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chandru1982b4u
Active Contributor

Join Date: Mar 2011
Location: INDIA
Posts: 10

Earthing wire for a motor

04/21/2011 12:37 PM

On what basis the size of the earthing strip or wire is calculated. whether by the full load amps or the source side amps. For eg for a 10HP motor with FLA of 12.5A must be earthed with min how much sized cable.
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wareagle
Guru
United States - Member - New Member

Join Date: Jun 2008
Location: Alabama
Posts: 1604
Good Answers: 63
#1

Re: Earthing wire for a motor

04/21/2011 2:32 PM

In the NEC the equipment grounding conductor is based on the size of the overcurrent device. Check this article and table 250.122 on page 2.

http://www.cable.alcan.com/NR/rdonlyres/19FC4111-742D-47B7-9A28-06DF8AB4023D/0/EquipmentGroundingConductor.pdf
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electricalexpert65
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Join Date: Oct 2009
Posts: 2061
Good Answers: 169
#2

Re: Earthing wire for a motor

04/25/2011 7:24 AM

The earth conductor must be sized based on the maximum single-phase to ground fault current anticipated at the moto end. This shall include the source impedance, the impedances of the starter components, the connecting cable. But, in a large installation such calculations are laborious. So, there are ready reckoners available - giving the earth conductor sizes for various HP Motors.
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