philippi Academy · Configurator

Calculate cable cross-section and fuse rating
In short

A cable on board is properly dimensioned if it meets two conditions simultaneously: the current-carrying capacity of the cross-section is higher than the continuous current, and the voltage drop across the outgoing and return lines remains below the permissible limit. The calculation uses voltage drop in volts = 0.0175 × current in amperes × double the cable length in meters ÷ cross-section in mm², where 0.0175 is the specific resistance of copper at 20 °C. The larger of the two cross-sections applies. The fuse is determined by the cross-section, not the device: it must be above the continuous current and below the cable's load capacity. Example at 12 V and a maximum voltage drop of 3 %: a consumer drawing 10 A, 5 m from the distribution and fused in the cable, needs 6 mm² and a 10 A blade fuse. An inverter drawing 150 A, 3 m from a lithium battery, needs 50 mm² and a Class T fuse directly at the terminal.

Welches ist das richtige Kabel?

Bordnetz
Angabe als
i

Der Wert, den der Verbraucher dauerhaft zieht — laut Typenschild oder Datenblatt. Anlaufströme gesondert gegen die Auslösekennlinie der Sicherung prüfen.

A
i

Einfach messen. Gerechnet wird mit Hin- und Rückweg, also der doppelten Strecke — außer bei Rückleitung über Masse.

m
Zulässiger Spannungsfall
Welche Geräte gehören wohin?

3 % — wichtiger Stromkreis

Laut Norm

  • Zuleitung zu Schalttafel oder Verteiler
  • Positions- und Fahrlichter
  • Elektronik: Navigation, Funk, Steuergeräte, Autopilot
  • Bilgen- und Lenzpumpe, Motorraumlüfter

Aus der Praxis

  • Kühlbox und Kompressorkühlschrank
  • Ladegerät, Ladebooster, Wechselrichter
  • Motor- und Startelektrik
  • Standheizung

10 % — unkritischer Stromkreis

Laut Norm

  • Beleuchtung im Innen-, Wohn- und Arbeitsbereich

Aus der Praxis

  • 12-V-Steckdosen und USB-Lader
  • Lüfter im Wohnraum
  • kurzzeitig laufende Verbraucher: Seilwinde, Ankerwinde, Wasserpumpe

Versorgt eine Leitung auch nur einen Verbraucher aus der 3-%-Spalte mit, gilt für die ganze Leitung 3 %.

Leitung
i

FLRY-B ist dünnwandig und bis 105 °C belastbar — die übliche Leitung im Fahrzeug-Kabelbaum. Im Shop in 1,5 bis 6 und 16 bis 50 mm², rot und schwarz.

Verlegung
i

Gebündelte Adern und warme Einbauorte geben Wärme schlechter ab, die Leitung darf dann weniger Strom führen.

Gebündelt heißt: mehr als 3 stromführende Adern liegen eng zusammen (Kabelbaum, Kabelkanal) oder die Leitung läuft durch den Motor- oder Maschinenraum. Plus- und Minusleitung eines Geräts zählen als 2 Adern.

Rückleitung
i

Zweipolig zählt die Strecke doppelt, Hin- und Rückleitung. Über Masse nur einfach — dann muss der Masseübergang großflächig, blank und korrosionsgeschützt sein.

Sicherung sitzt
i

Die Sicherung schützt die Leitung, ihr Nennstrom richtet sich nach dem Querschnitt. Am Batteriepol sind nur Bauarten mit hohem Abschaltvermögen zulässig, die kleinste beginnt bei 30 A.

Batterie
i

Lithium-Ionen-Batterien liefern im Kurzschluss extrem hohe Ströme. Am Pol kommen dann nur die Bolzensicherung SHB (10 kA) und Class-T (20 kA) in Frage.

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Häufige Fragen

What cable cross-section do I need for 12 volts?

At 12 volts, it is almost always the voltage drop that determines the cross-section, not heating: 3 percent is only 0.36 volts, and that is quickly used up over longer runs. Example: 10 amperes over 5 metres single length give a calculated 4.9 mm², so 6 mm² from the standard range — even though 1.5 mm² could carry the 10 amperes thermally. At 24 and 48 volts the ratio shifts, because the permissible voltage loss grows with the system voltage.

Is the calculation based on the one-way or the doubled cable length?

The calculation uses twice the distance, because the current flows from the positive terminal to the device and back via the negative cable; both conductors cause voltage drop. In the configurator you enter the one-way, measured length — the doubling is built into the formula. The negative cable gets the same cross-section as the positive cable, but no fuse of its own.

Is the fuse sized for the device or for the cable?

The fuse protects the cable, not the device — so its rating is set according to the conductor cross-section. It must be above the load's continuous current so that the fuse does not blow in normal operation, and below the cable's current-carrying capacity so that the cable does not give way before the fuse in a fault. Protecting the device is the device's job.

3 percent or 10 percent voltage drop — which applies when?

3 percent applies to everything that depends on a stable voltage: the feed to a switch panel or distribution, navigation lights, electronics such as navigation, radio and control units, bilge pumps and engine room blowers — and in practice also compressor fridges, chargers, DC-DC chargers, inverters and diesel heaters. 10 percent is only acceptable for non-critical circuits, such as interior lighting, 12 V sockets or short-duty winches. As soon as a cable also feeds even one important load, 3 percent applies to the whole cable. This classification follows ABYC E-11 and applies equally to boats and vehicles.

Why is the cross-section larger when cables are bundled?

Bundled cores shed their heat less well, so their permitted continuous load drops. The philippi catalogue has two rows for this: singly or up to three cores bundled at 30 °C, and more than three cores bundled or installation in the engine room at 60 °C. For 6 mm² that is 45 versus 35 amps. The configurator takes these values unchanged from the table.

Do lithium batteries need a different fuse?

Yes: at the terminal of a lithium battery only fuses with a high breaking capacity are permitted, i.e. the SHB bolt-down fuse (10,000 amperes) or Class T (20,000 amperes). Lithium banks deliver much higher short-circuit currents than lead-acid, AGM or gel; blade and strip fuses cannot safely interrupt this current. The configurator therefore only offers these two types at the terminal for lithium. Example: 150 amperes over 3 metres at 12 volts result in 50 mm² with a Class T fuse directly at the terminal.

How far from the positive terminal may the fuse be?

As short as possible, no more than 20 centimetres: the section between the battery terminal and the fuse is unprotected in a short circuit. The configurator does not allow a greater distance and warns from 17.8 centimetres, the limit according to ABYC E-11. Placing the fuse directly on the battery clamp avoids this risk entirely — there are holders for terminal mounting and stud-mount fuses for this.

philippi Engineering — Engineering department, philippi elektrische systeme GmbH

Written and technically reviewed by the engineering department of philippi elektrische systeme GmbH in Remseck am Neckar. If a standard or a product specification changes, the article is revised and the review date updated.

Development, manufacture and testing of on-board electrical components for over forty years

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