From the (E)Mailbag:

NEC SAYS YOU CANT' 'PARALLEL CONDUCTORS UNDER 1/0. I HAVE WORKED ON TWO
SCHOOLS AND BOTH PROJECTS HAVE PARALLEL NEUTRAL CONDUCTORS UNDER 1/0. CAN YOU
EXPLAIN THIS? THE BLUE PRINTS ARE ENGINEERED TO USE 200% NEUTRALS FOR PANELS
FEEDING COMPUTERS.
THANKS!

Was this the way it was drawn on the single line diagrams, or elsewhere in the Engineered planset [to use paralleled grounded conductors that are smaller than 1/0]??
wonder how that got through plancheck

Must have been the last day before Vacation - both times!

If this is not the way it was drawn up, then it's somebody's conception of an increased ampacity for the grounded conductor [200% of unbalanced load size].

Technically, it shouldn't be an immediate hazard, but it's not within the minimum requirements of the NEC, so that alone makes it something of consern.

I can see a possible change in the future NEC, as to this type of situation. This one wouldn't really have the expected calamity of a typical paralleled feeder. In this case, the ungrounded feeders [subfeed] would be sized correctly [we hope], using only single conductors. The grounded conductor is being supplimented with an additional conductor of equal size, to make the end result ampacity 200% of any single ungrounded conductor.

In other setups, the total amperage is paralleled across the individual feeders, and is protected by an OCPD that exceeds the ampacity of a single conductor by at least 100%.
Here I can see the need to use conductors of 1/0 or larger for overall integrety and strength.

The grounded conductor being supplimented is to cover the highest possible load on that conductor, which exceeds the ampacity of a single conductor. As long as the terminations are solid, the conductors are equal in size, length and type, and the feeder is derated properly for the 5 current carrying conductors - it really doesn't pose any danger as far as I can see. Others may not agree with me, so please post any comments.

I would not say the same if 4 smaller conductors were used to equal the 200% rating of a given ungrounded conductor. That's going back to paralleled conductors, instead of supplimented conductors.

Scott SET

Bill,
Is this an existing building? 310-4 EX 4 permits grounded conductors #2 and larger to be paralled in EXISTING installations.
Don(resqcapt19)

310-4 Ex4;

Under engineering supervision, grounded neutral conductors - - -

Glenn

Don,

This was a question sent in be Email. I don't have any further information. Sorry.

Bill

The FPN of 310-4 Ex4 seems to give a guide of only on triplen currents.

How much Plant ( location ) load study would involve allowing the Ex 4 to be utilized?

Glenn

I find several " engineering supervision " 's in the NEC. I do not find a definition.

Did I miss the definition ?

Glenn

I take it there is a reason, other than an arbitrary choice of 1/0 ,that the NEC is addressing.

I see no "engineering" or "supervision" definition, but as I am the son of an engineer I will give it a shot;

Engineer;
one who can , through factual analysis,pick apart anything in God's material universe down to sub-atomic levels...

( with apologies to Scott here)