Conduit size in metric

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m sleem

m sleem

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I am confused of the metric size of conduits in table 2/chapter 9,
e.g. in table 2, 16 metric vs 1/2", 1/2" approx equals to 13 mm also the outer diameter is approx 21 mm, so what is the meaning of 16 here?
 
The 'imperial' conduit sizes are 'nominal', meaning that they have some vague historical connection to the number. Something along the lines of 'pipe was specified in terms of its _inner_ diameter, but manufactured to a specific outer diameter. As manufacturing methods (and wall thickness) changed, the _outer_ diameter was held constant but kept the name. (Which makes sense; 1" pipe fittings will mate with 1" pipe, even if they are of different wall thicknesses.)

The 'metric' conduit sizes are also 'nominal', with a different history.

Rather more like names or words in a language than actual measurements. If you trace the 'etymology' back enough you might find a real measurement. If you want some fun look up the history of 'AWG' wire sizes....
 
winnie said:
The 'imperial' conduit sizes are 'nominal', meaning that they have some vague historical connection to the number. Something along the lines of 'pipe was specified in terms of its _inner_ diameter, but manufactured to a specific outer diameter. As manufacturing methods (and wall thickness) changed, the _outer_ diameter was held constant but kept the name. (Which makes sense; 1" pipe fittings will mate with 1" pipe, even if they are of different wall thicknesses.)

The 'metric' conduit sizes are also 'nominal', with a different history.

Rather more like names or words in a language than actual measurements. If you trace the 'etymology' back enough you might find a real measurement. If you want some fun look up the history of 'AWG' wire sizes....
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Those "nominal" pipe sizes were already established before we started using similar sized ones to pull conductors through.

1/2 inch pipe usually has ~1/2 ID, where 1/2 tubing often has ~1/2 OD, but 1/2 electrical raceway is going to have ID that is near 1/2 regardless of what type it is.
 
Metric sizes for both wire and conduit are not based on unit conversions, such as inches to millimeters. Rather, they are what the manufacturers actually make.
 
kwired said:
1/2 inch pipe usually has ~1/2 ID, where 1/2 tubing often has ~1/2 OD, but 1/2 electrical raceway is going to have ID that is near 1/2 regardless of what type it is.
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Um, no
The inside diameter of schedule 40 "half-inch" pipe is 0.62 inches.
The inside diameter of schedule 80 "half-inch" pipe is 0.55 inches.
The inside diameter of schedule 40 PVC conduit is 0.58 inches.
The inside diameter of "half-inch" EMT is 0.7 inches.
 
The nominal conduit size is really a minimum. What's the complaint with providing more space than the spec?

I wonder whether one can use actual size in a case where the needed area exceeds the table area by a fraction.
 
LarryFine said:
The nominal conduit size is really a minimum. What's the complaint with providing more space than the spec?

I wonder whether one can use actual size in a case where the needed area exceeds the table area by a fraction.
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I would think using standard fittings with typical sizes is always easier in terms of availability and replacement.
 
drcampbell said:
Um, no
The inside diameter of schedule 40 "half-inch" pipe is 0.62 inches.
The inside diameter of schedule 80 "half-inch" pipe is 0.55 inches.
The inside diameter of schedule 40 PVC conduit is 0.58 inches.
The inside diameter of "half-inch" EMT is 0.7 inches.
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The "~" does not mean "exactly" All the 1/2 inch nominal dimensions you gave were closer to 1/2 than 3/4 though.

Other difference I was trying to point out is some cases nominal is ID others it is OD. "pipe is usually nominal ID, "tubing" is usually OD, but some types of tubing are based on ID.

Electrical raceways is all based on ID for the nominal part, some luminaire parts may be OD based dimension at times.
 
Tilde. ~

approximately, or similarity In math.

it’s a not in logic, although I prefer to use an exclamation mark !
 
Hv&Lv said:
Tilde. ~

approximately, or similarity In math.

it’s a not in logic, although I prefer to use an exclamation mark !
Click to expand...

Mathematics uses a double tilde (≈), to mean approximately. It is common outside of formal mathematics to simply use the single tilde to mean the same thing, for typing simplicity. Like yourself, I also prefer the exclamation mark as the logical NOT operator.

To clarify "similarity in Math", it really means proportionality.
 
m sleem said:
I am confused of the metric size of conduits in table 2/chapter 9,
e.g. in table 2, 16 metric vs 1/2", 1/2" approx equals to 13 mm also the outer diameter is approx 21 mm, so what is the meaning of 16 here?
Click to expand...

Metric designators are based on RMC's interior diameter rounded to mm. You'll notice it is a lot closer to the actual size, than the nominal inch sizes, the actual sizes of today came before the metric designators were assigned.

Even though inner diameter varies among different types in the same nominal size, it will be the same metric designator. Like inch sizes, it is still just a name for the size, and will not necessarily equal any actual measurement of the raceway exactly.
 
Older members likely aware of this, younger or newer to the trade probably not - I think it was 1996 when they changed conduit fill table to multiple tables, one for each raceway type. Before then 1/2 EMT vs 1/2 RMC or FMC - all used the same area for fill calculations. I do believe you were supposed to look at markings on Schedule 80 PVC to get the internal area though, Sch 80 was and usually still is marked with it's internal area even though technically you now are supposed to use the table in NEC.
 
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