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The basic equation I've been using to calculate the amount of Amperage required to achieve a particular power output using 3-phase electricity is:

Amp. = Watts / (√3 × PF × Voltage)

where PF = Power Factor, which in my case is assumed to be 1.0 (heating elements). For example, to run 18000 on 230 volt 3-phase electricity would require 45.2~ amps.

Amp. = 18000 / (1.732 x 1.0 x 230) = 45.2 amps

However, if the voltage is High Leg Delta, even though the 3 phases are each measuring approx. 230 volts, but 2 of the legs are much lower than the third, should the same equation be used to anticipate required amperage?

BONUS QUESTION: In real-world applications, multimeters are obviously reading different amperages for each live wire, but on paper, the total amperage may in fact be found using the aforementioned equation. This poses a particularly difficult challenge when using Thermal Overload Relays that are used to set the overload threshold. We are seeing that for people using High Leg Delta, the setting isn't actually factoring in individual legs, but the overall amperage as the relay isn't being tripped when the legs excede the setting. The side effect of permitting this strange high amperage, but not really, to pass is that other equipment sensitive to amperage is being damaged. My inclination is to overcompensate, but figured this must something that has been addressed.

Any guidance is appreciated.

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Does using “High Leg Delta” 3-phase electricity require a different equation for calculating Amperage/Power?

No
Current and power are measured independently of neutral or ground on High-Leg Delta. However, transformer supplier may limit load imbalance.

should the same equation be used to anticipate required amperage?

Yes

If imbalance is specified not to exceed x% then differential current protection is required.

If power up surge is grossly imbalanced and caused low load phases to run over voltage then maintenance must improve load balance to prevent power up unbalanced surge voltages.

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The thing with high-leg delta is it's a compromise to also yield common household 120V single-phase in a way which is not unsafe.

By convention, 208V 3-phase is often used in "Wye" mode because one leg of the wye is good old familiar 120V. However, machines which use 240V 3-phase tend use it in "delta" mode. (not least because they tend to be jumperable for either 240V or the very common 480V).

240V "plain-wye" is rare because the resulting "wye" voltage would be the oddball and fairly useless 139V. And why build a machine to rely on that voltage, and lock yourself out of 240V/delta and 240V/high-leg markets? Likewise you'd be foolish to build a machine that depended on 240V/high leg, because it couldn't run on 240V delta or be jumperable for 480V.

So, your 3-phase 240V machine is almost surely using the 240V in delta mode. It is not using the neutral at all. So you don't care where neutral is. You are presenting a balanced 3-phase load to the transformers. The neutral never enters play, so it is not imbalanced at all.

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