PG&E Rule 20 and the Three-Utility Coordination Problem
Yes, you can run electric, gas, and communications in a single trench—but only if you follow PG&E Rule 20, NEC Article 300, and the specific separation requirements each utility enforces. I’ve designed dozens of these trenches in the Bay Area and seen projects delayed months because someone treated “utility coordination” as an afterthought. It’s not. Rule 20 requires written approval before you break ground, and that approval hinges on trench geometry that meets all three operators’ standards simultaneously.
Horizontal Separation: The 12-Inch Rule and Its Exceptions
NEC Table 300.5(C) sets a 12-inch minimum clearance between power and communications cables when both are in the same trench. That’s the federal baseline. But PG&E and AT&T often demand more. In our experience, PG&E’s Rule 20 permits 12 inches between their power conduit and communications conduit only when the power conduit is either rigid metal conduit (RMC) or encased in concrete. If you’re using PVC, the separation jumps to 18 inches minimum, sometimes 24 inches depending on the branch office reviewing your drawings.
AT&T and Comcast—the cable operators we work with most—typically specify 18 inches between their ducts and electric utility conduit. That’s their insurance against induction noise and physical damage during maintenance. Gas lines (PG&E or local district) must stay 12 inches from electric conduit per California Code of Regulations Title 8. We’ve never seen a gas operator demand more, but they will reject trench designs where thermal expansion or settling could compress that gap.
Vertical Stacking and Burial Depth Requirements
Most single trenches we design stack utilities vertically to save excavation cost. The standard configuration is gas on bottom (shallowest), power in the middle, and communications on top. That order matters—if you flip it, you’re asking maintenance crews to dig past communications to reach gas shutoffs, which creates liability. California requires all utilities in joint trenches to bury at least 18 inches deep in residential areas, 24 inches in commercial driveways and roadways per California General Plan standards. Gas lines in some districts demand 24 inches minimum, so we always assume 24 inches when designing for mixed-use sites.
The trench width typically runs 24 to 36 inches for three utilities. A 24-inch width gets tight—you’re relying on single conduits per utility, and there’s almost zero room for a second run later. We recommend 30 inches to give crews working room and allow one additional conduit per utility without redesign. That extra 6 inches of excavation adds roughly $15–20 per linear foot, not $100–150. The higher estimate assumes you’re widening the trench because the first design failed and you’re starting over.
Conduit Sizing and the 40% Fill Rule
NEC Article 352 caps PVC conduit fill at 40% of interior cross-section when you’re pulling three or more conductors. Most of our power runs use 2-inch PVC (interior area 3.26 square inches), which handles four 6 AWG conductors (0.26 sq. in. each) at about 32% fill—comfortable and inspectable. The mistake we see often is engineers pulling four or six circuits through the same conduit to save money on trench width, then hitting fill limits and needing conduit upsizing anyway.
Oversizing is cheap. A 2.5-inch PVC conduit (interior 4.72 sq. in.) costs maybe $2 more per foot than 2-inch but gives you 60% more capacity. If you’re designing for a site that might add circuits later, or if the property owner wants to avoid trenching a second time, that extra diameter is insurance. We typically spec 2.5-inch for mixed-use buildings and 2-inch for single-tenant industrial.
AT&T and Comcast Duct Standards: They’re Not the Same
AT&T requires duct to be Schedule 40 PVC minimum, 1.5 inches ID for their standard copper pair runs or 2 inches for fiber. Comcast specifies 1.25-inch corrugated HDPE for cable, but will accept PVC if it’s Schedule 40 or better. Both companies require an access point (handhole or service box) every 300–400 feet. We’ve had Comcast field crews reject ducts smaller than 1.25 inches because their pull winch can’t navigate tighter radii, so don’t spec 1-inch conduit thinking you’re saving money. You’ll be digging again.
Both utilities also require separate conduits for their services. You cannot run AT&T copper pair and Comcast cable in the same duct. That seems obvious, but we’ve reviewed designs from other firms showing both in one conduit with a note “utility will install separator.” Utilities won’t do that. It’s your trench, your design responsibility.
Getting Approvals at 65% Design, Not 90%
This is where we save schedules. At 65% design—meaning trench profile, conduit layout, and separation distances are locked but utility crossings and service box locations aren’t finalized—you submit a joint-trench coordination package to PG&E, AT&T, and Comcast simultaneously. Each one has a 10–15 business day review window officially, but in practice PG&E’s Rule 20 review takes 4–6 weeks because it routes through both the electric service planning group and the gas operations team. AT&T typically responds in 2–3 weeks. Comcast can take up to 8 weeks depending on the market.
If you wait until 90% design to request approvals, any feedback about separation distances or duct sizing forces redesign. We’ve seen projects held up four months because someone submitted at 90% and the utility flagged a 6-inch separation gap at a crossing point. The client then had to widen the trench and resubmit. Start the approval clock at 65%. That gives you room to absorb feedback and revise without crushing your permitting timeline.
Real Site Example: Oakland Mixed-Use Building
We designed a trench for a four-story mixed-use in downtown Oakland last year: 520 linear feet, serving electric service, gas, AT&T fiber, and Comcast cable. The standard config would’ve been 30 inches wide, 24 inches deep. PG&E Rule 20 required 18-inch separation between their power conduit (3-inch PVC, three 1/0 circuits at 28% fill) and gas duct (2-inch PVC). AT&T demanded 18 inches from their fiber conduit to electric. Comcast needed 18 inches to fiber.
We stacked it: gas 24 inches deep, power 6 inches above that, communications 6 inches higher. Horizontal separation achieved by offsetting each conduit 8 inches laterally. Total trench width 36 inches. Excavation cost $89 per linear foot, handhole installations (14 total) ran $1,200 each. PG&E Rule 20 approval took 34 days. AT&T took 18 days. Comcast took 52 days but their delay didn’t impact critical path because we’d finalized electric and gas details sooner. The site broke ground three weeks after final utility sign-off.
Ready to Coordinate Your Multi-Utility Trench?
We’ll pull your site details, coordinate with PG&E Rule 20, AT&T, and Comcast from day one, and submit at 65% design to keep your schedule on track. Contact us to start the approval process.