Among all the generator’s specs, the type of neutral bonding could be one of the most enigmatic for a person who only has little understanding of electrical engineering.
You may have come across the terms “floating” and “bonded” neutral. While they sound obscure to many people, they are an important factor when hooking up your portable generator to a transfer switch.
But, before we go into the specifics of a bonded and floating neutral, we must first explain what a “neutral” is, and how it is different from a “live/hot wire” and a “ground”.
Difference Between Live/Hot, Neutral and Ground
An electrical circuit consists of two wires. One carries the current from a source to a load, and the other returns this current back to the source. In any electrical circuit, this loop must be closed for the current to flow.
A wire that carries a current from a source to a load is called a “live/hot wire” – a current carrying wire.
A wire that returns the current back to the source is called a “neutral wire”, or simply neutral – current carrying wire again.
Contrary to these two, a “ground wire”, or just ground, is not a part of the electrical loop and only carries current in a fault condition, quite literally into the ground (which is why it is often also called earth). The ground wire is basically a protective wire, whose purpose is to ensure the safety of the system.
Bonded Neutral Generator
In a bonded neutral generator, the neutral is bonded to the generator’s frame. The transfer switch, must in this case transfer the neutral to meet the safety standards according to NEC (National Electrical Code) article 250.
The safety standards of the NEC article 250 require that the neutral must be grounded at the first means of disconnect (which means the nearest possible point), and cannot be grounded twice, to avoid induced transient voltages or currents.To elaborate, let’s take an example of a residential home circuit.
We are interested in connecting a generator to a home circuit. First, we must learn how a residential electricity system works.
In residential homes, all load is powered by a utility (an electricity distribution company). The power cables from the utility are connected to the electrical panel installed in homes (diagram below). The electrical panel distributes this power to each circuit within a home.
The electrical panel (also called distribution board) is a system that distributes current to different circuits within a home or a building. It contains separate fuses or circuit breakers for each circuit, often enclosed in a metal enclosure. All the neutral points are grounded at a single common point.
To shift the load from the home electrical panel to a generator, a transfer switch is used. The transfer switch always transfers (disconnects from the home electrical panel and connects to the generator’s terminal) the live/hot wire. So, only the neutral wire stays in question.
As per NEC standards, we cannot ground the neutral twice. Since the generator already contains a neutral bonded to the ground, the transfer switch must transfer the neutral from the home electrical panel to the generator’s neutral terminal (diagram below).
We can observe that the load is completely isolated from the home electrical panel and is mounted directly onto the generator. Thus, a bonded neutral generator behaves as an isolated or a stand-alone unit.
In a floating neutral generator, the neutral is not bonded to the generator’s frame. Therefore, the ground must be provided by the home panel. The transfer switch, in this case, does not transfer the neutral to comply with the NEC standards.
The load is transferred from the home electrical panel to the floating neutral generator in accordance to NEC article 250. As per the standard, neutral must be grounded once, and not more than once. However, the generator does not have a grounded neutral.
Therefore, the transfer switch does not transfer the neutral. It only connects the load to the generator’s neutral. The neutral from the generator is now connected to the load and is grounded via the home electrical panel (diagram below).
In a floating neutral generator, since the neutral is not bonded to the generator’s frame, both wires are normally current carrying wires. Therefore, both slots of the receptacles on the generator are considered live/hot receptacles.
If there is a short circuit between any slot of the receptacles and the generator’s frame, the return path, i.e. neutral wire, is not bonded to the frame. There will be no return path for the current to flow through. Therefore, all the short circuit current will flow through the metal frame into the ground and thus, providing safety from the short circuit current.
Floating neutral generators are used for systems which already have a grounded neutral, e.g. home electrical panels, recreational vehicles etc. The generator is connected by means of a transfer switch that does not transfer the neutral.
There are no standards for a specific type of generator to have a bonded or a floating neutral.
Summary of Differences Between Bonded and Floating Neutral Generators
|Bonded Neutral Generators||Floating Neutral Generators|
|Neutral is bonded to the frame.||Neutral is not bonded to the frame.|
|Transfer switch transfers the neutral.||Transfer switch does not transfer the neutral.|
|Protects from transient voltages.||Protects from short circuit.|
|Mostly used in stand-by and open frame generators.||Widely spread in portable inverter generators.|
|Can be used both as an isolated unit and for systems with a grounded neutral.||Used for systems with a grounded neutral.|
How to check for Floating Neutral or Bonded Neutral?
As a standard practice, the type of neutral, bonded or floating, is usually clearly mentioned near the portable generator’s outlets.
Another simple way to check for the type of neutral bonding is to consult the generator’s manual or visit its manufacturer’s website. You can also call the manufacturer and inquire directly.
The type of neutral bonding can also be detected using a continuity tester. For this, first, turn the unit off. Now insert one lead of a continuity tester in the neutral slot of any AC receptacle (you can consult the user manual to find it) and attach the other lead to its metal frame. Continuity means that the neutral is bonded, and in case of discontinuity, it is floating.
The electromechanics behind bonded neutral generators are in a stark contrast to floating neutral generators. Hence, their respective applications may differ.
Therefore, we must first carefully determine the type of generator that we need before hooking it up. Any misjudgment can be very dangerous.
You should never try to modify a generator on your own. It is not only dangerous but will also void the manufacturer’s warranty.