8.1 NETA ATS Table 100.18 and Repair-Priority Conventions

Severity classification turns a quantitative temperature measurement into a maintenance decision. For electrical equipment, the governing published criterion in North America is Table 100.18 of the ANSI/NETA ATS (Acceptance Testing Specifications) and ANSI/NETA MTS (Maintenance Testing Specifications). Level II thermographers are expected to reproduce that table accurately, know that it carries no priority numbers, and keep it separate from the contractor severity matrices that do.

The Actual ANSI/NETA Table 100.18

Thermographic Survey — Suggested Actions Based on Temperature Rise. Read one of the two ΔT columns, depending on which comparison you were able to make:

Three things about this table trip up candidates:

1. There are no priority numbers. NETA publishes recommended actions, not "Priority 1" through "Priority 4". Any exam answer or report that says "NETA Priority 2" is using someone else's label.
2. The two columns are not interchangeable. A 25 °C rise over ambient sits in the "monitor" row. A 25 °C rise over a similar component at similar load is well past the ">15 °C — repair immediately" row. Reading the ambient numbers against a similar-component measurement is the single most common misuse of this table.
3. The similar-component column has no third row. It goes 1–3, 4–15, then >15. The blank cell is in the standard.

NETA's own notes qualify the table: temperature specifications vary by equipment type, even within a class (cables of different ratings, for example); heating is generally related to the square of the current, so load has a major impact on ΔT; and the values are offered as reasonable guidelines in the absence of consensus standards for ΔT.

Test parameters NETA states alongside the table

Where the Priority Numbers Actually Come From

The Infraspection Institute Standard for Infrared Inspection of Electrical Systems & Rotating Equipment (2016 Edition), Section 11.1, is the document that attaches priority numbers to ΔT criteria — and it publishes four different criteria sets side by side. Section 11.1.1 states the key caveat explicitly: "The priority values listed in the above tables are provided for reference purposes and may not appear as part of the referenced document." That is the sentence that resolves the whole confusion. Priority 1 is the most severe in every one of these tables.

1. NETA criteria, as tabulated by Infraspection

2. Military Standard (MIL-STD-2194) criteria

A single ΔT column, not split by reference type, and far more permissive at the low end:

MIL-STD-2194 (Infrared Thermal Imaging Survey Procedure for Electrical Equipment, 1988, Notice 1 in 1995) is a cancelled Navy standard. It is no longer circulated, yet many thermographers still use it — which is precisely why you must name your criteria set in the report.

3. Experience-Based criteria (Level-II CIT Reference Manual)

For electrical and/or mechanical equipment:

4. Motor cores (on a test bench, not in service)

Why the set you pick changes the answer

The same measurement lands in different places depending on which criteria you applied — this is the highest-value comparison in Chapter 8:

An 17 C° similar-component finding is an immediate-repair Priority 1 under NETA and a next-routine-maintenance Priority 4 under MIL-STD-2194. Same physics, opposite urgency, entirely because of the criteria set. Level II therefore names the criteria set and edition in the report, every time. Infraspection's Section 10.3.5 requires exactly that: when using absolute criteria, record "the surface temperature of the exception and the standard temperature(s) referenced."

Common study-aid error: a widely copied table claims "NETA: P4 = 1–10 °C, P3 = 11–20 °C, P2 = 21–35 °C, P1 > 35 °C, versus a similar component." That table is wrong twice over — it reads NETA's ambient numbers against a similar-component comparison, and it truncates the top band at 35 C°. The nearest real table is the Experience-Based set (1–10 / >10–20 / >20–40 / >40 C°), which is not NETA at all.

Load Requirements: What NFPA 70B Actually Says

Resistive heating in connections scales roughly with I²R, so a survey at light load can miss a real defect entirely.

NFPA 70B (2023) states the rules directly:

The widely quoted "40 % minimum" is therefore a fallback permitted by NFPA 70B, not the target. The target is normal load. Below 40 percent, you have not met the recommended practice and should not present the finding as a standard-graded classification.

Exam classic: a 9 °C ΔT at 18 % load is not a clean classification of any kind. The correct action is to re-inspect at normal loading (or at least 40 percent of nominal) and to document the loading either way.

Why this matters numerically: at 20 % current, heating power is only 4 % of the full-load I²R for the same resistance. Incipient loose joints may produce no measurable ΔT at all — a false negative. Conversely, a large ΔT already visible at low load may be far worse at rated current.

Measurement Preconditions Before You Classify

Severity labels do not rescue bad radiometry. Before assigning any action level:

1. Emissivity and reflected apparent temperature set correctly (Chapters 5–6), especially on oxidized versus shiny connectors
2. Spot size / IFOV adequate for the lug, fuse, or bolt head
3. Focus, range, and optical path correct (windows, distance, angle)
4. Similar load on the reference component — never compare a loaded feeder to an open spare
5. Steady conditions — wait after load steps; inrush is not a severity snapshot

If two phases carry unequal current by design, a phase-to-phase ΔT may reflect load imbalance, not connection resistance. Level II distinguishes current-imbalance patterns from single-point connection hotspots (Chapter 9).

Worked Classification Scenarios

Scenario A — Fuse block. Three identical fuses, same circuit, at normal load. Readings 28 °C, 28 °C, 45 °C. ΔT = 17 °C versus peers. NETA Table 100.18: >15 °C — major discrepancy, repair immediately. A contractor matrix would call the same finding Priority 3. Report the criterion you used; if the specification names NETA, this is an immediate-repair finding, not an outage-window item.

Scenario B — MCC lug. Suspect lug 68 °C, sister phase 30 °C at similar measured current. ΔT = 38 °C. Major discrepancy under NETA; Priority 1 under the contractor matrix. Both agree: act now.

Scenario C — Ambient-only comparison. A unique single cable termination has no peer. Measured 62 °C, ambient 30 °C → ΔT over ambient = 32 °C. That is the 21–40 °C "monitor until corrective measures can be accomplished" row of the ambient column — not the ">15 °C repair immediately" row, which belongs to the similar-component column.

Scenario D — Low load. ΔT = 12 °C at 15 % load. Do not classify as though the survey were valid. Re-inspect at normal loading per NFPA 70B 7.4.4, and document the loading.

Scenario E — Numbering trap. A handout labels "Priority 1: 1–10 °C, possible deficiency." That handout numbers priorities in the opposite direction. The physics band is mild; do not order a shutdown because the digit is "1".

Reporting Checklist for Electrical Findings

Common Traps

Summary for Recall

ANSI/NETA Table 100.18 grades findings with four recommended actions, read against either a similar-component ΔT column (1–3 / 4–15 / >15 °C) or an ambient-air ΔT column (1–10 / 11–20 / 21–40 / >40 °C), and assigns no priority numbers. Priority numbers come from the Infraspection Institute standard, which tabulates four different criteria sets — NETA, MIL-STD-2194, Experience-Based, and motor cores — all numbered with Priority 1 as most severe, and warns that those numbers may not appear in the referenced document itself. The widely copied "NETA P4 1–10 / P3 11–20 / P2 21–35 / P1 >35 °C" table matches none of them. NFPA 70B 7.4.4–7.4.6 requires normal circuit loading, permits a 40 percent floor when normal load is infeasible, and requires the loading to be documented.