conductor sizing

Status
Not open for further replies.

KMOORE001

Member
Location
Hutto Texas
I was taught that all conductors less that 100 amps were based on 60 degree chart because of 110-14 (C). Now most devices, disconnects, and breakers are dual rated at 60 and 75 degree. We have done this because of sometimes getting into older items as listed above that are not dual rated. We believe in if there is a error we would prefer it to be a error for safety. Are we outdated and everyone is using 75 degree for these applications or not. Please respond.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I would use the 75? C rating if all of the components were already rated for 75? C. The conductors and equipment are tested to operate at that temperature so using the 60? C ampacity is IMO a waste of someone's money.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
I err on the side of caution but only after due dilligence. If we have as-builts, field survey, or some other means of knowing the rating of the devices/terminations then proper engineering can be done; when such is not the case it is proper to use the more conservative values.
 

Volta

Senior Member
Location
Columbus, Ohio
One Code problem that can easily occur would be sizing the conductor based on the 60 deg rating, and then later find that the termination points are rated 75.

That can often mean that a smaller conductor could have been used, so the conductors would have been effectively "increased in size", which means we undersized the EGC per 250.122(B).

Wishing for a new chart showing EGC size per ungrounded conductor size.
 

kwired

Electron manager
Location
NE Nebraska
One Code problem that can easily occur would be sizing the conductor based on the 60 deg rating, and then later find that the termination points are rated 75.

That can often mean that a smaller conductor could have been used, so the conductors would have been effectively "increased in size", which means we undersized the EGC per 250.122(B).

Wishing for a new chart showing EGC size per ungrounded conductor size.

Equipment grounding conductors are not sized to the ungrounded conductors they are sized to the overcurrent device. If the ungrounded conductors are increased in size for some reason such as voltage drop then you increase the EGC by the same ratio that you increased the ungrounded. ECG size is not affected by temperature, or number of conductors in a raceway either.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Equipment grounding conductors are not sized to the ungrounded conductors they are sized to the overcurrent device. If the ungrounded conductors are increased in size for some reason such as voltage drop then you increase the EGC by the same ratio that you increased the ungrounded. ECG size is not affected by temperature, or number of conductors in a raceway either.


I think that the point he was trying to make is that if you used the 60? C ampacity to size the conductors and everything was actually rated for 75? C then you have actually increased the ungrounded conductor size. This would then require an increase in the size of the EGC. Augie's graphic shows this. If you used the #6 condcutors on the 75? C rated terminations you have actually increased the conductor size.

6_15_34_3_2.gif
 

kwired

Electron manager
Location
NE Nebraska
I think that the point he was trying to make is that if you used the 60? C ampacity to size the conductors and everything was actually rated for 75? C then you have actually increased the ungrounded conductor size. This would then require an increase in the size of the EGC. Augie's graphic shows this. If you used the #6 condcutors on the 75? C rated terminations you have actually increased the conductor size.

6_15_34_3_2.gif

1. I see the example as a bad example - the same size grounding conductor is required for both circuits.

2. The conductor on the left of the example was not increased. It started at a different size than the one on the right, place more conductors in the raceways in the example and we may need to talk about increasing conductor sizes and as a result EGC size.

3. How can you increase the size of a conductor before you determine what the absolute minimum size required is?

The first step in sizing a conductor is to determine what minimum size is required. Terminal temperature has to be known before you can select from the appropriate column. Then you can derate for number of conductors in a raceway, temperature, voltage drop or whatever.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
1. I see the example as a bad example - the same size grounding conductor is required for both circuits.

2. The conductor on the left of the example was not increased. It started at a different size than the one on the right, place more conductors in the raceways in the example and we may need to talk about increasing conductor sizes and as a result EGC size.

3. How can you increase the size of a conductor before you determine what the absolute minimum size required is?

The first step in sizing a conductor is to determine what minimum size is required. Terminal temperature has to be known before you can select from the appropriate column. Then you can derate for number of conductors in a raceway, temperature, voltage drop or whatever.


I agree that the same size EGC can be used for either installation. The question is if you had all 75? C equipment, like the receptacle on the right and you sized the conductors using the 60? C ampacity then haven't you increased the conductor size from #8 to #6?
 

kwired

Electron manager
Location
NE Nebraska
I agree that the same size EGC can be used for either installation. The question is if you had all 75? C equipment, like the receptacle on the right and you sized the conductors using the 60? C ampacity then haven't you increased the conductor size from #8 to #6?

you did not increase anything you sized two different circuits that have different characteristics that required a different minimum conductor size.

Once that was done you then sized the EGC to the breaker. Since no conductor adjustments were made from the minimum size required you do not have to adjust the EGC either.

you can not adjust a conductor ampacity until you have a minimum conductor size to adjust from.

If you were sizing the same circuit to 60 deg and never even give 75 deg a thought or actually have 60 deg equipment you would not say to yourself when sizing the EGC "if I had sized the conductors for 75 deg they would have been smaller" you would have just sized the EGC to the breaker with no other adjustment.
 

kwired

Electron manager
Location
NE Nebraska
maybe a simpler explanation of what I am trying to say is that choosing a 60 or 75 degree conductor is a part of conductor selection process and not a conductor deration or adjustment process.

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 the circular mil area of the ungrounded conductors.

like I have tried to say before how can it be increased in size before a base or starting size has been determined?
 

Volta

Senior Member
Location
Columbus, Ohio
Ok. So you have a load, say it is 50 amps. We use THHN. We are limited to the 75 degree rating of the termination provisions for the devices to be used even though THHN is rated as 90 degree C. We find that we can use #8.

Conductor has been selected, #8 THHN.

For some reason, any reason, or no good reason at all, we install a #6 for one of the ungrounded conductors. We have increased the size. We have provided the capacity for more fault current to flow than the original conductor would have allowed. Per 250.122(B), we need to proportionately increase the size of the EGC.

For ungrounded: could've use 16510 cmills, did use 26240. Increased by 1.5893%.
For EGC: per 250.122(A) for 50 amps, could've been #10 (10380 cmills), but per 250.122 increase by 1.5893% to 16497 cmills, so must be larger than #10. Use #8 EGC.

Certainly not perfect, if the load is allowed to be protected at 60 amps, we replace the OCP and presto! We've not increased the size all of a sudden. The section needs work, which is why they should throw T250.122 out and fashion a new one after the logic of 250.66.
 

Volta

Senior Member
Location
Columbus, Ohio
Or you have a load, say it is 50 amps. We use THHN. We are limited to the 60 degree rating of the termination provisions for the devices to be used even though THHN is rated as 90 degree C. We find that we need to use #6

Conductors have been selected, #6 THHN.

The load is later replaced with a new version. Now we have 75 degree termination ratings throughout. Silly, but same NEC Section applies anyhow, and now the conductors are larger than needed.

The physics haven't changed, no danger has been increased, but the way it reads now the EGC if #10 would be too small.

So remove it from the EMT, bring it to the scrap yard, and feel like you've complied. Didn't need that green wire anyhow. :grin:
 

jaylectricity

Senior Member
Location
Massachusetts
Occupation
licensed journeyman electrician
In what situation would the EGC need to be larger for a smaller circuit? Say you have #12 ungrounded conductors and a #12 EGC. If you could have sized the ungrounded conductors at #14, why would you end up needing anything more than a #12 EGC?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
you did not increase anything you sized two different circuits that have different characteristics that required a different minimum conductor size.

Once that was done you then sized the EGC to the breaker. Since no conductor adjustments were made from the minimum size required you do not have to adjust the EGC either.

you can not adjust a conductor ampacity until you have a minimum conductor size to adjust from.

If you were sizing the same circuit to 60 deg and never even give 75 deg a thought or actually have 60 deg equipment you would not say to yourself when sizing the EGC "if I had sized the conductors for 75 deg they would have been smaller" you would have just sized the EGC to the breaker with no other adjustment.


My question is if the equipment is all rated for 75? C and you size the wire for 60? C haven't you increased the size of the ungrounded conductor?
 

Volta

Senior Member
Location
Columbus, Ohio
In what situation would the EGC need to be larger for a smaller circuit? Say you have #12 ungrounded conductors and a #12 EGC. If you could have sized the ungrounded conductors at #14, why would you end up needing anything more than a #12 EGC?

You wouldn't. You have already increased the size of the EGC proportionantely, complying with 250.122(B).

The EGC also is not required to be larger than the ungrounded conductors per 250.122(A).
 
Status
Not open for further replies.
Top