jar546
CBO
Some inspectors and contractors still believe that adding a ground rod at a machine or piece of equipment is a good idea. Maybe it’s a habit from the old days, maybe it just feels like extra grounding equals extra safety. But in reality, sticking a ground rod into the dirt near a CNC machine, large motor, or even a pool pump, and calling it “extra protection” is not only technically wrong, it can actually make the situation more dangerous.
Let’s start with what the code actually says. NEC 250.54 addresses auxiliary grounding electrodes, formerly known as supplemental grounding electrodes. These are not required by code. You are allowed to install one, but it’s entirely optional. And more importantly, if you do install one, it is not required to comply with the grounding electrode system rules under 250.50 or the resistance requirement under 250.53(A)(2) Exception. It’s not required to be part of the main grounding system and is not counted as one of the required grounding electrodes.
Now contrast that with NEC 250.53(A(2), which requires a supplemental electrode only when a single rod, pipe, or plate electrode fails to achieve a resistance of 25 ohms or less to earth. In that case, a second rod is required and must be at least six feet away. That’s not what we’re talking about here. We’re not talking about meeting the required grounding electrode resistance threshold. We’re talking about people adding extra ground rods next to equipment or machines, thinking it helps, when it doesn’t. In fact, it hurts.
These auxiliary rods do not improve grounding. They introduce problems. In sensitive equipment, like CNC machines or modern VFD-driven motors, they can create a voltage differential between the equipment ground and the rest of the facility’s grounding system. That voltage differential becomes a path for noise, surges, and circulating current. You’ve just created a ground loop.
Ground loops wreak havoc. They corrupt control signals, introduce transient voltages, and damage sensitive electronics. CNC manufacturers have documented this, and many have issued guidance specifically warning not to install ground rods at the machine. But the problem persists. Whether from habit, misunderstanding, or a false sense of security, contractors still install them, and some inspectors still demand them.
This isn’t just limited to manufacturing floors. The same bad practice happens around pools. You’ll sometimes see a ground rod driven near the pool pump or near the perimeter surface while the equipotential bonding grid is being installed. That’s a big mistake. The pool bonding grid is not a grounding electrode. It’s part of an equipotential bonding system designed to eliminate voltage gradients that could injure or kill someone. The system is meant to equalize, not ground. Introducing a separate ground rod without bonding it properly back to the system can compromise that design and make things worse, not better.
And just to be clear, the NEC doesn’t use the earth as a return path for fault current. NEC 250.4 is crystal clear about this. Grounding is not for clearing faults. That’s the job of the equipment grounding conductor. The earth has high impedance and does not reliably trip overcurrent protection. That means these auxiliary rods are not enhancing safety. They’re bypassing it.
If a piece of equipment needs grounding, it needs to be connected to the equipment grounding conductor. That EGC must be bonded to the building’s grounding electrode system, which includes any required electrodes such as ufer grounds, ground rods, metal water pipes, or building steel. Once that’s done correctly, you’re done. There’s no need and no benefit to jamming another ground rod into the earth unless you’re specifically trying to meet the 25-ohm rule for a single rod electrode under 250.53(A)(2), and that only applies when using rod, pipe, or plate electrodes as part of the required grounding electrode system.
Let’s stop pretending that more is better when it comes to grounding. It’s not. More ground rods don’t mean more safety. They mean more chances for interference, damage, and liability. Inspectors need to stop asking for them unless they are required by code, and contractors need to stop installing them out of habit. The code is written for a reason. If you follow it, your system will be safer and your equipment will last longer.
Let’s start with what the code actually says. NEC 250.54 addresses auxiliary grounding electrodes, formerly known as supplemental grounding electrodes. These are not required by code. You are allowed to install one, but it’s entirely optional. And more importantly, if you do install one, it is not required to comply with the grounding electrode system rules under 250.50 or the resistance requirement under 250.53(A)(2) Exception. It’s not required to be part of the main grounding system and is not counted as one of the required grounding electrodes.
Now contrast that with NEC 250.53(A(2), which requires a supplemental electrode only when a single rod, pipe, or plate electrode fails to achieve a resistance of 25 ohms or less to earth. In that case, a second rod is required and must be at least six feet away. That’s not what we’re talking about here. We’re not talking about meeting the required grounding electrode resistance threshold. We’re talking about people adding extra ground rods next to equipment or machines, thinking it helps, when it doesn’t. In fact, it hurts.
These auxiliary rods do not improve grounding. They introduce problems. In sensitive equipment, like CNC machines or modern VFD-driven motors, they can create a voltage differential between the equipment ground and the rest of the facility’s grounding system. That voltage differential becomes a path for noise, surges, and circulating current. You’ve just created a ground loop.
Ground loops wreak havoc. They corrupt control signals, introduce transient voltages, and damage sensitive electronics. CNC manufacturers have documented this, and many have issued guidance specifically warning not to install ground rods at the machine. But the problem persists. Whether from habit, misunderstanding, or a false sense of security, contractors still install them, and some inspectors still demand them.
This isn’t just limited to manufacturing floors. The same bad practice happens around pools. You’ll sometimes see a ground rod driven near the pool pump or near the perimeter surface while the equipotential bonding grid is being installed. That’s a big mistake. The pool bonding grid is not a grounding electrode. It’s part of an equipotential bonding system designed to eliminate voltage gradients that could injure or kill someone. The system is meant to equalize, not ground. Introducing a separate ground rod without bonding it properly back to the system can compromise that design and make things worse, not better.
And just to be clear, the NEC doesn’t use the earth as a return path for fault current. NEC 250.4 is crystal clear about this. Grounding is not for clearing faults. That’s the job of the equipment grounding conductor. The earth has high impedance and does not reliably trip overcurrent protection. That means these auxiliary rods are not enhancing safety. They’re bypassing it.
If a piece of equipment needs grounding, it needs to be connected to the equipment grounding conductor. That EGC must be bonded to the building’s grounding electrode system, which includes any required electrodes such as ufer grounds, ground rods, metal water pipes, or building steel. Once that’s done correctly, you’re done. There’s no need and no benefit to jamming another ground rod into the earth unless you’re specifically trying to meet the 25-ohm rule for a single rod electrode under 250.53(A)(2), and that only applies when using rod, pipe, or plate electrodes as part of the required grounding electrode system.
Let’s stop pretending that more is better when it comes to grounding. It’s not. More ground rods don’t mean more safety. They mean more chances for interference, damage, and liability. Inspectors need to stop asking for them unless they are required by code, and contractors need to stop installing them out of habit. The code is written for a reason. If you follow it, your system will be safer and your equipment will last longer.
