Dim Lights Traced To POCO Transformer Not House Cross-System Decision Tree

Why this matters

A house with chronically dim lights, slow-heating appliances, and electronics that misbehave is often assumed to have an internal wiring problem. In a meaningful share of cases the cause is upstream of the meter: a loaded or failing utility (POCO) transformer, an undersized or aged service drop, or a primary-side voltage issue. No amount of panel work fixes a utility-side low-voltage condition. The electrician's job is to prove the boundary, demonstrate whether the low voltage exists at the service point or only inside the house, and hand the utility a documented case when the fault is theirs. This avoids re-pulling feeders that were never the problem.

Symptom presentation

• Lights are dim across the whole house, not on one circuit, and dim further when large loads run.
• Voltage measures low at every outlet, often well below 114 V line to neutral.
• Symptoms worsen at peak times (evenings, hot afternoons) when the neighborhood draws more.
• Motors run hot and slow; electronics reset or brown out.
• The neighbor reports the same symptoms, pointing to a shared transformer.

Quick checks

• Measure line-to-line and line-to-neutral voltage at the main panel with the service under typical load; compare to the acceptable range (roughly 114 to 126 V on a 120 V nominal leg per ANSI C84.1).
• Capture voltage with a min/max meter while large loads cycle to see the sag depth.
• Read voltage at the meter base/service point versus inside the panel to locate the drop.
• Ask whether neighbors on the same transformer have the same complaint; a shared sag is a strong utility-side indicator.

Isolation tree

Step 1. Measure voltage at the service point (line side of the meter) under load.

• Low at the service point already (below the ANSI low limit): the deficiency arrives from the utility. Go to Step 2.
• Normal at the service point but low inside: the loss is in the service entrance or the house. Go to Step 4.

Step 2. Compare both legs to neutral at the service point and watch the neutral.

• Both legs low and roughly balanced, sagging together under neighborhood load: an overloaded or undersized transformer or a long, undersized service drop. Document and refer to the utility.
• One leg high and the other low, with neutral-to-ground voltage climbing under load: a failing service neutral on the utility side. This is a hazard; refer to the utility urgently.

Step 3. Confirm the pattern across time.

• Sag tracks neighborhood demand (worse at peak): transformer loading or primary-side regulation. Provide the utility your logged min/max readings and timestamps.
• Sag is constant regardless of demand: a degraded service-drop conductor or a high-resistance utility connection. Refer to the utility to inspect the drop and connectors.

Step 4. House-side path: meter voltage at the meter base, the main lugs, and a far outlet under load.

• Voltage drops across the main lugs or the meter base: a loose or corroded service-entrance termination. De-energize and retorque or replace the connection.
• Voltage good at the panel but low at a far outlet: undersized branch conductors or a long run with excessive voltage drop, or a loose neutral on that circuit. Correct conductor sizing or re-terminate.
• Voltage low everywhere inside but fine at the service point: a high-resistance main neutral or main lug inside the panel. Inspect and retorque the main connections.

Confirming diagnosis

A confirmed utility-side cause shows low voltage already present at the service point under load, balanced legs sagging with neighborhood demand, and neighbors with matching symptoms. A confirmed house-side cause shows normal voltage at the service point and a measurable drop across an internal termination or an undersized run. The boundary test, service-point voltage versus inside voltage, is the deciding evidence. Document timestamped min/max readings at both points; that record is what moves a utility to act.

Reading the leg balance

The relationship between the two legs is a fast discriminator. On a healthy split-phase service both legs sit within a couple of volts of each other to neutral, and both ride up and down together as load changes. A transformer or drop that is simply overloaded pulls both legs down roughly equally, keeping them balanced. A failing neutral, by contrast, lets the legs diverge: as load shifts to one leg, that leg sags while the opposite leg climbs above nominal, because the missing neutral forces the two halves of the 240 V into a series divider across the loads. Watching the legs while a large 120 V load cycles, one going up as the other goes down, is the single clearest sign of an open neutral and separates it from a benign loaded-transformer sag where both move together.

Remediation

• Utility transformer overload, undersized drop, or primary regulation: refer to the utility with documented readings; the fix is a larger transformer, a new drop, or regulator adjustment on their side.
• Failing utility service neutral: refer to the utility urgently as a hazard.
• Loose or corroded service-entrance termination (house side of the meter): de-energize, inspect, and retorque or replace.
• Undersized branch run or internal voltage drop: upsize conductors or correct the run; re-terminate loose neutrals.

References

• ANSI C84.1, Electric Power Systems and Equipment, Voltage Ratings (Range A and B service voltage limits).
• NFPA 70 (NEC) Article 230, Services (service-entrance conductors and connections).
• NFPA 70 (NEC) Article 250.24, Grounding Service-Supplied AC Systems (service neutral integrity).
• NFPA 70 (NEC) Article 215, Feeders (voltage-drop guidance).
• NFPA 70 (NEC) Article 110.14, Electrical Connections (termination integrity).