Thread: Service entry neutral was #2 & Hot was 3/0

Inspected house today with service entry neutral cable a #2 and the hot ungrounded service entry cables were #3/0, this cant be correct ?
Does anyone know if this was allowed in 1973. TKS

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Originally Posted by Sam Morris

Inspected house today with service entry neutral cable a #2 and the hot ungrounded service entry cables were #3/0, this cant be correct ?

I believe 3/0 THW is good to 200 amp. 2 THW good to 115. NEC table 310.16. Dependent on run.

Decades ago the code specifically allowed the neutral to be two wire sizes smaller than the hot conductors. Now, however, the neutral conductors are required to be sized for their load, meaning that (typically) the neutral will be the same size as the hot conductors unless the electrical contractor shows load calculations to justify a smaller size neutral ... sometimes, the neutral conductor needs to be larger than the hot conductor due to harmonics, which can lead to increased current on the neutral.

Originally Posted by Jerry Peck

Decades ago the code specifically allowed the neutral to be two wire sizes smaller than the hot conductors. Now, however, the neutral conductors are required to be sized for their load, meaning that (typically) the neutral will be the same size as the hot conductors unless the electrical contractor shows load calculations to justify a smaller size neutral ... sometimes, the neutral conductor needs to be larger than the hot conductor due to harmonics, which can lead to increased current on the neutral.

This house was built in 1973, I don't know what year was used for codes.

Originally Posted by Sam Morris

This house was built in 1973, I don't know what year was used for codes.

The neutral was allowed to be two sizes smaller then ... of course, though, two sizes smaller than 3/0 is ... 2/0 (one size smaller) then 1/0 (two sizes smaller).

Originally Posted by Jerry Peck

The neutral was allowed to be two sizes smaller then ... of course, though, two sizes smaller than 3/0 is ... 2/0 (one size smaller) then 1/0 (two sizes smaller).

Great stuff>>> Thank You

For residential single phase services in my area the neutral is typically two sizes smaller. Pretty much all SEU and URD I can buy is like this from the factory. 4/0, 4/0, 2/0AL for 200A services, and 2,2,4AL for 100A.
4/0,4/0,4/0 & 2,2,2 are available, but not readily.

In this case however I can't think of any code that would have allowed it. Although, I was pretty young in '73.

im just a lurker around here but:

Section 220.22 : the neutral demand load shall be the maximum unbalance computed load between the neutral and any one ungrounded conductor.

the neutral doesnt see the same loads as the hots.it doesnt see the load of the dryer or (to a extent) the stove or the hvac.that leaves you lighting and receptacles and what little you get from a stove neutral.so could of been smaller,at least in the past.















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Originally Posted by Sam Morris

Inspected house today with service entry neutral cable a #2 and the hot ungrounded service entry cables were #3/0, this cant be correct ?
Does anyone know if this was allowed in 1973. TKS

One thing I did not see you include is what size main breaker this has. If it's only rated for 150 amps then the neutral would be the correct size for the panel.

Originally Posted by Robert Meier

I can't remember, was the two size smaller rule that Jerry mentioned based on the OCPD size or the feeder conductor size?

It was based on the feeder size, so if the overcurrent device was lower rated than the feeder hot size, the neutral could still only be two sizes smaller than the actual feeder size.

Originally Posted by Robert Meier

I was leaning that way but couldn't be sure. Thanks for the clarification.

Robert,

Think of it this way - if the neutral was two sizes smaller than allowed for the overcurrent rating but the feeder hots were larger than necessary for the overcurrent rating, and someone were to install a higher overcurrent rating main to match the rating of the feeders - the neutral would now be woefully undersized.

Originally Posted by Jerry Peck

Robert,

Think of it this way - if the neutral was two sizes smaller than allowed for the overcurrent rating but the feeder hots were larger than necessary for the overcurrent rating, and someone were to install a higher overcurrent rating main to match the rating of the feeders - the neutral would now be woefully undersized.

But it would be compliant as installed. Before any upgrade was done all parts should be checked for proper sizing. The code rarely makes provisions for future changes. This is not one of them.

Originally Posted by Jim Port

But it would be compliant as installed.

Not as I recall the wording in the code. The wording in the code was, as I recall, that the neutral was allowed to be 'two sizes smaller than the ungrounded conductors', not that the neutral was allowed to be 'two sizes smaller than the required size of the ungrounded conductors' - makes a big difference with that different wording.

True, I was watching football and did not pull out the books.