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Conductor Width Nomograph

Determine the capacity of a conductor

Conductor Width Nomograph


This nomograph will help you determine the maximum allowable current capacity (in amperes) of a conductor. Reprinted from IPC-2221 (and MIL-STD-2118), the nomograph shows current for various conductor thicknesses, width, and temperature rises.

Using the nomograph

  • Locate the width of the conductor on the left side of the bottom chart. 
  • Move right horizontally, until you intersect the line of the appropriate conductor thickness. Move down vertically to the bottom of the chart to determine the cross-sectional area of the conductor.
  • Move up vertically, until you intersect the line of the appropriate allowable temperature rise. This is the increase in temperature of the current-carrying conductor. Conductor temperature should not exceed 105°C. For example, if the ambient temperature might reach 80°C, the temperature rise above ambient of the conductor should be less than 25°C (105°C - 80°C). In this case use the 20°C curve.
  • Move left horizontally, to the left side if the chart to determine the maximum allowable current. Reverse the order of these steps to calculate required conductor width for a given current.

Conductor aspect ratio

For best producibility, design conductors to be at least five times as wide as they are thick. For example, with 2 oz. Copper (0.0028"/50μm) design the conductors to be 0.0140" (0.36mm) or wider. In tight situations Minco is successful in achieving 2.5:1 ratio conductor widths.

Assumptions

  • The nomograph is valid only for conductors with a polyimide cover layer — not exposed conductors.
  • The conductor thickness includes copper plating. Be aware that plating may add 0.0005" (13μm) to 0.0014" (36μm) of thickness. Selectively plated circuits do not have significant plating over conductors. The nomograph does not apply for plated metals other than copper.
  • Derate current by 15% for conductor thicknesses greater than 3 oz. (0.0042”/75μm).
  • The temperature rise curves only recognize heat generated by the conductor itself. Heat from power dissipating components or nearby conductors on other layers is not included.
  • It is assumed that conductor coverage is relatively small; i.e. there is enough free space between conductors for lateral heat dissipation. Groups of closely spaced parallel conductors on the same layer can be treated as one large conductor. Add all the cross sectional areas together and all the currents together to determine the temperature rise.
  • Current ratings are for still air environments. Forced air cooling will increase the maximum allowable current. Operating circuits in a vacuum will greatly decrease the maximum allowable current.

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