Tap Rule for feeder

[Zapp20]

Hi All,
I have a Chiller with a MCA of 1192 amps and a 2000 amp MOP and I need to tap into existing Switchgear to install a new section of gear which will house new breaker. There is no room to install a breaker in the existing Switchboard nor tie in a new section so the new section will be along side of the old section.. I'm trying to figure out the size of wire needed to feed my new breaker. Started to read code 240.21 but it got confusing. thanks hopefully this makes sense.

 

[Strathead]

Hi All,
I have a Chiller with a MCA of 1192 amps and a 2000 amp MOP and I need to tap into existing Switchgear to install a new section of gear which will house new breaker. There is no room to install a breaker in the existing Switchboard nor tie in a new section so the new section will be along side of the old section.. I'm trying to figure out the size of wire needed to feed my new breaker. Started to read code 240.21 but it got confusing. thanks hopefully this makes sense.

Two pieces of information are needed, I believe. First, what is the size of the main circuit breaker in the existing switchboard and will you be able to limit the length of the tapped wires to less than 10 feet? If yes to the second, then the wire size will be whatever parallel combination you choose that adds up to a MINIMUM circuit ampacity of 2000 amps (you can't use the "next higher breaker" rule), As long as the main breaker is not 10 times the 2000 amps, which I doubt it is.

 

[kwired]

How long is the tap conductor?

Also remember even if you have a 10 foot or less tap conductor doesn't mean you can't apply the 25 foot tap rules.

Either rule you need at least 2000 amps of conductor for the tap conductor though since you have a 2000 amp overcurrent device.

Only way you need larger would be if you had over 20,000 amp feeder that you are tapping from for the 10 foot rule or over 6000 amps feeder being tapped from for the 25 foot rule.

The rest of the details are a little more minor but different for each rule.

ETA: If you are tapping into service conductors 240.21 doesn't apply. Your conductors would only need to be at least the MCA yet could still have a 2000 amp OCPD.

 

[Carultch]

Hi All,
I have a Chiller with a MCA of 1192 amps and a 2000 amp MOP and I need to tap into existing Switchgear to install a new section of gear which will house new breaker. There is no room to install a breaker in the existing Switchboard nor tie in a new section so the new section will be along side of the old section.. I'm trying to figure out the size of wire needed to feed my new breaker. Started to read code 240.21 but it got confusing. thanks hopefully this makes sense.

240.21(B) is the section of rules that matter. Generally speaking, you need at least as much amperes of wire as you have OCPD into which that you terminate the tap conductors. You need that anyway, when the OCPD is greater than 800A.

There are several length/circumstance limiting rules in 240.21(B), and you follow whichever one applies to your case. Each has corresponding context rules to include as well. 10 ft / one tenth, 25 ft / one third, outside unlimited length, etc. They all have context rules. The "one tenth" or "one third" means that you need at least that fraction of ampacity of the main feeder in your tap conductors, which you will probably end up having anyway in large size taps. So if you make a 30A tap off of a 600A feeder where the 10 ft rule applies, you can't get away with just 30A of wire. You'd need at least 60A of wire. Go to 25 ft, and you'd need 200A of wire even though it is just a 30A tap.

 

[wwhitney]

Do (2011) 240.21(F) and 430.28 bear on this application? At first glance, they seem to allow a tap with an ampacity of only 1192 amps.

Cheers, Wayne

 

[infinity]

The 2000 amp rating is for starting current the actual conductors only need to be 1192 amps. Does this change anything?

 

[Strathead]

The 2000 amp rating is for starting current the actual conductors only need to be 1192 amps. Does this change anything?

In my opinion no, because the code section states the ampacity is... "not less than the device supplied by the conductor." Not that I don't see the question. Similarly, when you have conductors from a OCPD to a fused disconnect, what really says you can size those conductors to code 430 since, unless there is a section that specifically addresses this, it is only the conductors from the fuses on that are the motor branch conductors.

 

[kwired]

I should have looked at 430 before my last post.

I think 430.28 allows the feeder taps to be same size as the branch circuit conductors, and still has the same 1/10 or 1/3 capacity limits as the general 10 and 25 foot tap rules.

If you already have a ~1200 amp conductor, you need an awful big feeder to violate the 1/10 or 1/3 ampacity restriction.

 

[Zapp20]

Two pieces of information are needed, I believe. First, what is the size of the main circuit breaker in the existing switchboard and will you be able to limit the length of the tapped wires to less than 10 feet? If yes to the second, then the wire size will be whatever parallel combination you choose that adds up to a MINIMUM circuit ampacity of 2000 amps (you can't use the "next higher breaker" rule), As long as the main breaker is not 10 times the 2000 amps, which I doubt it is.

The switchboard is rated 2000amp and yes I can keep the run under 10 feet. I want to tap the buss on the gear.

 

[augie47]

The switchboard is rated 2000amp and yes I can keep the run under 10 feet. I want to tap the buss on the gear.

Tapping the buss can create its own set of problems. Many inspectors would not pass such an installed based on alteration of equipment and getting factory permission to do so it about as rare as hen's teeth.

 

[Strathead]

Tapping the buss can create its own set of problems. Many inspectors would not pass such an installed based on alteration of equipment and getting factory permission to do so it about as rare as hen's teeth.

More often than not the bus is parallel copper which also creates tapping dilemma