Last month, we discussed the meaning and purpose of grounding and bonding requirements in NFPA 70, National Electrical Code. This article looks at the requirements for when the NEC requires the system to be grounded and what that means.

To find the requirement for which systems need to be grounded, we need to investigate Part II of Article 250. This is where we find sections 250.20 and 250.21. These two sections cover what systems are and are not required to be grounded. Notice that we have dropped the “bonded” term now as all systems are required to have the bonding component to create effective ground-fault current paths, and the bonding requirement was already discussed in Section 250.4 for grounded and ungrounded systems.

So, this means sections 250.20 and 250.21 specifically deal with the requirement to connect the electrical system to the earth, which involves making a physical connection with one of the system conductors. We will discuss which conductor this is going to be in a little bit. First, let’s look at the system characteristics that warrant making this connection. Section 250.20 is broken up into three different groupings, with specific system parameters that dictate the requirement to ground the system.

Less than 50V

The first group is for AC systems operating at less than 50V. This is primarily going to apply to control systems and other small electrical systems fed through smaller transformers such as control transformers. These systems must be grounded if the system exhibits any of the following characteristics:

• Supplied by transformers if the transformer supply system exceeds 150V to ground
• Supplied by transformers if the transformer supply system is ungrounded
• Installed outside as overhead conductors

Some examples of systems under 50V that would be required to be grounded include control transformers that transform 480V into 24V or a transformer that turns an ungrounded system at 120V into the 24V control voltage. Part of the reason we ground an electrical system is to limit the effect of lightning on the system, and thus, systems operating at under 50V installed outside as overhead conductors also must be grounded.

50V to 1,000V

The next grouping is for systems between 50V and 1,000V. This section is going to include far more systems than those under 50V. These systems must be grounded if:

• The system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150V.
• It is a three-phase, 4-wire, wye-connected system in which the neutral conductor is used as a circuit conductor.
• It is a three-phase, 4-wire, delta-connected system in which the midpoint of one phase winding is used as a circuit conductor.

These requirements focus on a couple of key concepts. The first is whether the system can be grounded, so the voltage-to-ground can be stabilized and limited to less than 150V across all other system conductors. This covers systems such as the common 240V single-phase system in most residential applications. That system can be grounded at the midpoint of the winding to ensure that the maximum voltage to ground between either leg of the system is 120V.

This would also apply to a 208V wye-connected system, as the common point of this system can be grounded to limit the phase-to-ground voltage to 120V. 

However, if this 208V wye system is going to be supplying line-to-neutral loads, we also have the second bullet point to discuss, as this is a prime example of the three-phase, 4-wire, wye-connected system that uses a neutral conductor. This also covers the 480/277V and 600/347V wye-connected systems.

The final bullet describes systems that are connected in a delta configuration yet still want to use a line-to-neutral option. The only common system that meets these parameters is the 120/240V, three-phase delta system.

Over 1,000V

For systems over 1,000V, Section 250.20 sends us to Section 250.188 for mobile and portable equipment; otherwise, it simply says that systems above 1,000V are permitted to be grounded. Systems above 1,000V are much more complicated. They require the oversight of engineering studies and must meet certain design requirements for the designing engineer to make this decision. Also, systems above 1,000V are not typically installed in areas where they will be supplying utilization equipment accessible to people other than those qualified to work around that voltage level, so the rules are different and beyond the scope of someone just learning the ropes.

Permitted, but not required

Next, we must also have a brief discussion involving Section 250.21, which covers the systems between 50V and 1,000V that are permitted, but not required, to be grounded. One key statement made in this section is that if a system is not specifically required to be grounded by 250.20, then it is by default not required, but can be if the design calls for it. A prime example of this is 480V, three-phase systems that supply zero line-to-neutral loads. These systems cannot be grounded to limit the voltage-to-ground to less than 150V, nor do they use a neutral as a circuit conductor.

This system could be left ungrounded if the installation called for it to be that way. One reason might be a desire to limit the fault current during ground-fault conditions due to a continuous industrial process where an unexpected shutdown could create a danger to the safety of those in the facility or to the public. 

However, 250.21 also requires the use of ground-fault detectors on these systems because if there is a ground-fault, it essentially turns the system into a grounded system and can cause the level of fault currents we are trying to avoid upon a second ground-fault. 

So, we need the detectors to inform us that an undesired condition has occurred so we can plan the orderly shutdown to repair it before it causes the condition we want to avoid. Section 250.21 also spells out three specific applications that are permitted, but not required, to be grounded due to the nature of the installation.

Finally, we need to discuss what conductor is the one that gets the special treatment and must be grounded. For this we can look in Section 250.26. This section states that if a system is to be grounded, whether it is required or not, there are specific conductors that get grounded. If the system uses a neutral conductor, that will be the grounded conductor. If the system does not use one, one of the legs of the system gets grounded.

Keep in mind that grounding of electrical systems serves so many critical functions, and it is imperative to understand when we need to apply these requirements. Understanding the “when” is just as important as understanding the “why” behind these critical safety rules. 

Next month’s article will look at what it takes to ground a typical AC service supplied premises wiring system and dive deep into the requirements of 250.24. Until then, stay safe and remember to always test before you touch.