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Old 07-12-2019, 04:16 PM   #1
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Default Underground service sizing questions.

Before I get in trouble for asking dumb questions, I am a Mechanical engineering in a small M/E office but I spent my summers while in school working as an electricians apprentice/laborer, so no I did not go to school for this stuff.

On a recent project the office EE asked how I would interpret the code conductor sizing for an underground service.

The service consists of a weather head on the pole with teck 90 in PVC down to the meter base mounted on a 6x6 post near the pole. From the meter base the teck 90 is run 100' as direct burial to the house where it enters just above the concrete foundation and down into the top of the panel.

For the pole to the meter base I am assuming the use of table 4 for the allowable amperage.

I am trying to determine when I can and cant use table d 10b for the underground portion of the service. is there a specific length the cable can be run above ground and still use d 10b before I need to use table 4 for the amperage .
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Old 07-12-2019, 06:06 PM   #2
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I would tell the office EE not to worry, the electrician that installs it will know exactly what to do
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Old 07-13-2019, 08:29 AM   #3
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Quote:
Originally Posted by emtnut View Post
I would tell the office EE not to worry, the electrician that installs it will know exactly what to do
Even when the EE's drawings are wrong.
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Old 07-13-2019, 08:38 AM   #4
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The EE will not know what the utility will require. Have the LEC determine what is required based on the utility's requirements; this has nothing to do with the code book.

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Old 07-13-2019, 08:44 AM   #5
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This is why there are always problems. The electrical engineer doesn't know his own job, so he went to the mechanical engineer who in turn asks an internet forum that spends most of it's time talking about Home Depot.
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Old 07-15-2019, 02:39 PM   #6
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Quote:
Originally Posted by TMacNeil View Post
Before I get in trouble for asking dumb questions, I am a Mechanical engineering in a small M/E office but I spent my summers while in school working as an electricians apprentice/laborer, so no I did not go to school for this stuff.

On a recent project the office EE asked how I would interpret the code conductor sizing for an underground service.

The service consists of a weather head on the pole with teck 90 in PVC down to the meter base mounted on a 6x6 post near the pole. From the meter base the teck 90 is run 100' as direct burial to the house where it enters just above the concrete foundation and down into the top of the panel.

For the pole to the meter base I am assuming the use of table 4 for the allowable amperage.

I am trying to determine when I can and cant use table d 10b for the underground portion of the service. is there a specific length the cable can be run above ground and still use d 10b before I need to use table 4 for the amperage .
In general you need to use the LESSER of table 4 OR table D10/D11. On an underground service, there is always a short section where it is not underground. Don't forget to apply the 75deg derating (0.886) to the D10/D11 table values.
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