Does anyone know any code violation to splice AL cable underground with 750 Kcmil and 600 Kcmil? The original spec said for 500 Kcmil copper but it was subtituted with 600 Kcmil AL at 90C. All underground cables had been installed but it violated NEC to have 90C rated cable installed on 75C rated switchgear. Now, in order to fix the problem, the contractor wants to splice 600 Kcmil AL cable in the closest pull box with 750 Kcmil to the switchgear. I believe it is a code violation based on taps rule (NEC 240.12). Is there any code violation? Please correct me if I'm wrong. Thanks.
Splicing AL underground
- Thread starter ee77
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Provided you are able to find an approved/listed means, it would be legal.ee77 said:
There is no violation using a 90? cable under a 75? termination - as long as you use the 75? rating of the cable...600KCM AL is rated at 340Aee77 said:
750 KCM AL @ 75? is rated at 385A.ee77 said:
240.4(B) would allow the 600's at 75? to be used on a 400A CB.
240.12 Electrical System Coordination is not the correct article to attempt to cite.ee77 said:
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
Tap rules are about overcurrent protection. Conductors _must_ be protected from overcurrent; tap rules come into play if the OCPD _supplying_ a conductor is greater than its ampacity, and the conductor is being protected from overcurrent by the OCPD that it feeds.
For example, if a conductor has an ampacity of 100A, but it is supplied by a 200A breaker and feeds a 100A breaker, then tap rules apply.
If a conductor, in its conditions of use, is properly protected by its supply OCPD, then you don't need to worry about tap rules for that conductor.
If the 600kcmil Al conductors are properly protected by the supply OCPD, then this is not a tap situation. The ampacity of 600kcmil Al with 90C insulation is greater than the ampacity of 500kcmil Cu with 75C insulation, so if you really have 90C insulation, and if _everything_ connected to the conductor is rated for 90C (this includes the splices, the supply side lugs and gear, etc), and if the original 500kcmil Cu was properly protected, then this 600kcmil Al should also be properly protected.
As an aside separate from the original question: IMHO aluminium conductors spliced underground is asking for trouble. The conductors _will_ get wet, and there _will_ be corrosion. But as celtic says, if the splice is listed and approved, then this is legal.
-Jon
For example, if a conductor has an ampacity of 100A, but it is supplied by a 200A breaker and feeds a 100A breaker, then tap rules apply.
If a conductor, in its conditions of use, is properly protected by its supply OCPD, then you don't need to worry about tap rules for that conductor.
If the 600kcmil Al conductors are properly protected by the supply OCPD, then this is not a tap situation. The ampacity of 600kcmil Al with 90C insulation is greater than the ampacity of 500kcmil Cu with 75C insulation, so if you really have 90C insulation, and if _everything_ connected to the conductor is rated for 90C (this includes the splices, the supply side lugs and gear, etc), and if the original 500kcmil Cu was properly protected, then this 600kcmil Al should also be properly protected.
As an aside separate from the original question: IMHO aluminium conductors spliced underground is asking for trouble. The conductors _will_ get wet, and there _will_ be corrosion. But as celtic says, if the splice is listed and approved, then this is legal.
-Jon
Thanks for the explanation. According to manufacturer warranty, 75C ampacity rating shall be used for all cables connected to switchgear. The cable could have 90C rating but only 75C ampacity rating shall be used. In this case, 750Kcmil AL cable at 75C is equal to 500Kcmil at 75C. Change in size of grounded conductor is considered tap conductors according to NEC 240.23. Celtic mentioned code section 240.4 (B), is 750Kcmil AL cable the closest one to 400A circuit breaker? Or 600Kcmil is allowed to be protected by 400A circuit breaker since 400A is more common than 350A circuit breaker. I would think 750Kcmil AL is the closest one instead of 600Kcmil AL. If I had 350A going through this run (750Kcmil AL at 75C spliced with 600Kcmil Al at 90C), how do I determine the ampacity rating of the cable? based on 75C or 90C? I would think that the cables have to be rated at one temperature rating, not both. What do you guys think? Thanks once again.
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- Windsor, CO NEC: 2017
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- Service Manager
I disagree. There is a standard size 350A OCPD mentioned in 240.6(A), so the 340A-rated 600 AL conductor would have to be protected by a 350A breaker, not a 400A breaker.celtic said:
You're thinking of it kinda backwards. You start with your calculated load, then choose the conductor size, and then choose the OCPD.ee77 said:
It's not entirely accurate to start at an OCPD and work backwards, you end up repeating yourself until you get the right answer. So, it's not "is the conductor size closest to a 400A OCPD" the question should be, "what is the conductor size closest to my calculated load?"
I'm not going into the tap rules, because I don't have confidence in my knowledge of them, to be honest. I prefer to figure it out without taps if at all possible.
winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
240.23 does not define tap conductors in any way. It just permits you to have taps in the grounded conductors to match the conductor size change in the ungrounded conductors. Look at the rules for tap conductors. These rules come into play when conductors are being used with larger OCPD than would protect them. There are _no_ restrictions on size changes if the OCPD feeding the conductor properly protects that conductor.
If you had a 20A breaker feeding a circuit, then you could start with 10ga Cu conductors, switch to 6ga Cu conductors, and then end the run with 12ga conductors. None of these size changes would be taps.
You are trying to use pliers in place of a screwdriver. Evaluating this installation using tap rules is just confusing you, and not helping you figure out if these conductors are properly used. If these conductors are too small for the OCPDs protecting them, then tap rules won't apply; the conductors will simply be too small and will need to be replaced.
You seem to be getting hung up on the 90C temperature rating thing. The permitted temperature for conductors depends upon _several_ factors. The first, of course, is the temperature rating of the insulation itself. If the conductor has a 75C rating, then it can only be permitted to heat up to 75C. The next factor that influences the permitted temperature are the temperature limits of everything that the conductor touches.
For example, if the conductor terminates on a circuit breaker with a 75C terminal rating, then you can only permit this conductor to get to 75C. Or if the conductor is in a panelboard with an over-all 75C rating, then you can only use this conductor at its 75C rating. Or if the splice device that you use on this conductor has a 75C rating, etc.
But it is _very_ common to use the 90C rating if the conditions that would cause the cable to heat up to 90C are not present at the terminations. The most common example of this: conductors derated for bundling in a conduit. Say you have 90C conductors being used in conduit and a 75C rated panel. The ampacity _in the panel_ must be based on the 75C temperature rating, but the ampacity in the _conduit_ can be based upon the 90C conductor rating. This is useful, for example, if the conduit goes through a hot attic and you need to derate the conductors for high ambient temperature, or if there are many conductors in the conduit and you have to derate for conduit fill.
The point is that you don't have to use a single temperature rating over the entire circuit. You have to meet the temperature ratings at each and every portion of the circuit separately.
The section of 600kcmil can be calculated based upon its 90C ampacity if _everything_ that it touches is rated for 90C. This means that the 750kcmil conductors, the splices, the pull box, the conduit, and everything else that the 600kcmil touches has to have a 90C or better rating. Using the 750kcmil conductors at 75C because of the 75C switchgear, and using the 600kcmil conductors at 90C because everything they touch is rated at 90C, is in and of itself an acceptable (if IMHO sketchy) approach.
-Jon
If you had a 20A breaker feeding a circuit, then you could start with 10ga Cu conductors, switch to 6ga Cu conductors, and then end the run with 12ga conductors. None of these size changes would be taps.
You are trying to use pliers in place of a screwdriver. Evaluating this installation using tap rules is just confusing you, and not helping you figure out if these conductors are properly used. If these conductors are too small for the OCPDs protecting them, then tap rules won't apply; the conductors will simply be too small and will need to be replaced.
You seem to be getting hung up on the 90C temperature rating thing. The permitted temperature for conductors depends upon _several_ factors. The first, of course, is the temperature rating of the insulation itself. If the conductor has a 75C rating, then it can only be permitted to heat up to 75C. The next factor that influences the permitted temperature are the temperature limits of everything that the conductor touches.
For example, if the conductor terminates on a circuit breaker with a 75C terminal rating, then you can only permit this conductor to get to 75C. Or if the conductor is in a panelboard with an over-all 75C rating, then you can only use this conductor at its 75C rating. Or if the splice device that you use on this conductor has a 75C rating, etc.
But it is _very_ common to use the 90C rating if the conditions that would cause the cable to heat up to 90C are not present at the terminations. The most common example of this: conductors derated for bundling in a conduit. Say you have 90C conductors being used in conduit and a 75C rated panel. The ampacity _in the panel_ must be based on the 75C temperature rating, but the ampacity in the _conduit_ can be based upon the 90C conductor rating. This is useful, for example, if the conduit goes through a hot attic and you need to derate the conductors for high ambient temperature, or if there are many conductors in the conduit and you have to derate for conduit fill.
The point is that you don't have to use a single temperature rating over the entire circuit. You have to meet the temperature ratings at each and every portion of the circuit separately.
The section of 600kcmil can be calculated based upon its 90C ampacity if _everything_ that it touches is rated for 90C. This means that the 750kcmil conductors, the splices, the pull box, the conduit, and everything else that the 600kcmil touches has to have a 90C or better rating. Using the 750kcmil conductors at 75C because of the 75C switchgear, and using the 600kcmil conductors at 90C because everything they touch is rated at 90C, is in and of itself an acceptable (if IMHO sketchy) approach.
-Jon
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