Parallel Emergency Loads

[JimmyJunior]

Please consider the following:

1. Existing E/G system is a random access, parallel, three E/G switchgear.
2. A 3000AF drawout power breaker on E/G switchgear is presently serving a 480Vac, 1200A load using 4/phase 600MCM Al conductors. The linear distance is approx. 1000ft.

3. A new 3000A, 3 phase, 4W Cu busduct feeder was installed to replace the Al conductor feeder.

4. A last minute decision was made to retain the exisiting Al feeder fed from the 3000AF breaker and conncect the Cu busduct to the same breaker using 600MCM Cu conductor. To do this 6 new Al/Cu lugs/phase will be added to the breaker output. Each feeder will be serving separate loads. The power breaker rating plug will be increased to 3000A.

Some of us feel that the busduct feeder should utilize a separate breaker. This will provide an opportunity for better coordination, diversity, selectivity, etc.

Opinions please.

 

[rbulsara]

Besides being a completely unprofessional and stpiud idea there are other Code issues too if you are anywhere in the USA or NEC domain.

Just too many issues and I would simply distance myself from such installations or design. If you are a junior engineer working with bunch of "experienced" people who come up with such clap-trap ideas, sooner you leave them better would be the prospect of your future career!

1. What is a random access E/G system? (not that it matters here).

2. It appears that 3000A busduct was installed replace the 1200A feeder so it is going to the same general destination as the 1200A feeder. Most you can get out is 3000A so what good does the 1200A feeds do?

I hate to give free professinal advice to such "designers" but here it is: Why not install a distributio board with at end of the new 3000A busduct and install a 1200A brach breaker in that distribution board to feed the 1200A load?. Reuse the part of the original 1200A condcutors as necessary to save money.

3. The arramgement you describe would be considered a Tap of the feeder. Refer to tap rules in the NEC article 240, and none of which apply here. Most that can be inferred is the tap must end in its own set of fuses or a breaker.

4. Are you upgrading the 1200A feed itself to 3000A by adding more conductors? (Dont ask me why on earth you would do that) You cannot protect a 1200A feed by a 3000A breaker.

 

[JimmyJunior]

rbulsara:

Initially, the 3000AF had a 1200AT plug to comply with NEC and protect the 1200A Al Feeder. This feeder services a dedicated load of HVAC equipment (smoke mode control exhaust fans, elevators, central station lighting).

A 3000A busduct was installed and will feed a separate dedicate load of similar equipment and systems.

The total load on the E/G system will be within its continous and starting load ratings.

The decision to re-plug (3000AT) the 3000AF power circuit breaker and feed both the 1200A feeder and the 3000A busduct was made by management to save money.

Some of us are trying to convince management that a separate appropriately sized breaker must be used to allow us to provide the proper, coordinated, protection.

Concerning your comment, it is the other way around, the folks trying to convince management to use a separate breaker are experienced graduate engineers (I am the only one with a PE though); the group that has convinced management to use the same breaker are relatively young electricians with electrical construction and installation experience under their belts.

 

[davidbeach]

There is no permissible means of protecting a 1200A feeder with a 3000A breaker. The "best" (??) you could do would be to run the 1200A feeder outside the gear to a free standing 1200A breaker and have meet the tap rules for the 1200A feeder between the 3000A breaker and the 1200A breaker. But since you still have the expense of both the 3000A breaker and the 1200A breaker (no way around that for a safe and legal installation) the far better way would be a 1200A and a 3000A breaker in the gear.

 

[rbulsara]

Jimmy:

Well now you have some more ammunition. At the least put your opinion in writing to the "management".

Wish you well.

 

[waross]

Are you subject to the NEC?
Does this installation have to be inspected?
I would estimate the cost of doing this installation plus the cost of changing it when the AHJ rejects it and compare that to the cost of doing it right in the first place in my submission to management.
Consider your liability as the only PE around if there is an issue and people are looking for someone to blame.

Bill
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"Why not the best?"
Jimmy Carter