Could someone please help me understand the logic

[GeorgeKoehl]

Just recently we had to take out 500 MCM and replace it with 600 MCM for a 2000 amp service. The reason was due to the 75 degree schedule for the connectors. I understand it is code but it really seems counterintuitive to me ? here is why:

1. 500 MCM at 90 degrees can carry 430 amps
2. 600 MCM at 90 degrees can carry 475 amps
3. The lugs at the cabinet are rated a 75 degrees
4. upsizing the wire does not change the 90 degree rating of the wire nor does it increase the rating of the lugs; in fact:
5. it would seem that by upsizing 90 degree wire you could actually allow more amperage to be present on the 75 degree lugs and could more readily cause lug failure

I?m sure there is a good reason for this but for the life of me I can not figure it out.

 

[Dennis Alwon]

You are never allowed to put more than 420 amps on a 600 KCM wire because the ampacity is based on 75C not 90C because of connections as you stated. Likewise the 500 KCM is only good for 380 amps.

So if you parallel 600KCM you would need 5 runs to get 2000 amps.
5 runs of 500KCM is only good for 1900 amps. You cannot upsize to the next size breaker since the OCPD is more than 800 amps.

The 90C is basically used for derating ability-- I am not sure I understand what you are saying.

 

[mcclary's electrical]

Just recently we had to take out 500 MCM and replace it with 600 MCM for a 2000 amp service. The reason was due to the 75 degree schedule for the connectors. I understand it is code but it really seems counterintuitive to me ? here is why:

1. 500 MCM at 90 degrees can carry 430 amps
2. 600 MCM at 90 degrees can carry 475 amps
3. The lugs at the cabinet are rated a 75 degrees
4. upsizing the wire does not change the 90 degree rating of the wire nor does it increase the rating of the lugs; in fact:
5. it would seem that by upsizing 90 degree wire you could actually allow more amperage to be present on the 75 degree lugs and could more readily cause lug failure

I?m sure there is a good reason for this but for the life of me I can not figure it out.

If your calculated load goes over the rating of your 500's at 75 degrees, you have no choice but to change to 600's

 

[iwire]

If your calculated load goes over the rating of your 500's at 75 degrees, you have no choice but to change to 600's

He has no choice anyway.

 

[GeorgeKoehl]

The 90C is basically used for derating ability-- I am not sure I understand what you are saying.

I guess my understanding about changing the amperage rating of the wire from 90 to 75 was to accommodate the lower rating of the lug (which I assumed had to occur to prevent failure) It seemed to me that having a larger wire could possible allow more amperage to be present at the lug as opposed to using the smaller wire.

 

[JES2727]

I guess my understanding about changing the amperage rating of the wire from 90 to 75 was to accommodate the lower rating of the lug (which I assumed had to occur to prevent failure) It seemed to me that having a larger wire could possible allow more amperage to be present at the lug as opposed to using the smaller wire.

It's the OCPD that "allows" the amperage to be present on the lug, not the wire. The larger wire assures that the conductor and terminations will not overheat from the amperage that is being "allowed" to pass.

 

[hurk27]

I guess my understanding about changing the amperage rating of the wire from 90 to 75 was to accommodate the lower rating of the lug (which I assumed had to occur to prevent failure) It seemed to me that having a larger wire could possible allow more amperage to be present at the lug as opposed to using the smaller wire.

Ever noticed that when a conductor gets over loaded, where is the first place that conductors fails?

yep at the termination point, and it is the most common place to see burnt insulation, because the wire can not conduct the heat away from the connection point effectively, so put more copper/ aluminum in the connection and you get more heat transfer. no more burnt wire at the termination point.

 

[hurk27]

Heres one that was discussed on here before, put a 75? rated wire in the termination and after 6" change to a 90? rated wire with a connector rated for 90? run to the load and change back to a 75? rated wire again with a connector rated for the 90? rating, and all is good.

edited to add, when I say a 75? rated wire I mean sized per 310.16 to the 75? table as to the load it will carry, same as the 90? rating.

 

[dicklaxt]

I think it more easily understood if you were to say that a 500MCM conductor will allow 380 amps to flow without insulation degradation when the temperature is not greater than 75 degrees.

If you go on to the next step and ask yourself the question, what happens if I use the 90 degree insulation conductor and have an amp flow greater than 380 then what ? Well as I see it the termination point
will quickly increase in temp at the termination juncture because of the increased amp flow which would generate heat beyond the 75 degree rating and cause the termination point and the 90 degree insulation to fail.

I just recently sorted this out and finally got it thru this old hard head with another post.I did it wrong for many many years and know many others who are still doing it wrong,there must be a pretty good multplier that keeps things in check as it never reared it's head as a problem that I know of.I can see that it may very well be a problem under certain scenarios.The use of FLA X 125% also helps as the actual amp flow is less thus less heat.

dick

 

[Sierrasparky]

Boy was that an expesive mistake!
What did you do with all the scrap?
You can send it over my way