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Size of bonding wire for 200-amp load center

Posted By Michael Johnston, Monday, September 01, 2008
Updated: Monday, February 04, 2013

Question

For a 200-amp load center used for feeding charging equipment that is approximately 400 ft from main switchboard, what size of bonding wire is required? — T. C.

Answer

The question appears to deal with a voltage-drop situation, but information is missing. I will answer the question by inserting the minimum necessary information. The first item to clarify is the terminology. I presume you are referring to the size of the equipment grounding conductor with the feeder and not a bonding wire which is undefined by the NEC. The next bit of information needed is the size of the ungrounded phase conductors intended to be installed for this feeder. Note: if a voltage-drop calculation is performed for this 400-ft feeder, then the system voltage and phase configuration of the feeder are necessary for the voltage-drop calculation. I will answer the question without the voltage-drop calculation information since this information is missing.

First, the question is restructured as follows:

What is the minimum size equipment grounding conductor required for a feeder (400 feet in length) supplying a panelboard for charging equipment?

For a 200-ampere feeder, a 3/0 copper conductor is generally required by the minimum requirements of the NEC. The minimum size equipment grounding conductor required for a 200-ampere feeder is not less than 6 AWG copper (see 250.122 and Table 250. 122). Now if the feeder ungrounded conductors have been increased in size from 3/0 copper to 250 kcmil copper (for example) for voltage drop reasons, then the required equipment grounding conductor must also be increased in size proportionately [see 250.122(B)]. This is done as follows using Table 8, chapter 9 of the NEC:

Using Table 8, Chapter 9
Required size 3/0 AWG = 167,800 cm
Adjusted size 250 kcmil = 250,000 cm

Adjusted size phase conductor ÷ Required phase conductor size = 250,000 ÷ 167,800 = 1.49
The multiplier is 1.49. The size of the ungrounded circuit conductors has been increased 149%; therefore, the equipment grounding conductors must be increased 149% as well, per 250.122(B).

Required equipment grounding conductor = 6 AWG copper (Table 250.122 based on 200 ampere overcurrent protection for the feeder)

Using Table 8, Chapter 9
6 AWG = 26,240 cm × 1.49 = 39,097.6 cm

According to Table 8, a 4 AWG copper is required = 41,740 cm

The minimum size equipment grounding conductor for this 200-ampere feeder is a 4 AWG copper conductor. The answer is based on NEC-2008, Sections 250.122, Table 250.122, and Chapter 9, Table 8 (conductor properties).

I hope this helps with you questions concerning sizing equipment grounding conductors for feeders. As always, consult the local authority having jurisdiction for any local electrical code rules that may be necessary in addition to the minimum requirements of the NEC. — Michael J. Johnston, CMP-5

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Tags:  Focus on the Code  September-October 2008 

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