Feeder & Service Load Troubleshooting

Feeder / Service Load Problems Quick Answer

Use this when calculated feeder amps force an unwanted upsize, the main trips despite a green sheet, demand factors were applied to the wrong loads, or an addition finally made an old service look undersized.

1. Calculated Feeder / Service Load Comes In Higher Than Expected

What it looks like: The takeoff “should have been” a 100 A or 125 A feeder. After continuous and demand steps, the calculator pushes you to 150 A or 200 A and an unwanted conductor upsize.

Likely causes:

• Continuous 125% applied twice — Demand VA was already conservative, then every line got another 1.25. Continuous adjustment belongs on the continuous portion after diversification, not as a second multiplier on everything.
• No demand where demand is allowed — Treating all lighting and receptacle VA at 100% when Table 220.42 / 220.44 apply for that feeder.
• Wrong voltage / phase math — Dividing VA by 240 on a 208 V three-phase feeder, or forgetting √3 on three-phase, inflates amps.
• Largest motor 125% stacked incorrectly — Adding 125% of every motor instead of 125% of the largest plus 100% of the rest.

Fixes: Rebuild the sheet in order: connected loads → allowed demand factors → motor contribution → continuous ampacity (215.2) → amps at system voltage. Re-enter the cleaned list in the feeder calculator and compare to the demand chart.

2. Main or Feeder Breaker Trips Despite a Calc That “Passes”

What it looks like: Permit sheet or calculator says spare capacity. After move-in or a shop addition, the main or feeder breaker trips when dryer, range, HVAC, and shop tools stack.

Likely causes:

• Diversified paper vs real coincidence — Article 220 allows demand factors. Real peaks can still hit hard for short periods, especially with EVSE, welders, or compressors.
• Continuous equipment undercounted — Signage, heaters, Level 2 EVSE, or process loads run for hours and heat the breaker even when average demand looked fine.
• Loose lugs / high resistance — Heat and nuisance trips that look like “overload” but are connection problems. Clamp amps first; then inspect terminations.
• Wrong OCPD vs conductor pairing — Conductor ampacity passed, but the breaker was rounded in a way that does not protect the real continuous load.

Fixes: Clamp the feeder during the trip condition. If measured amps exceed the sheet by a wide margin, fix the inventory. If measured amps are below rating and the enclosure is hot, chase terminations and continuous classification before upsizing copper again.

3. Demand Factors Applied Incorrectly

What it looks like: Lighting got 35% when it should stay at 100% for the first slice, receptacles got demand that belongs to lighting, or EVSE / motors / HVAC were diluted with general receptacle factors.

Likely causes:

• Continuous vs noncontinuous confusion — Mixing “demand factor” with “already continuous-safe.” Demand diversifies calculated load; 215.2 still sizes continuous portions at 125%.
• Motor load at 125% on every motor — Only the largest motor contribution typically needs the 125% adder under 220.50 / motor feeder rules; the rest are not all multiplied the same way.
• HVAC vs lighting demand — Applying lighting table percentages to space heating, or summing heat and A/C when noncoincident rules allow the larger only.
• Wrong occupancy path — Using dwelling optional 100%/40% tiers on a feeder that should follow standard feeder takeoff, or the reverse. That is a method fork — settle it on the residential or commercial troubleshooting pages if the building type is wrong.

Fixes: Tag each line as lighting, receptacle, fixed appliance, motor, HVAC, EVSE, or continuous process. Apply only matching Article 220 factors, then finish with 215.2 continuous math. Use the chart for the common tables and the calculator for the arithmetic.

4. Double-Counting or Omitting Loads on Additions / Renovations

What it looks like: The addition calc double-counts the existing range and dryer “for safety,” or it forgets EVSE, heat pump, shop welder, and laundry that already live on the service. Reviewer and field numbers disagree by a full panel size step.

Likely causes:

• Copy-paste inventory — New sheet includes existing large appliances twice.
• Omitted future / new continuous loads — Level 2 EVSE, hot tub, ADU feeder, or shop compressor never entered the takeoff.
• Nameplate vs “typical” watts — Catalog guesses instead of appliance labels.
• Panel schedule not updated — Feeder calc used an old directory; breaker finder would have shown the missing circuits.

Fixes: Walk the existing panel with a circuit finder, build a nameplate list, then add only the new loads once. Re-run the feeder calculator with the honest combined inventory.

5. Existing Service Looks Too Small After the Addition Calc

What it looks like: The house ran fine for years on 100 A or 150 A. The garage / ADU / EV / shop addition calc shows the service is out of spare capacity.

Likely causes: The old service was never oversized for modern continuous loads. The addition is honest about EVSE and shop motors. Or the addition sheet forgot that the main still carries the dwelling load — you cannot size the garage feeder in isolation and ignore service headroom.

Fixes: Re-run the whole-service demand with existing + new loads. Then decide whether the answer is a service upgrade, a carefully sized subpanel feeder that still fits spare capacity, or load management where listed and allowed. That fork is covered on the decision page — not by inventing friendlier demand factors.

Decision: Fix the Sheet, Upsize the Service, or Add a Subpanel?

If the inventory and continuous math are wrong, fix the sheet first. If the sheet is honest and spare capacity is gone, choose between growing the service and adding a subpanel feeder that the existing service can still support. Do not treat those as the same project.

Frequently Asked Questions

Is this the same as residential or commercial load troubleshooting?

No. Those pages diagnose dwelling optional-method and Part III occupancy sheets. This page diagnoses feeder/service takeoffs: continuous 125%, demand on the wrong load types, measured trips, and addition inventory errors for the feeder calculator.

Do demand factors replace the continuous-load rule?

No. Demand factors diversify calculated load. NEC 215.2 still requires feeder ampacity of noncontinuous + 125% of continuous unless the assembly is listed for 100% continuous use.

Why did my calc jump a whole breaker size?

Usually continuous classification, motor 125% handling, wrong phase/voltage conversion, or omitted large loads. Rebuild the order of operations before changing wire size.

Should I upsize every time the main trips?

Not first. Clamp the feeder, verify inventory and terminations, then decide whether copper, connections, or capacity is the real problem.