Equipment Grounding Vs Voltage Drop

Status
Not open for further replies.
B

binwork91

Senior Member
Location
new york
Occupation
electrical engineer
I know we need to increase the EGC size due to voltage drop.
For example. 50A OCPD need to have #8 per phase and #10 CU G. If we upsize the conductor from #8 to #3/0 due to voltage drop,
then 85.01 (the size of a #3/0 AWG conductor) ÷ 8.367 (the size of a 8 AWG conductor) = 10.16,
5.261 (the size of a 10 AWG conductor) x 10.16 (the ratio found above) = 53.45
then we need #1/0 CU G.

Question: we know 200A OCPD only need #6 CU. G per table250.122. if we change the 50A OCPD to 200A OCPD with #3/0 per phase in the example above, other condition stay same (include the load) . then the EGC will #6.
in this case, do we use #1/0 as EGC or #6 as EGC? Why?
 
It's kind of a chicken-and-egg issue. I share your perspective.

Which came first, the over-sized conductors or the under-sized breaker?

In your question as asked, the #6 could be used.

But, is it worth it to actually replace a 1/0 with a #6? You'd have to decide.
 
The answer as to the actual size needed can likely be found on an engineer's desk.
250.122 is the MINIMUM size equipment ground needed based on the ove-rcurrent device.
250.4 notes that the ground path should be such to facilitate the operation of the over-current device and for long runs some engineering expertise would be advised.
 
This is one of the sections of the NEC that they just cannot get right. If you're using a metal raceway like EMT you don't need any wire type EGC so why should you ever have to increase the EGC when you're increasing the ungrounded conductor size for VD compensation? It should simply say that if the raceway qualifies as an EGC then use whatever size redundant EGC that gets you all hot and bothered.
 
LarryFine said:
It's kind of a chicken-and-egg issue. I share your perspective.

Which came first, the over-sized conductors or the under-sized breaker?

In your question as asked, the #6 could be used.

But, is it worth it to actually replace a 1/0 with a #6? You'd have to decide.
Click to expand...
I would like to use 1/0. However, I don't like to oversize the EGC to increase the cost. Especially, when I am doing Value engineering. That's why I want to find out the answer.
 
binwork91 said:
I would like to use 1/0. However, I don't like to oversize the EGC to increase the cost. Especially, when I am doing Value engineering. That's why I want to find out the answer.
Click to expand...
So is the bold the actual question? The answer is #6 from T250.122.
Question: we know 200A OCPD only need #6 CU. G per table250.122. if we change the 50A OCPD to 200A OCPD with #3/0 per phase in the example above, other condition stay same (include the load) . then the EGC will #6.
in this case, do we use #1/0 as EGC or #6 as EGC? Why?
Click to expand...
 
You answered your own question here:
binwork91 said:
If we upsize the conductor from #8 to #3/0 due to voltage drop,

in this case, do we use #1/0 as EGC or #6 as EGC? Why?
Click to expand...
You say the reason for the wires being larger than the breaker is voltage drop, thus you need the #1/0.
 
LarryFine said:
You answered your own question here:

You say the reason for the wires being larger than the breaker is voltage drop, thus you need the #1/0.
Click to expand...
If I want to save money from conductor, I decide to replace 50A OCPD to 200A OCPD, then it will be #6. If #6 is sufficient grounding for 200A OCPD, then it should be sufficient grounding for 50A OCPD. Then why not using #6? I understand base on code we need #1/0 due to voltage drop.
 
infinity said:
So is the bold the actual question? The answer is #6 from T250.122.
Click to expand...
I try to ask If #6 is sufficient grounding for 200A OCPD, then it should be sufficient grounding for 50A OCPD. Then why not using #6 EGC for 50A OCPD with #3/0 phase conductor.
 
The amp rating of the wires has little to do with the EGC size. A foot long piece of #10 will probably trip a 200A breaker, but a 500 foot long piece of #6 may not (or at least not very quickly). Wire length adds resistance which can increase impedance enough that a breaker won't trip on magnetic trip and will have to rely on thermal which is much slower.

If you oversize wires because of voltage drop, then you need to increase the EGC size. A #6 EGC on a 50A breaker may be too small if the circuit is very very long. Increasing a 50A circuit from #8 to 3/0 is a huge increase in size and implies that it is very long.
 
binwork91 said:
I try to ask If #6 is sufficient grounding for 200A OCPD, then it should be sufficient grounding for 50A OCPD. Then why not using #6 EGC for 50A OCPD with #3/0 phase conductor.
Click to expand...
Apparently, it's because of the reason for the mismatch. That's why I said it depends on whether you're up-sizing the wire or down-sizing the breaker. Who can prove what your reasoning was?
 
steeltubeinstitute.org

GEMI Analysis | Steel Tube Institute

Download the GEMI Analysis Software to learn how steel conduit can shield against the effects of EMF in your electrical distribution systems, about its use as an equipment grounding conductor, and more.
steeltubeinstitute.org steeltubeinstitute.org
this free software program allows you to determine the size of the EGC and tge effects of voltage drop, and why the EGC size needs to be increased
 
binwork91 said:
I try to ask If #6 is sufficient grounding for 200A OCPD, then it should be sufficient grounding for 50A OCPD. Then why not using #6 EGC for 50A OCPD with #3/0 phase conductor.
Click to expand...
It should be if there is no voltage drop. If there is VD then the EGC needs to be large enough to open the OCPD.
 
This is far and away the worst rule in the book.

If I don't upsize and just accept a larger voltage drop, then I can use a smaller EGC and the breaker won't trip. But if I upsize the ungrounded conductors, making it easier to trip the breaker, then the smaller EGC becomes inadequate. Absolutely unscientific. And if the EGC is inadequate without upsizing then the rule doesn't address the saftey issue.

Not to mention that they removed the specific reference to voltage drop. So now if we upsize for future expansion, or terminal size, or because #1 wire was in short supply, we can still be hit with this stupidly written rule when there's absolutely no science based physical reason for it whatsoever.

The rule should be entirely rewritten to only apply to wire lengths above some minimum (500ft?), and then require calculation via formula. And as mentioned above, non-flexible metallic raceway methods should be entirely exempt.
 
jaggedben said:
This is far and away the worst rule in the book.

If I don't upsize and just accept a larger voltage drop, then I can use a smaller EGC and the breaker won't trip. But if I upsize the ungrounded conductors, making it easier to trip the breaker, then the smaller EGC becomes inadequate. Absolutely unscientific. And if the EGC is inadequate without upsizing then the rule doesn't address the saftey issue.

Not to mention that they removed the specific reference to voltage drop. So now if we upsize for future expansion, or terminal size, or because #1 wire was in short supply, we can still be hit with this stupidly written rule when there's absolutely no science based physical reason for it whatsoever.

The rule should be entirely rewritten to only apply to wire lengths above some minimum (500ft?), and then require calculation via formula. And as mentioned above, non-flexible metallic raceway methods should be entirely exempt.
Click to expand...
The only possible justification that I can constuct for requiring the proportional upsize of the EGC would be to reduce the voltage on the "grounded" metal at the far end during the time it takes for the breaker to trip.
If the hot and EGC are the same size, the transient exposed voltage will be limited to half the line voltage. If the resistance difference is high the exposed voltage can approach the line voltage.
But this argument fails pretty badly if the Code allows the EGC to be smaller than the ungrounded conductor(s) in the first place.
 
jaggedben said:
This is far and away the worst rule in the book.

If I don't upsize and just accept a larger voltage drop, then I can use a smaller EGC and the breaker won't trip. But if I upsize the ungrounded conductors, making it easier to trip the breaker, then the smaller EGC becomes inadequate. Absolutely unscientific. And if the EGC is inadequate without upsizing then the rule doesn't address the saftey issue.
Click to expand...
The code also requires you to have an effective ground fault path, so even if you do not upsize for voltage drop reasons, you still have to provide a means to generate enough current in the event of a ground fault that the OCPD will open and clear the fault.

250.4 General Requirements for Grounding and Bonding.
(A) Grounded Systems.
....
(3) Bonding of Electrical Equipment. Normally non–current carrying
conductive materials enclosing electrical conductors
or equipment, or forming part of such equipment, shall be
connected together and to the electrical supply source in a
manner that establishes an effective ground-fault current path.
(4) Bonding of Electrically Conductive Materials and Other
Equipment. Normally non–current-carrying electrically
conductive materials that are likely to become energized shall
be connected together and to the electrical supply source in a
manner that establishes an effective ground-fault current path.
(5) Effective Ground-Fault Current Path. Electrical equipment
and wiring and other electrically conductive material
likely to become energized shall be installed in a manner that
creates a low-impedance circuit facilitating the operation of the
overcurrent device or ground detector for high-impedance
grounded systems. It shall be capable of safely carrying the
maximum ground-fault current likely to be imposed on it from
any point on the wiring system where a ground fault may occur
to the electrical supply source. The earth shall not be considered
as an effective ground-fault current path.
Click to expand...
Subparagraph (B) provides for similar requirements for ungrounded systems.
 
jaggedben said:
This is far and away the worst rule in the book.
Click to expand...
But the new exception added in 2020 lets you substitute your own judgement on upsizing if you are "qualified."

Exception: Equipment grounding conductors shall be permitted to be sized by a qualified person to provide an effective ground fault current path in accordance with 250.4(A)(5) or (B)(4).


So if you are upsizing the ungrounded conductors because, say, in the future you want to be able to upsize the supply breaker, and not due to length, then you can choose to use the a smaller EGC (than the proportional upsizing, but presumably sufficient for the future upsized breaker).

Cheers, Wayne
 
wwhitney said:
But the new exception added in 2020 lets you substitute your own judgement on upsizing if you are "qualified."

Exception: Equipment grounding conductors shall be permitted to be sized by a qualified person to provide an effective ground fault current path in accordance with 250.4(A)(5) or (B)(4).


So if you are upsizing the ungrounded conductors because, say, in the future you want to be able to upsize the supply breaker, and not due to length, then you can choose to use the a smaller EGC (than the proportional upsizing, but presumably sufficient for the future upsized breaker).

Cheers, Wayne
Click to expand...
Thanks I wasn't aware of that 2020 revision yet. That goes along way toward fixing it. Now to convince AHJs I'm qualified. :-D
 
Status
Not open for further replies.
Top