Quick Hits

🗓️ MAY 23, 2026
⚡ GROUNDING & BONDING — EP 13

The “Bottleneck” Trap: Sizing Your Supply-Side Bonding Jumper  ⚠️
joshthesparky4 · Josh The Sparky

⚡ OPENING HOOK

You spent all day pulling 500 kcmil service conductors, and now you’re reaching for a #6 copper scrap to bond the cabinet?

Stop right there.

If you treat bonding as a “one size fits all” afterthought, you’re creating a bottleneck that can fail under fault conditions. On the supply side, the rules change—and your tape measure won’t save you.

🧠 CORE IDEA

⚡ The Supply-Side Bonding Jumper (SSBJ) is not just a ground wire.

Because it’s located on the supply side of the service overcurrent protective device (OCPD), it is unprotected by any breaker or fuse. If a fault occurs, this conductor must carry extremely high current until the utility transformer protection operates.

📏 SIZING RULE — NEC 250.102(C)(1)

You do not size this based on the breaker (there isn’t one on this side).

You size it based on the largest ungrounded service conductor(s).

⚡ COPPER CONDUCTOR SIZING GUIDE

• Up to 2 AWG → 8 AWG Copper

• 1 AWG to 2/0 AWG → 6 AWG Copper

• 3/0 AWG to 250 kcmil → 4 AWG Copper

• Over 1100 kcmil → 12.5% of the area of the largest conductor

⚡ THE 12.5% RULE (PARALLELED SERVICES)

If conductors are paralleled or extremely large (e.g., 4000A service):

⚡ You must calculate 12.5% of the total cross-sectional area of the largest phase conductor.

Example:
If you have 2000 kcmil per phase → SSBJ must be ≥ 12.5% of that total area.

⚠️ BIG MISCONCEPTION

“I’ll just use the same size as my Grounding Electrode Conductor (GEC).”

Wrong.

The GEC (to rods/Ufer/etc) is limited by NEC 250.66 (often capped at 3/0 copper).

But the Supply-Side Bonding Jumper can be significantly larger in high-amperage services.

These are not interchangeable.

🔥 WHY IT MATTERS

⚡ Massive Fault Current Exposure
There is no upstream OCPD on the supply side. The conductor must survive until utility protection operates.

⚡ Path of Least Resistance Control
If undersized, fault current may divert through conduit, building steel, or unintended paths.

⚡ Fire Risk Under Fault Conditions
An undersized bonding jumper becomes a thermal weak point in a high-energy fault.

📌 CORE TAKEAWAY

⚡ Bonding is the bridge—if it’s too small, the fault current finds another path.
⚡ Always verify NEC 250.102(C)(1) before sizing.
⚡ On the supply side, conductor size is driven by the source capacity, not the load.

⚡ FINAL LINE

A service is only as strong as its weakest link.

Don’t let your bonding jumper become the fuse.

📱 SOCIALS

Josh The Sparky: https://joshthesparky.com
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⚡ Educational content based on NEC 250 concepts. Reference materials based on Mike Holt Enterprises resources.
#NEC250 #Electrician #ServiceEntrance #BondingJumper #ElectricalCode #SparkyLife