Main Panel Relocation Feeder Voltage Drop: Outdoor Meter-Main to Indoor Distribution Panel
When an outdoor service disconnect turns the old indoor main into a feeder panel, size the new feeder with real voltage-drop numbers, ampacity, and neutral-isolation checkpoints.
Main panel relocation projects fool people because the breaker size looks familiar. Once an exterior meter-main or service disconnect is added, the old indoor main is no longer acting like service equipment. It becomes a feeder-supplied distribution panel, and that means conductor sizing, bonding details, and voltage-drop performance all deserve a fresh check.
The practical question is simple: how much current will the new feeder really carry, how far is the route from the outdoor disconnect to the interior panel, and how much voltage will the house, workshop, or light commercial load lose on the way? For NEC work, useful checkpoints are NEC 230.70, NEC 250.24(A)(5), NEC 310.16, and the familiar design guidance from NEC 215.2(A)(1) and 210.19(A)(1). For IEC-style work, IEC 60364-5-52 remains the practical cable-sizing reference. NEC distribution board panel basics
Why Panel Relocation Feeders Need Their Own Voltage-Drop Review
An outdoor disconnect often adds 20 to 100 ft or more of feeder that did not exist before, so the indoor panel can become the weakest voltage point in the building.
Homeowners and DIY remodels often size the feeder from the old main rating instead of from actual demand, conductor impedance, and route length.
Once the indoor panel becomes downstream equipment, neutral isolation and grounding details change, so this is the right moment to recalculate instead of copying the old service conductors.
Electricians and engineers see the same field symptom repeatedly: the upgrade passes inspection, but long HVAC, range, EV, or workshop loads now feel softer because the new feeder consumed the voltage-drop margin.
Code and Design References Worth Marking on the Plan
- NEC 230.70: when the service disconnect is moved outdoors, the indoor panel downstream is no longer the service disconnecting means.
- NEC 250.24(A)(5): the grounded conductor is bonded at the service disconnect, not re-bonded in the interior distribution panel that is now fed as a feeder.
- NEC 310.16: conductor ampacity still comes first. The feeder must be thermally legal before voltage-drop optimization means anything.
- NEC 215.2(A)(1) and 210.19(A)(1) informational notes: designers still commonly target about 3% on a feeder or branch segment and about 5% total to the utilization equipment.
- IEC 60364-5-52: confirm installation method, grouping, ambient conditions, and allowable voltage drop before accepting the final cable size for relocated main-board layouts.
Planning Cases for Common Panel Relocation Layouts
These are planning numbers, not a substitute for the actual load calculation, utility requirements, or final code review. They are useful because panel relocation jobs often become voltage-drop limited faster than people expect.
| Scenario | Distance and load | Approx. result | Design note |
|---|---|---|---|
| 200A meter-main to indoor distribution panel | 120/240V, 160A design load, 35 ft one-way | 4/0 Al is about 3.6%; 250 kcmil Al is about 3.0% | A 200A feeder is rarely short enough to ignore. Use actual load, not only service rating. |
| 125A panel relocation during service upgrade | 120/240V, 90A design load, 70 ft one-way | 1/0 Cu is about 1.3%; 2/0 Al is about 2.0% | This is where a modest upsizing decision can avoid weak HVAC and kitchen loads later. |
| 100A remote interior board in IEC layout | 230V single-phase, 55 m one-way | 25 mm2 Cu is about 3.5%; 35 mm2 Cu is about 2.5% | IEC sizing must still include installation method, grouping, and ambient correction. |
| 225A outdoor disconnect to interior commercial panel | 208Y/120V three-phase, 140A design load, 28 m one-way | 3/0 Cu is about 1.4%; 4/0 Al is about 2.2% | Three-phase feeders usually buy margin, but long indoor routes still deserve a calculator check. |
Worked Examples with Specific Numbers
200A residential upgrade, 160A design load, 35 ft one-way
A 4/0 aluminum feeder at roughly 0.321 ohm per 1000 ft lands near 8.2V, about 3.4% at 240V when the design load is 160A. Moving to 250 kcmil aluminum drops the result closer to 7.1V, about 3.0%. If the house already has long branch circuits to HVAC or EV loads, that extra margin is worth taking seriously.
125A relocation feeder, 90A design load, 70 ft one-way
With 1/0 copper at roughly 0.122 ohm per 1000 ft, the feeder drop is about 1.5V, or 0.6% at 240V. A 2 AWG copper option climbs closer to 2.4V, about 1.0%. Both can be acceptable, but the lower-drop feeder leaves more room for kitchen small-appliance and compressor loads downstream.
100A IEC indoor board, 230V, 55 m one-way
A 25 mm2 copper feeder at about 0.727 ohm per km drops roughly 8.0V, or 3.5% at 230V. A 35 mm2 copper feeder drops about 5.8V, or 2.5%. In a board feeding heat pumps, ovens, or long final circuits, that 1% difference is very noticeable.
Field Checklist Before You Approve the Relocation Feeder
- Start with the real demand or design load, not only the label on the old or new main breaker.
- Measure the actual one-way route from the outdoor service disconnect or meter-main to the interior panel lugs.
- Confirm whether the indoor panel is now feeder-fed equipment and keep neutral isolation, grounding, and bonding details separate from the voltage-drop math.
- Check the longest downstream branch circuits before finalizing the feeder size, because feeder drop plus branch drop is what the equipment actually sees.
- If the result is already near 3% on the feeder by itself, upsize before drywall, trench backfill, or finish work make the decision expensive.
FAQ
Why does moving the main outside change the feeder calculation?
Because the old indoor main is no longer acting as service equipment. Once it becomes a feeder panel, the new conductors between the outdoor disconnect and indoor board must be checked as a real feeder for ampacity, neutral isolation, and voltage drop.
Should I size the relocation feeder from the full service rating?
Not by habit alone. Start with the applicable load calculation or design load, then review conductor ampacity and voltage drop. A 200A service upgrade does not automatically mean every new interior feeder should be judged only at the full breaker label.
Is 3% feeder drop a hard NEC rule on panel relocation jobs?
No. The common 3% feeder and 5% total guidance comes from informational notes, not a mandatory NEC performance limit. It remains a very practical design target because the branch circuits still need some of that voltage-drop budget.
What is the IEC equivalent check for this kind of feeder?
IEC 60364-5-52 is the practical reference for conductor selection, installation method, grouping, ambient correction, and allowable voltage drop. The engineering logic is the same: make the cable thermally legal first, then check delivered voltage under load.
Run the Relocation Feeder Before the Panel Gets Moved
Enter the feeder current, system voltage, phase, conductor material, candidate size, and one-way distance so you can compare conductor options before the exterior disconnect and interior panel are locked in.