We are in the process of installing a new section of the plant and I am running into a code problem. The engineer has calculated that to supply a 1200A sw. gear that we need to run triple 500mcm in 3 DIFFERENT conduits. After checking the tables and finding no deration factors apply since there are not more then 3 current carrying conductors in each raceway, this should produce the desired 1200 amps. The Electrical inspector also agrees, but since this plant is located at a Quarry, it falls under MSHA regulation and they are saying that it has to be derated to 80% and the triple 500mcm is large enough to supply the sw. gear. They also state that this is a NEC requirement, but I can't seem to find it anywhere. I want to know if this is something I am just missing, or is this something that is just a pet peave of this particular MSHA inspector?
Doby Industrial Electrician
- Thread starter Doby
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- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
First, welcome to the forum. Secondly, three sets of 500 MCM is not adequate for a 1200 amp service. Each is only good for 380 amps, so three would be good for 1140 amps. Third, I have to presume you are talking about the three wires in each conduit being a Phase A, a Phase B, and a Phase C (i.e., you are not putting all three Phase A wires in one conduit, etc.). Finally, the NEC does not require any derating under the conditions you describe. I suggest asking the MSHA Inspector to provide a reference for his 80% derating.
weressl
Esteemed Member
charlie b said:
Perhaps the derating that is required for 40C* at the lowest rise of 60C*, became a "rule of tumb" 80%?
On the other hand three 500kCMil @ 90C* rise rating without ambient adjustment would be 1290A.
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petersonra
Senior Member
- Location
- Northern illinois
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- Semi-retired engineer
Couldn't they just set the trip at 1140 amps?charlie b said:
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
That would be fine. But that is not what the OP described.petersonra said:
- Location
- Massachusetts
weressl said:
If you can find any devices (breaker, panels, disconnect switches etc.) under 600 volts that have 90 C terminals I would love to know about them. :smile:
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- Location
- Massachusetts
Doby said:
310.7?
What year and code are you looking at?
- Location
- Massachusetts
Did you mean B.310.7?
If so then yes the engineer can use that, electricians can not.
If so then yes the engineer can use that, electricians can not.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I agree with Bob. Furthermore, if you are trying to use Table B.310.7, then you are in a "3 Electrical Ducts" configuration, not the "1 Electrical Duct" configuration. Without further review of the exact circumstances, I would start with an ampacity of 341 times 3, or 1023 amps total.
weressl
Esteemed Member
iwire said:
The OP did not define the end connection points.
There are 90C* compression lugs or barrel connectors.
Distribution transformers secondary connections may be equipped with 90C* connectors.
600V switchgears do have 90C* rated connection points.
MSHA has their own regulations, which may differ from the NEC. The MSHA
inspectors also have access to written explanations on how to interpret those regulations. They will not show you those interpretations, just the regulations, which your company/engineer? should have.
inspectors also have access to written explanations on how to interpret those regulations. They will not show you those interpretations, just the regulations, which your company/engineer? should have.
BAHTAH
Senior Member
- Location
- United States
1200Amps?
1200Amps?
As pointed out the 500kcm will be good for 1140amps. The breaker lugs will be rated for 75-deg c, so I don't see how you can terminate the 500kcm using a current value higher than the 1140 even if 310.7 gives you a higher rating. Why not install (4) 350kcm which will give you 1240 amps and use a 100% rated breaker if your table 310.7 gives you a value on the 350K that when derated to 80% is still at 1200. I am still looking for the table. 1200amp breakers have lugs for (4) 350's.
1200Amps?
Doby said:
As pointed out the 500kcm will be good for 1140amps. The breaker lugs will be rated for 75-deg c, so I don't see how you can terminate the 500kcm using a current value higher than the 1140 even if 310.7 gives you a higher rating. Why not install (4) 350kcm which will give you 1240 amps and use a 100% rated breaker if your table 310.7 gives you a value on the 350K that when derated to 80% is still at 1200. I am still looking for the table. 1200amp breakers have lugs for (4) 350's.
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- Location
- Massachusetts
weressl said:
Of course, that is why I did not ask about those products. :grin:
I will happily bet none of those will be used.
That is news to me but I will take your word for it.
AFAIK the only terminals rated for ampacity calculations at 90 C where above 600 V.
weressl
Esteemed Member
iwire said:Of course, that is why I did not ask about those products. :grin:
I will happily bet none of those will be used.
That is news to me but I will take your word for it.
AFAIK the only terminals rated for ampacity calculations at 90 C where above 600 V.
Industrial LV switchgear employs withdrawable circuit breakers. The incoming and outgoing cables land on secondary bus extensions equipped with holes to accept any of the varied type bolted connections. There are MANY 600V/90C* rated connectors that can be installed at these points.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
weressl said:Industrial LV switchgear employs withdrawable circuit breakers. The incoming and outgoing cables land on secondary bus extensions equipped with holes to accept any of the varied type bolted connections. There are MANY 600V/90C* rated connectors that can be installed at these points.
The termination connection point is the important item not the connector.
I know of no UL listed overcurrent protective devices (i.e. UL489 breakers) rated for use with conductors sized using the 90? column even if they use drawout construction.
Switchgear built to ANSI standards is often available for use with conductors sized using the 90? column.
weressl
Esteemed Member
jim dungar said:
That is why is pointed out the bus extentions.
The breakers have nothing to do with the issue since the stationary plug in component is part of ther switchgear and rated to 50C* rise at 40C* ambient.
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
All of the circuit components are required to be rated for 90 degree C, that would also mean the enclosure.
I do not know the MSHA requirements, so disregarding those and ANSI standards, the listing is what will determine the credibility of being able to use the 90 degree C column.
Also, if the engineer stamps his calculations, it is a moot point.
I do not know the MSHA requirements, so disregarding those and ANSI standards, the listing is what will determine the credibility of being able to use the 90 degree C column.
Also, if the engineer stamps his calculations, it is a moot point.
weressl
Esteemed Member
Pierre C Belarge said:
That is just plain nonsense. The enclosure is not a current carying part.
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