EGC size

[don_resqcapt19]

If you have a installation with 12 #10 copper current carrying conductors on 20 amp OCPDs in a raceway, and your are installing a common EGC, what size is the EGC required to be? The #10s were used because of the required 50% derating for the 12 current carrying conductors.
Don

 

[bphgravity]

Re: EGC size

On first glance, I would say only a number 12 would be required based on 250.122(C).

However, 250.122(B) requires the EGC to be increased with the ungrounded conductors and does not specify for WHAT reasons you increase the ungrounded conductor.

So in that case, I would say a #10 to keep the porportion of increased ungrounded conductor size.

 

[dereckbc]

Re: EGC size

What is this Don a test? I know you know the answer. 10 AWG. 250.122(B)&(C)

 

[iwire]

Re: EGC size

Due to the wording of 250.122(B)

(B) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors.

I would say there is no choice but to increase to 10 AWG in this case.

But I have to ask why? If the ungrounded and grounded conductors have only been increased in size for derating as per 310.15(B)(2)(a) what would be the need to increase the size of the grounding conductor?

[ June 11, 2003, 04:03 PM: Message edited by: iwire ]

 

[pierre]

Re: EGC size

Hello Don,

250.122(B) Increased in Size. Where ungrounded conductors are increased in size equipment grounding conductors, where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors.(quote from the 2002 NEC)

In the Handbook, there is a formula to follow, I used to just upsize the EGC, until I came across this formula. (the handbook is a better example than here, this post is not verbatim from the Handbook)

Size ratio = upsized conductor (circular mils) divided by the original sized conductor(circular mils).
This gives you a ratio (larger than 1), and you multiply this by the size of the EGC of the original sized circuit (also circular mils). The circular mils can be found in Chapter 9, Table 8.
This method may require going up more than 1 size.

In your case: 10380(cm) divided by 6530(cm) = 1.59(ratio)

multiply 1.59 x 6530 (your original # 12 for the circuit conductor) = 10,383

10 AWG = 10380 which is technically too small and you should use 8 awg for the EGC. The difference of only 3 is splitting hairs, but this is actually a good example.

Let me know what you would do

Pierre

 

[iwire]

Re: EGC size

Originally posted by pierre:

In your case: 10380(cm) divided by 6530(cm) = 1.59(ratio)

multiply 1.59 x 6530 (your original # 12 for the circuit conductor) = 10,383

10380 / 6530 = 1.5895865237366003062787136294028

1.5895865237366003062787136294028 x 6530 = 10380

 

[don_resqcapt19]

Re: EGC size

Do you really have to use #10? The ampacity or the #10 is only 20 amps under the conditions of the installation. The 20 amp OCPD is the maximum permitted for the conductor in this case. I think that you can still use the #12. 250.122(B) is not completely clear. Increased in size from what? I think it means where the circuit conductor has an ampacity greater than its overcurrent protectiive device that you have to increase the EGC size. Here the ampacity of the #10s is not greater than the rating of the OCPD and, in my opinion, the EGC is not required to be increased in size.
Don

 

[dereckbc]

Re: EGC size

Don, due to the wording of 250.122(B) I disagree. 122B does not specify for what reason you would increase the size. No if you look in the 2002 Handbook at the example text that follows 250.122(B)it does specify voltage drop, but it is not the code language.

I think the intent of the panel was for voltage drop, but they did not word it that way. If the conductors were incresed for derrating purposes and not voltage drop I think its OK, but until the wording is hanged you are still required to increase the size.

 

[bphgravity]

Re: EGC size

Hey Bob!

Did you do that calculation in your head?!!

WOW!

 

[tom baker]

Re: EGC size

First time I've ever seen Don post a question.
In the 1999 NEC the wording for table 250.122 EGC was "may be required to be increased in size". the 2002 NEC changed it to shall be. The orginal concern was increasing for voltage drop, but there are other reasons to increase the size of the ungrounded condcutors other than volt drop.

 

[bennie]

Re: EGC size

Don makes a good point from a technical aspect. The equipment ground conductor should be sized larger when fault clearing is the issue.

When heat dissipation is the only concern the No. #12 will carry all fault current likely to be imposed.


A 500 Kcmil TW conductor in an ambient temperature of 131? F. has ampacity of 131 amps.
Sizing the equipment ground based on the breaker time/trip curve and total impedance would not be any larger than a No. #6 cu.

 

[bphgravity]

Re: EGC size

This goes back to 90.1(A) Practical Safeguarding.

Using a #12 may be practically safe but not the best choice in my opinion.

If you were getting ready to board an airplane or an amuzement park ride, would you get on if the attendent said "Everything is practically safe, buts thats it."

All the values of 310.16, 250.66 and 250.122 are bare minimums that provide practical safety!

 

[don_resqcapt19]

Re: EGC size

In the '99 code the EGC only had to be increased in size if the circuit conductors were increased in size to reduce the voltage drop.

('99 code) (b) Adjustment for Voltage Drop. Where conductors are adjusted in size to compensate for voltage drop, equipment grounding conductors, where installed, shall be adjusted proportionately according to circular mil area.

In the '02 code the "voltage drop" wording was removed. One of the reasons was that there is no way for the inspector to prove that the circuit conductors were increased in size because of voltage drop. The substantiation for this proposal, 5-264, is shown below.

SUBSTANTIATION: The current text is limited to voltage drop only and is subject to abuse and misinterpretation (e.g. it was done per the plans, not for voltage drop). The manufacturers directions often call for conductor to be increased in size, with no explanation for why the ungrounded conductors size is increased, with no corresponding requirement for the equipment grounding conductor to be increased.

There is nothing in the code section that specifies the starting point for the circuit conductor ampacity. The only reasonable assumption is that the starting point is to be based on the maximum permitted ampacity in 310.15. The adjustments in 310.15(B)(2) change the ampacity of the conductor. In this case the maximum permitted ampacity of the #10s is 20 amps. The wire is protected at its ampacity and has not been increased in size.
Don

 

[bennie]

Re: EGC size

Don: How does manufacturers instructions fit into the technical design of a circuit? Leave it to the manufacturers and all wire will be #10 minimum.

Like the old saying "the best way to sell more beer is to not put a big head on it".

Ampacity of the equipment ground wire is irelevant, only the impedance is a factor.

[ June 11, 2003, 10:09 PM: Message edited by: bennie ]

 

[iwire]

Re: EGC size

Hi Bryan Of course I did that in my head.

On using the 12 AWG ground in the circuit that Don is describing why is it "not the best choice".

The ground is not normally carrying current and when it does it is much more than the current it is rated for.

I understand the need on long runs to increase the grounding conductor size, but I am at a loss to see a benefit in this application.

As Bennie pointed out impedance is the important factor, not ampacity.

 

[pierre]

Re: EGC size

Good Morning to All,

It is raining AGAIN, so no fishing this morning.

How about the note to Table 250.122? I have not seen anybody mention this. My experience in the field is that a lot of installations are not as concerned with the EGC, as long as the 'lights are on'. Me, if I am going to oversize any conductor it is going to be the one that will keep me out of trouble.

Pierre

 

[bphgravity]

Re: EGC size

I agree with Pierre.

I just feel an electrician should never ask himself "what will be good enough to work" or "what is all I have to do". I just think it is a bad attitude.

If your planing to take an exam, or simply want to know what exactly the code minimum is, then fine. But if you are asking what an electrician should really install in this situation, I believe all of us would just upsize to #10.

I know Don is trying to further our understanding of this requirement and I appreciate the thought experiment. Now that I know an #12 is really all I need, I feel much better about installing a #10.

This would make for a really good lecture!

Where do I sign up, Don?

 

[bennie]

Re: EGC size

It is admirable to want to upsize some systems. I once felt that way. Now I look back at all the conduit and wire that is still just laying there and will never see an amp of a load.

I now feel that minimum is the way to go in most installations.

The biggest waste of our resources is the
u-ground receptacles in homes. I have 65 receptacles in my house. Only four of them have a three wire attachment cord.

 

[pierre]

Re: EGC size

Bennie,

What would you do without those three you are using? With your experience and knowledge I don't need an answer to that.... But most people using the electrical system on their premises haven't got a clue, and that is okay because WE in the industry do and we care so they can be safe 'WITHOUT A CLUE'.

 

[charlie b]

Re: EGC size

I got into this one late, but here's my contribution.

Last week, I heard a presentation from an expert who offered the following explanation for upsizing the EGC: No matter what reason led to you upsize the phase conductors, by doing so you have lowered the impedance of one half of the fault current path. From the source to the load (i.e., the hot-to-case fault point), the impedance will be lower. But that will not (he said) be enough. You need to reduce the impedance of the other half of the path (i.e., the EGC). Otherwise (again, he said), there will not be enough fault current to trip the OCPD.

I made a quick attempt to calculate one sample fault, and could not substantiate his statements. However, at least in the case of upsizing for voltage drop (i.e., for long runs), it makes sense to me that a larger EGC will reduce the impedance of the fault path, and result in a faster trip of the OCPD.