 1 appliances must be grounded, whereas class 2 (double-insulated) appliances do not need to be. I know that many electric instantaneous water heaters are class 1 and therefore must be grounded; I do not know whether all instantaneous water heaters are class 1. Regardless, it is easy to determine the appliance class once the water heater is on hand. A class 1 appliance will have a grounding terminal to connect the grounding wire. A class 2 appliance will not have a grounding terminal, and it will be clearly marked with either the words “class 2”, “double-insulated”, or with the double-insulation symbol (a square within a square—look on a cell phone charger or battery charger to see an example of these markings). If you ever see a water heater that has no grounding terminal and is not marked as being a class 2 appliance, throw it away.

What is the proper way to ground a class 1 water heater? There are many elements required to properly ground any appliance. To start, the grounding wire (called the equipment grounding conductor in the regulation, but I’ll keep calling it the grounding wire here) must be connected to the grounding terminal on the water heater. The grounding wire must run back to the consumer unit (the box housing the circuit breaker for the water heater) along with the other two wires from the water heater (the phase and neutral wires). Inside the consumer unit, the grounding wire must be connected to the ground busbar.

What size must the grounding wire be? Depends on the ampere rating of the circuit breaker for the water heater. If the circuit breaker is 16 amperes, the grounding wire must be 1.5 mm2 or larger. For a 20-ampere circuit breaker, 2.5 mm2 or larger. For 25 and 32-ampere circuit breakers, 4 mm2 or larger. (If the wire type is VAF-GRD, which is a flat multi-core cable with a white sheath, then you don’t get to choose the grounding wire size independently of the phase and neutral wire sizes. For this wire type, you size the phase and neutral wires first and accept the size of the grounding wire in that cable.)

Now we have the water heater grounding wire properly connected to the ground busbar in the consumer unit. But this does not necessarily properly ground the water heater. For the water heater to be properly grounded (and in fact for anything in the house to be properly grounded), the consumer unit itself must be properly grounded. So how must the consumer unit be grounded? I will give a short overview below for a single-phase consumer unit. Don’t try to wire according to this description—it’s not detailed enough and there are exceptions (for example if the consumer unit housing the water heater circuit breaker is a subpanel, then proper grounding is more complicated than described below). With the following information, I’m only trying to convey that simply connecting an appliance grounding wire to something (a ground busbar, a ground rod) does not properly ground the appliance.

To ground the consumer unit, the ground busbar must be connected to a ground rod (other grounding electrodes are allowed by regulation, but they’re almost never used in domestic installations). Regulation requires that the ground rod be at least 2.4 meters long, 5/8-inches in diameter (which is about 15 mm), and made of solid copper or copper-clad steel (electroplated, not a copper sheath pressed onto a steel rod). The size of the wire that connects the ground rod to the ground busbar in the consumer unit depends on the size of the service conductors (the mains), but for almost all domestic installations it must be not smaller than 10 mm2.

The last element for a properly-grounded appliance is this: the neutral service conductor (the neutral wire running from the meter to the consumer unit) must be grounded at the consumer unit. How? The neutral service conductor must connect to the ground busbar in the consumer unit. Then, a short wire the same size as the neutral service conductor must be run from the ground busbar to the neutral terminal of the main breaker. This is called the main bonding jumper in Thai- and USA-regulation parlance, and you can look it up on the internet to read more about its importance in grounding.

All of the above are necessary, and required by regulation, for proper grounding. In Thailand, most installations do not have an adequate ground rod and the neutral service conductor is not usually grounded at the consumer unit. These two failings render the grounding system for the house ineffective and the electrical system can be significantly more dangerous as a result. Note that grounding a water heater (or any appliance) by connecting it to its own ground rod, as is sometimes seen in Thailand, is dangerous and prohibited by regulation. All grounding wires for appliances, luminaires, and receptacle outlets must run back to the consumer unit and connect to the ground busbar.

Earth Leakage Breaker Instantaneous water heaters have a built-in earth leakage breaker (ELB), also referred to on some units as an earth leakage circuit breaker (ELCB) or residual current device (RCD). I’ll use the term RCD because that’s the term used in the Thai electrical code. The built-in RCD was required by the 2545 version of the electrical code. Revision 1 to the code, which is now in force, has a more a stringent requirement. It is no longer enough to have an RCD built into the water heater—now, the entire branch circuit must be protected by an RCD. This requirement pertains to whirlpool tubs also, incidentally. There are a few ways to do this. First, the entire consumer unit can be protected by an RCD either with an external RCD, an RCD built into the consumer unit, or an RCD main breaker. Second, an RCD circuit breaker can be installed for the water heater branch circuit. Third, an RCD can be installed in the water heater branch circuit wires outside the consumer unit.

By regulation, the RCD must have a trip current not greater than 30 milliamperes (mA). All RCDs marketed for domestic use in Thailand meet this requirement. However, better to use an RCD with a lower trip current if one is available. RCDs with lower trip currents are available from some manufacturers. Square D markets RCD circuit breakers with a 10 mA trip current, and Safe-T-Cut has RCD-protected consumer units with trip currents adjustable down to 5 mA, for example.

Wire Size One widespread problem with wiring in Thailand is undersize wires. What size wires (the phase and neutral wires—the grounding wire was covered above) are required by regulation for a water heater? The required wire size depends on the ampere rating of the circuit breaker for the water heater. It also depends on the type of wire used and the wiring method, which complicates things. I will give the required wire sizes for one common wire type and wiring method—wire type VAF-GRD run on the surface of the wall or embedded in the wall plaster. For a circuit breaker of 20 amperes or less, the wire must be 2.5 mm2 or larger. For a 25-ampere circuit breaker, 4 mm2 or larger. And for a 32-ampere circuit breaker, 6 mm2 or larger.

Safety Breaker It is customary in Thailand to install a switch for the water heater outside the bathroom. The switch is usually a so-called safety breaker. This switch is not required by regulation.

Recommendations for Fixing an Installation In a house with significant wiring deficiencies, what can be done to make instantaneous water heaters more safe? The easiest way to reduce the electrical shock hazard from a water heater is to install an RCD to protect the water heater branch circuit. If there is currently a safety breaker installed outside the bathroom for the water heater, it will be easiest and cheapest just to swap it out for an RCD safety breaker. Haco markets RCD safety breakers under the name Earth Leakage Breakers. Use a 16-ampere circuit breaker for a 3500-watt water heater, a 25-ampere breaker for a 4500-watt heater, and a 32-ampere breaker for a 6000-watt heater. Alternatively, if you have a Square D consumer unit, you can replace the circuit breaker for the water heater with a Square D RCD circuit breaker with a 10 mA trip current. Use the circuit breaker ampere ratings given above for the different sizes of water heaters, except that for a 4500-watt heater you may use a 20-ampere breaker (as of last year, Square D did not market a 25-ampere RCD circuit breaker). Note that replacing a circuit breaker involves opening the consumer unit and working close to live wires. The service conductors (mains) remain live even when the main breaker is switched off. Don’t attempt to replace a circuit breaker unless you understand the hazards and how to avoid them, as well as having the equipment you need to be safe. And the final alternative for providing RCD protection for the water heater branch circuit is to install an RCD main breaker (they all have 30 mA trip currents). This is the most expensive option and the most trouble, as it entails working with live wires. Replacing the main breaker should only be done by an electrician.

Test all RCDs monthly. RCDs can go bad or be damaged by electrical surges from, for example, nearby lightning strikes.

Repairing the grounding system can be difficult or almost impossible (for one who lives in an apartment building, for example). It requires some expertise to assess the current state of the grounding system and to effect any repairs. But also doing these things can be dangerous because it requires opening and working within the consumer unit. It’s frankly beyond the scope of this post. Anyone who is serious about installing a proper grounding system probably needs to hire an electrician or engineer who knows what they’re doing to help. Not sure how to find a knowledgeable electrician or engineer in Thailand. And on that discouraging note, I’ll end this post.