My aluminum underground service entrance conductors are not marked
with their size on the visibile part of their insulation. And the
local utility claims not to have the size information on file. So I
took a closer look at the conductors. Each one appears to have 19
strands, as 6-7 strands are visible around half the perimeter. The
outer diameter of the bundle of strands is 0.36" +/- 0.01", measured
with plastic calipers.
I cross-referenced this info with one current manufacturer's data, and
I came up with a size of 1/0. Does this sound right? Are the
stranding and total OD of stranded wire standard across the industry,
or is it possible that this cable is actually something smaller than
1/0? My information is that these wires were installed in 1989.
1/0 aluminum is good for 100-125a depending on which table you use.
Does that seem to match your main breaker?
Using the residential feeder table a 2/0 (.418") would be good for
150a. Turn your calipers to measure the high points
The existing main breaker is 100A, but the service conductors appear
to be sized for 125A, hence my confusion. I'm replacing my combo
meter base and main panel, and I would like to put in a 125A breaker
in the new main disconnect if possible.
The basic question I have is this: if you can't see the writing on a
conductor's insulation, can you determine the conductor size from the
number of strands and the outer diameter of the strand bundle? For
example, Anixter's "Wire and Cable Handbook" says their Class B
concentric 1/0 stranded conductors are 0.373" in outer diameter and
have 19 strands. Does this mean that any manufacturer's Class B
concentric 1/0 stranded condcutors will be the same?
With aluminum you have to be sure whether it is regular stranded or
compact conductor. The stranding is not as obvious on compact since
they squeeze all the air out. There are tables in the back of the NEC
that have conductor diameters and the various diameters of insulated
wires so measuring is a valid option.
The number of strands really only determines the class of sizes you
have. The bigger the size the more the strands but several sizes
Right, but if I assume that it is concentric (regular stranded) and
impute a wire gauge from its OD, then if it is in fact compact
conductor with the same OD, it will be a higher wire gauge than might
imputed wire gauge. So its ampacity will only be higher than I
assume, and all is well.
True. It happens that 1/0 is the smallest size that uses 19 strands,
for the manufacturer I mentioned previously. Will this be universally
If these conductors are truly "yours", they are subject to NEC rules,
however if they are owned by the utility company, they decide how much
amperage they care to pull through them. For example in NY, you may have a
400 amp service with 500 MCM copper on the owners end, connected to a set of
1/0 aluminum conductors on the utilities end
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