ECN Forum Forums General Topics of the Trade General Discussion Area Voltage drop & over sized conductors

200KAIC Available!! Jeeze! Hope that's only at the Transformer's terminals Would be like a 1500 KVA 600 VAC 0.5% Z transformer - plus a gang load of motor contribution to boot!

If the 200KAIC was available after the main service, and at least 1/2 as high at the first sub panel, I would not even think of using breakers for anything - even the branch circuits!!
Would be more like RK5 fuses for everything!!!

On the serious side now, I am just messing around here, not trying to be a jerk, or make fun of your message.
You have a valid point here. SCA is a major factor with larger conductors. So is the Skin effect.
I wouldn't imagine the fault levels could be that extreme, especially at some distance from the transformer [not using LRA motor contributions], but they could reach as high as 65KAIC if there were large subfeeders [like 500 KCMIL] and there was a lot more available at the service.

The branch circuits would be limited by their conductor sizes and lengths, but even then with 65K available at that sub panel, the branch breakers would need to be 65K also.

That NEC article about voltage drop is not a mandatory thing, it's more a suggestion. Some equipment manufacturers have specific tolerances that they also suggest their equipment be connected to. They are usually within 10% +/- of the RMS voltage at x temperature, and so on. Some have less tolerance, like +/- 1% maximum. All depends on the integrety of the equipment and the cost to replace burned out pieces [overhead, downtime and the part it's self].

Too much voltage drop on conductors creates too much heat generated in the conductors them selves, so there is a limit that should be set to where one will say that the voltage drop is becoming excessive.

A happy medium between limiting the voltage drop to a good enough level that will not overheat the conductors, while keeping the SCA within some safe area and trying to avoid too much skin effect is the way to go.
With the SCA, upsizing one or two sizes wouldn't make the available level majorly higher, but if the sizes were upgraded twice the ampacity size, that would have a major factor in the overall SCA.

On the other hand, skin effect would probably assist in limiting the SCA in that situation...


Feel free to launch napalm on this message

Scott SET