10.3 Electrical Wire Sizing, Conductor Temperature & Load Analysis

10.3 Electrical Wire Sizing, Conductor Temperature & Load Analysis

[!NOTE] Where this lives: AC 43.13-1B, Chapter 11, Aircraft Electrical Systems. Section 1 covers inspection and care of electrical systems; the later sections cover wire selection, protection, routing, and bonding. Chapter 12 covers avionics installation. For alterations, AC 43.13-2B supplies the installation guidance and the electrical load analysis expectations.

Electrical questions on the IAR test come in three shapes: what size wire, how hot will the conductor get, and may this load be added. All three are answered from charts, and all three reward knowing which variables move the answer.

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Selecting a Wire Size

Four inputs drive wire selection from the AC 43.13-1B chart:

1. Circuit current in amperes.
2. Conductor length — the total run, and for a single-wire (ground-return) installation, the length from the bus to the load.
3. System voltage — 14 volt or 28 volt.
4. Duty cycle — continuous or intermittent. Intermittent loads permit a smaller conductor for the same current because the wire has time to cool.

The chart is entered with the current and the length, and read against the appropriate voltage and duty-cycle curve to give an AWG size. Two secondary checks then apply:

• Bundle derating. A wire in a large bundle or in conduit runs hotter than one in free air and may require upsizing.
• Circuit protection coordination. The wire must be protected by the circuit breaker or fuse, not the other way round: the protective device is sized to protect the wire.

How the variables move the answer:

A worked shape of the classic question: a 16-foot run carrying a continuous 10-ampere load at 28 volts is entered on the continuous-duty side of the chart at 10 amperes and 16 feet, and read out to an AWG size in the mid-teens. Change "continuous" to "intermittent" and the chart permits a smaller wire; extend the run and it demands a larger one.

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Allowable Voltage Drop

Wire selection is really a voltage-drop problem in disguise. Commonly applied allowable drops in the conductor for a continuous load:

Intermittent loads are allowed a larger drop. This is why a 28-volt system tolerates a smaller conductor for the same wattage: the current is halved for the same power, and the allowable drop is doubled.

The practical consequence for an IA reviewing an alteration is that length matters as much as current. An avionics box drawing modest current at the far end of a long tail-mounted run can require a surprisingly heavy conductor, and an installer who sized the wire by current alone will have undersized it.

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Estimating Conductor Operating Temperature

A conductor carrying current heats up. The relevant question is not "does it get warm" but "does the estimated conductor temperature stay below the wire's rated temperature?" AC 43.13-1B Chapter 11 supplies a chart for estimating conductor temperature, and it is driven by three things:

1. Ambient temperature of the space the wire runs through.
2. The conductor's rated temperature — set by the insulation specification. A MIL-W-22759 wire family member may be rated 150 °C, 200 °C, or higher depending on the dash number.
3. The ratio of actual circuit current to the wire's maximum rated current (I / I-max).

The behavior to understand is the shape of the relationship: temperature rise above ambient grows with the current ratio, and it grows faster than linearly. A wire operated at a small fraction of its rated current runs barely above ambient; the same wire near its rated current runs far hotter.

So in a problem giving an ambient of 43 °C, a conductor rating of 150 °C, a circuit current of 5 amperes, and a maximum rated current of 11 amperes, the current ratio is under one half, the temperature rise above ambient is modest, and the estimated conductor temperature lands well below the 150 °C rating — the installation is acceptable. An answer that exceeds the conductor's rating is the signal that the wire is undersized for the application.

Two derived rules an inspector uses:

• Ambient matters as much as current. The same wire and load is acceptable in a cool avionics bay and unacceptable routed near an exhaust or in an unventilated engine compartment.
• Insulation rating is not optional information. Substituting a lower-temperature wire specification into an installation designed around a high-temperature one is an unapproved change.

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Electrical Load Analysis for an Alteration

When an alteration adds electrical equipment, the installer must show the generating system can carry it. The standard expectation is that the total continuous electrical load does not exceed 80 percent of the generator or alternator rated capacity under normal operating conditions, leaving margin for battery charging and transient loads.

An acceptable load analysis for a Form 337 package includes:

[ ] Alternator/generator rated output (each, and total for multi-source systems)
[ ] Existing continuous loads, itemized
[ ] Existing intermittent loads, itemized and identified as intermittent
[ ] New equipment loads being added
[ ] Total continuous load vs. rated capacity, with margin shown
[ ] Battery capacity and the emergency (alternator-out) load analysis
[ ] Circuit protection sized to protect the added wire
[ ] Bus assignment showing which bus the load is on

The emergency load analysis is the half installers forget: after an alternator failure the aircraft runs on battery, and the analysis must show which loads remain and for how long the battery supports them.

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Routing, Support, Separation, and Bonding

These are pure inspection items, and AC 43.13-1B Chapter 11 states several with numbers:

• Separation from mechanical control cables: wires and electrical cables must be separated from mechanical control cables. In no instance should wire be able to come closer than 1/2 inch to such controls when light hand pressure is applied to the wires or controls. Where clearance is less, adequate support must prevent chafing.
• Slack: bundles are installed with enough slack to permit ease of maintenance, prevent mechanical strain on wires, terminations, and supports, and allow equipment to be shifted for servicing.
• Bend radius: coaxial and triaxial cable is bent at a radius no less than 6 times the outside diameter of the cable, since crushing the dielectric changes the cable's impedance.
• Unused wires are individually dead-ended, tied into a bundle, and secured to permanent structure — strands cut even with the insulation and capped with a pre-insulated closed-end connector or a one-inch piece of insulating tubing folded back and tied.
• Routing above fluid lines: wire bundles are routed above fuel, oil, and hydraulic lines wherever possible, so that a leaking line does not drip onto electrical connections.
• Bonding and shielding: Appendix D of Part 43 requires bonding and shielding to be inspected for improper installation and poor condition at every annual and 100-hour inspection, under the radio group at paragraph (i)(3).

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What the IA Checks on an Electrical Alteration

1. Is there a load analysis, and does the continuous load stay within the 80 percent guideline?
2. Is the wire sized for current and length, with voltage drop within limits?
3. Is the conductor's temperature rating appropriate for the ambient of its route?
4. Is the circuit protection sized to protect the wire, and is it accessible in flight if the design requires it?
5. Is routing clear of controls (1/2-inch rule), above fluid lines, and free of chafe points through the full travel of everything nearby?
6. Was the work done to approved data? Adding equipment that requires structural modification or that creates substantial bus loading is a major alteration requiring approved data and a Form 337.

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High-Yield Exam Traps

• Length drives wire size as much as current does. Sizing by amperes alone undersizes long runs.
• Continuous versus intermittent changes the chart curve and therefore the answer.
• Conductor temperature = ambient plus a rise that grows with the I/I-max ratio. Compare the result with the conductor's rating.
• The 1/2-inch separation from control cables is measured under light hand pressure, not at rest.
• Coaxial cable bends at no less than 6 times its outside diameter.
• The circuit breaker protects the wire, not the equipment.