Safety switch bonding/grounding

ktm400sx · Nov 21, 2005

This is my scanerio:
400 amp breaker feeds a machine center....uses 2 x 2" conduits with 3/0 wire (2 per phase, 3 phase)
We have to put in a safety switch that disconnects the mains.
We ran 4 x 2" conduits to the safety switch and are planning on pulling in 3 x 3/0 into each pipe.
My question is:
Do I need to run a 1/0 grounding/bonding conducter in each pipe?
Thanks for any help

jbwhite · Nov 21, 2005

Re: Safety switch bonding/grounding

250.122(F)

augie47 · Nov 21, 2005

Re: Safety switch bonding/grounding

what size overcurent device, if any, is ahead of your feed? curious as to the 1/0

[ November 21, 2005, 03:29 PM: Message edited by: augie47 ]

pierre · Nov 21, 2005

Re: Safety switch bonding/grounding

KTM
the equipment ground conductor is not sized to the phase conductors, but to the overcurrent device. If you are protecting the disconnect with an 800 amp overcurrent device, yes you will need to install a 1/0 EGC in EACH parallel raceway, as per JB's post.

jbwhite · Nov 21, 2005

Re: Safety switch bonding/grounding

i was wondering why 4 3/0 on a 400amp breaker?

pierre · Nov 21, 2005

Re: Safety switch bonding/grounding

I saw 4 sets of 3/0, and ASSUMED 800 amp overcurrent device. If it is not an 800 amp device, and it is 400 amp, the EGC would be a 3 AWG conductor.

"We ran 4 x 2" conduits to the safety switch..."

jbwhite · Nov 21, 2005

Re: Safety switch bonding/grounding

the original post said 400 amp. so why the 4 sets?

hey, but i wanna know. if for voltage drop, does the ecg need to be increased?

pierre · Nov 21, 2005

Re: Safety switch bonding/grounding

Yes it does, as per 250.122(B)

jbwhite · Nov 21, 2005

Re: Safety switch bonding/grounding

if you dont mind helping. how would i apply that?what chart do i use to look up the circular mil area?

charlie b · Nov 21, 2005

Re: Safety switch bonding/grounding

Originally posted by jbwhite: What chart do i use to look up the circular mil area?

Table 8. It's in the back of the book, in the Chapter 9 Tables.

jbwhite · Nov 21, 2005

Re: Safety switch bonding/grounding

charlie b, thanks for the help, but i think you took my question out of context.

there are several posts above the one that you replied to that are relivant.

jbwhite · Nov 21, 2005

Re: Safety switch bonding/grounding

i am totally amazed. i have a journeyman licence in PA, and a masters in NC yet i still do not know everything that there is to know about the NEC.

i am always trying to learn more so i am glad to find a fourm like this one where others are willing to help.

to reword my question: 250.122 (B) says that if the size of the ungrounded conductors is increased then the size of the EGC needs to be increased based on the circular mill area of the conductors. What tables, and or calculations do i need to make this adjustment?

iwire · Nov 21, 2005

Re: Safety switch bonding/grounding

I can't take credit (or blame

) for the following, it is from the NECH

Equipment grounding conductors on the load side of the service disconnecting means and overcurrent devices are sized based on the size of the feeder or branch circuit overcurrent devices ahead of them. If the ungrounded conductors are increased in size to compensate for voltage drop, the equipment grounding conductors must also be increased proportionately.

Example
A 240-volt, single-phase, 250-ampere load is supplied from a 300-ampere breaker located in a panelboard 500 ft away. The conductors are 250 kcmil copper, installed in rigid nonmetallic conduit, with a 4 AWG copper equipment grounding conductor. If the conductors are increased to 350 kcmil, to what size must the equipment grounding conductor be increased?

Solution

Step 1.
Calculate the size ratio of the new conductors to the existing conductors:

Step 2.

Calculate the cross-sectional area of the new equipment grounding conductor. According to Chapter 9, Table 8, 4 AWG, the size of the existing grounding conductor has a cross-sectional area of 41,740 circular mils.

Step 3.

Determine the size of the new equipment grounding conductor. Again, referring to Chapter 9, Table 8, we find that 58,436 circular mils is larger than 3 AWG. The next larger size is 66,360 circular mils, which converts to a 2 AWG copper equipment grounding conductor.

[ November 21, 2005, 04:38 PM: Message edited by: iwire ]

charlie b · Nov 21, 2005

Re: Safety switch bonding/grounding

Originally posted by jbwhite: charlie b, thanks for the help, but i think you took my question out of context.

I did not take your question out of context. I answered your question correctly, if not completely. Table 8 will give you the circular mill area of each size of conductor. Here is the process:

Figure out what size phase conductor you need to handle the load. Look up the circular mil area of this size conductor (From Table 8). Call this value "A."
Figure out what size EGC is needed for the circuit (based on the overcurrent protection device). Look up the circular mil area of this size conductor (From Table 8). Call this value "B."
Figure out what size you wish to use for the phase conductors, based on voltage drop or any other considerations. Look up the circular mil area of this size conductor (From Table 8). Call this value "C."
Divide "C" by "A," and multiply by "B." Call the results "D."
Put another way, D = (C/A) x B
Look at Table 8, and pick the smallest conductor that has a circular mil area at least as high as "D." Use that conductor for your EGC.

Here's an example:

Assume a 200 amp load.
From Table 310.16, a 3/0 conductor can handle this current.
From Table 8, a 3/0 is 167,800 circular mils,
From Table 250.122, for a 200 amp overcurrent device, the minimum EGC is #6.
From Table 8, a #6 is 26,240 circular mils,
Because of voltage drop, you wish to increase the phase conductors to 250 MCM.
You don't need Table 8 for this. A 250 MCM is (not surprisingly) 250,000 circular mils.
Calculate: (250,000 / 167,800) x (26,240) = 39,094
The smallest conductor that is at least 39,094 circular mils is #4 (41,740 circular mils).
Therefore, you use a #4 EGC for this application.

Is this what you were asking for?

augie47 · Nov 21, 2005

Re: Safety switch bonding/grounding

jb: never doubt charlie's answers... if you think he's wrong, you better read again.
The fellows told you correctly..250-122 (B) tells you to increase the size porportionly,,,Table 8 gives you the area. Only "formula" requies you to do the math,,,,

[ November 21, 2005, 04:47 PM: Message edited by: augie47 ]

charlie b · Nov 21, 2005

Re: Safety switch bonding/grounding

OK, Bob, you beat me to the draw again. That is what I get for checking my math and my spelling one more time, before posting.

iwire · Nov 21, 2005

Re: Safety switch bonding/grounding

Originally posted by charlie b:
OK, Bob, you beat me to the draw again.

It is an empty victory, I cheated, there is no glory in a cut and paste.

augie47 · Nov 21, 2005

Re: Safety switch bonding/grounding

me too
sorry guys..amazing what all happens while you are thinking/typing/editing,,,

jbwhite · Nov 21, 2005

Re: Safety switch bonding/grounding

thanks all. i appreciate the help.

ktm400sx · Nov 21, 2005

Re: Safety switch bonding/grounding

Originally posted by jbwhite:
i was wondering why 4 3/0 on a 400amp breaker?

Its 2 incoming and 2 outgoing......2 from machine center and 2 going back to machine center. So 2 x 3/0 per phase.
Is my 1/0 ground sized correctly....?
I did what Charlie B is doing....I think

Safety switch bonding/grounding

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