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I've been checking some online calculator (this one and this one) to determine the width of a PCB trace for a given current capacity, but I not sure if the value specified for current refers to direct or alternating current.

Do these calculators assume one type of electrical current or is the result applicable to both?

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    \$\begingroup\$ A DC current is equivalent to an RMS AC current ...so treat 1 Amp as either 1 Amp DC or AC RMS. Remember that if you are talking about line voltage (110-240) then there are other rules about track spacing that become critical. \$\endgroup\$ – Jack Creasey Apr 29 '17 at 2:13
  • \$\begingroup\$ Note that for very high frequencies, the skin effect may come into play--but typically, you won't have to worry about that. \$\endgroup\$ – Hearth Apr 29 '17 at 2:30
  • \$\begingroup\$ Above 4Mhz (skin depth for 1.4mil, 35 micron 1 ounce/foot^2 standard CU foil), you should think about the thinning of the useful region. At 400MHz (e.g the 4rth harmonic of 100MHz MCU lines) the resistance is 5 milliOhms/square. \$\endgroup\$ – analogsystemsrf Apr 29 '17 at 3:35
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Both, AC and DC.

Your copper traces will fail by overheating, therefore it's the heating effect of the current that's important.

The calculator gives the RMS value of the current, which is the heating effect. For DC, this is just the same as the steady value. For AC, it's the square Root of the Mean of the Square of the current. A very 'pulsy' waveform, for instance the charging current from a bridge rectifier into the big electrolytic capacitors in a mains power supply, could have quite a high peak, but relatively low RMS.

The calculator must use the correct copper thickness. Note that at high frequency, the effective cross section of the trace reduces, due to the skin effect. Neither of those tools you've linked to appears to address this, so for high frequency (starting at several MHz for typical copper thicknesses) you would need to allow for this yourself. It's an unusual amateur design that uses high currents at high frequencies (ham radio transmitters perhaps), certainly forget about frequency for power supplies and audio.

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