De-Mystifying Type 4 PoE Nominal Current Specifications

“P = IV is a fundamental engineering formula that describes power in terms of its relationship to current and voltage where:

          P = power, measured in watts (W)
          I = current, measured in amps (A)
          V = voltage measured in volts (V)

Those familiar with this equation and IEEE Std 802.3bt™ might observe that there seem to be inconsistencies between the nominal highest Type 4 PoE current per pair, Class 8 power sourcing equipment (PSE) output power, and PSE output voltage specifications in the amendment.

Table 145‑11 of IEEE Std 802.3bt specifies 90W as the output power of a Class 8 PSE and 52V is the nominal output voltage of all IEEE 802.3 PoE‑compliant injectors, regardless of Type. Inserting these values into the power formula above results in the maximum current for a Type 4 PoE system being:

I = P / V = 90W / 52V = 1.73A

This result is then divided by 2 because two pairsets (a pairset consists of a positive and a negative pair) share the Type 4 PoE current load, which yields:

Maximum Type 4 current per pair = 1.73A / 2 = 866mA

This begs the question – why then does Table 145‑1 of IEEE Std 802.3bt specify the nominal highest current per pair for a Type 4 PSE to be 960mA? The answer is related to the fact that, in the early stages of IEEE Std 802.3bt development, specifications were derived using a maximum Limited Power Source (LPS) value of 100VA. This is equivalent to 100W in dc systems and specified by IEC 60950‑1 as safe for consumer access without the need for specialized tools or certification. While it’s possible for an engineered PoE system to deliver 100W of power and still comply with LPS requirements, a system designed to deliver 90W provides additional operating margin to ensure that the 100VA limitation is never exceeded in any real‑world PoE deployment condition.

Today, an application of PoE that utilizes a maximum current per pair specification of 960mA would be considered a specialized and non-commercial “extended power” implementation having greater powering capacity than permitted by Class 8 parameters.”