Let's break down NEC 250.102(C)(1) and the 12.5% rule to clarify when it applies to the grounded conductor versus the bonding jumper. The "OR" in the rule is the key to understanding the differentiation.

NEC 250.102(C)(1)

This section deals with the sizing of equipment bonding jumpers on the supply side of the service and the main bonding jumper. It also affects the sizing of the grounded conductor where run with service conductors.

Key Parts of 250.102(C)(1)



General Rule

The equipment bonding jumper on the supply side of the service and the main bonding jumper must not be smaller than prescribed by 250.28.

Exception

For services with multiple sets of service conductors or multiple sources, the main bonding jumper and supply-side bonding jumper size is determined by the largest ungrounded service conductor(s) in the service entrance.

Note 1 (This is where the 12.5% comes in)

Note 1 provides an optional method for sizing the grounded conductor and/or the bonding jumper if the ungrounded conductors exceed 1100 kcmil copper or 1750 kcmil aluminum.

Understanding the "OR" and Application of the 12.5% Rule

The "OR" in Note 1 means you can choose to apply the 12.5% rule to either the grounded conductor OR the bonding jumper (or both, if desired), when the ungrounded conductors exceed 1100 kcmil Cu or 1750 kcmil Al. It does not mean you must apply it to both. It provides flexibility based on design considerations.

When to Apply 12.5% to Grounded Conductor vs. Bonding Jumper

Here's the important decision-making process:1.

First, Determine if You Exceed 1100 kcmil Cu or 1750 kcmil Al

If the ungrounded conductors do not exceed these sizes, the 12.5% rule is not applicable. You size the grounded conductor and bonding jumpers based on the standard rules in 250.24(C)(1) and 250.28 respectively.
2.

If You Exceed 1100 kcmil or 1750 kcmil

Now the 12.5% rule becomes optional. You have the choice of applying it or not.3.

Applying the 12.5% Rule

This is where you choose whether to apply it to the grounded conductor, the bonding jumper, or both. There are several things to consider when deciding which to apply it to.

Grounded Conductor

If you choose to size the grounded conductor based on the 12.5% rule, it must be at least 12.5% of the area of the largest ungrounded service-entrance conductor.

Bonding Jumper

If you choose to size the bonding jumper based on the 12.5% rule, it must be at least 12.5% of the area of the largest ungrounded service-entrance conductor.

Why Choose One Over the Other?

There's no single "right" answer. It depends on the specific installation and engineering judgment. Practicality and Cost

Sizing the grounded conductor to 12.5% might be more practical and cost-effective in some installations, especially if it results in a standard conductor size.

Fault Current Path

Sizing the bonding jumper to 12.5% could be preferred if a lower impedance fault current path is desired, especially in systems with high available fault current.

Voltage Drop

A larger grounded conductor might help reduce voltage drop, although it is not sized for voltage drop but rather fault current carrying capability and ground fault return.

Example

Let's say you have a service with three 1200 kcmil copper conductors.1.

Exceeds Threshold

1200 kcmil > 1100 kcmil, so the 12.5% rule is optional.
2.

Choice

You can choose to: Size the grounded conductor based on 250.24(C)(1). Size the bonding jumper based on 250.28. Size the grounded conductor to at least 12.5% of 1200 kcmil (which is 150 kcmil). Size the bonding jumper to at least 12.5% of 1200 kcmil (which is 150 kcmil). Size both the grounded conductor and the bonding jumper to at least 150 kcmil.

In Summary: The 12.5% rule in Note 1 of 250.102(C)(1) is optional when ungrounded conductors exceed 1100 kcmil Cu or 1750 kcmil Al. The "OR" means you can apply the 12.5% rule to either the grounded conductor OR the bonding jumper (or both). The decision of which to apply it to depends on practical considerations, fault current requirements, and voltage drop concerns.Always consult with a qualified electrical engineer to determine the appropriate sizing of conductors and bonding jumpers for your specific installation. Flag for review