I need help sizing a large disconnect

[wehaveplans]

Hi everyone. We are installing two 2000 amp aluminum electrical busway (480Y) systems that will be feed from one 2500 kVa transformers. The busway taps all have over current protection that we have sized to allow a maximum amperage capacity of 1400 amps to ever travel though the busway. Between the transformer and the busway we of course need to have proper over current protection.We are looking at feeding two disconnects rather than large distribution gear since the disconnects are smaller and can be installed indoors for ease of access in case of an emergency.

I'm not fully educated as to how to properly size the disconnects and I need a bit of help. Essentially, the 600v rating is the only confusing part for me. A 1200 amp 600v disconnect is a common size, whereas a 1600 amp seems to be a custom and/or expensive unit. Mathematically speaking, we are running a maximum load of ~1,164 kVa. 1200 amp @ 600v gives a mathematical maximum load of ~1,247 kVa. Of course we are running 480v though the system, but does the fact that the gear rating is 600v change the sizing calculation?

 

[gadfly56]

Hi everyone. We are installing two 2000 amp aluminum electrical busway (480Y) systems that will be feed from one 2500 kVa transformers. The busway taps all have over current protection that we have sized to allow a maximum amperage capacity of 1400 amps to ever travel though the busway. Between the transformer and the busway we of course need to have proper over current protection.We are looking at feeding two disconnects rather than large distribution gear since the disconnects are smaller and can be installed indoors for ease of access in case of an emergency.

I'm not fully educated as to how to properly size the disconnects and I need a bit of help. Essentially, the 600v rating is the only confusing part for me. A 1200 amp 600v disconnect is a common size, whereas a 1600 amp seems to be a custom and/or expensive unit. Mathematically speaking, we are running a maximum load of ~1,164 kVa. 1200 amp @ 600v gives a mathematical maximum load of ~1,247 kVa. Of course we are running 480v though the system, but does the fact that the gear rating is 600v change the sizing calculation?

No, of course not. It just affects whether or not you can use it at your particular voltage. For the same VA, all things being equal, as you drop the voltage your current must increase.

 

[Jraef]

Current ratings have nothing to do with voltage ratings. You don't size the disconnect based on kVA at the switches voltage rating, you size it based on the amperage it will see at whatever the voltage is that will be going through it. So if the transformer secondary is 480V, then you size the disconnect for the amperage at 480V.

The voltage rating of the switch is only about separation to avoid flash-over and creepage between poles. They come in 300V (for 240V systems) and 600V for everything between 240V and 600V. We mostly use 480V here in the States, Canadians use a lot of 600V distribution, so one size fits all.

1200A is the physical limit of what we call a simple "safety switch" based upon a knife switch mechanism. Any more than that and the switch can't hold itself together safely during a fault. So above 1200A, it changes to what's called a "bolted pressure switch" (BPS) design, also called a "Pringle" switch or "Bolt-Lok" switch because of common manufacturer's names. they are a lot more expensive because at that much energy, they can be a lot more dangerous when something goes wrong.

If you want to get full 2000A from each busway, you will need a 2000A fused switch and that means a BPS or Power Circuit Breaker.

 

[kmh]

I need help sizing a large disconnect

No, 600 V is merely the insulation level and should not be used in your calculations. 480V is your distribution voltage.

 

[augie47]

Since you are dealing with a 480Y system keep in mind NEC 230.95 and 240.13 require GF protection in most cases where the overcurrent device is 1000 amps or greater. If you meet the criteria for the requirement your overcurrent device will need to incorporate GF protection

 

[wehaveplans]

Current ratings have nothing to do with voltage ratings. You don't size the disconnect based on kVA at the switches voltage rating, you size it based on the amperage it will see at whatever the voltage is that will be going through it. So if the transformer secondary is 480V, then you size the disconnect for the amperage at 480V.

The voltage rating of the switch is only about separation to avoid flash-over and creepage between poles. They come in 300V (for 240V systems) and 600V for everything between 240V and 600V. We mostly use 480V here in the States, Canadians use a lot of 600V distribution, so one size fits all.

1200A is the physical limit of what we call a simple "safety switch" based upon a knife switch mechanism. Any more than that and the switch can't hold itself together safely during a fault. So above 1200A, it changes to what's called a "bolted pressure switch" (BPS) design, also called a "Pringle" switch or "Bolt-Lok" switch because of common manufacturer's names. they are a lot more expensive because at that much energy, they can be a lot more dangerous when something goes wrong.

If you want to get full 2000A from each busway, you will need a 2000A fused switch and that means a BPS or Power Circuit Breaker.

Thanks for the very detailed response, it's quite a lot of help. I'm seeing lots of BPS's and Power Circuit Breakers in that size, which device is preferable? If they are comparable, is one easier to install and service than the other?

 

[wehaveplans]

Since you are dealing with a 480Y system keep in mind NEC 230.95 and 240.13 require GF protection in most cases where the overcurrent device is 1000 amps or greater. If you meet the criteria for the requirement your overcurrent device will need to incorporate GF protection

Our feeders are coming underground into the building and will only be around 4 to 6 feet in length. If I recall correctly from our conversation, this design will work just fine since the out of ground wire length is less than 10 feet.

I'm not 100% and I'm going to give those codes a look though when I have time to do some research.

 

[augie47]

It might be worth your while to give a bit more detail and obtain input from the Forum. There are some potential pitfalls looming.
For instances: are these services or feeders and if they feeders how are they presently protected.
(The requirement for GF protection has nothing to do with conductor length in tne building)

 

[jim dungar]

It might be worth your while to give a bit more detail and obtain input from the Forum. There are some potential pitfalls looming.

Some people on the forum have conniptions over a non-electrictian installing a device faceplate, but it appears they have no problem with DIY engineering.

People design electrical systems for a living. Most vendors will be glad to help you select equipment. But as Augie said, What are you missing?

 

[ActionDave]

Some people on the forum have conniptions over a non-electrictian installing a device faceplate, but it appears they have no problem with DIY engineering.

People design electrical systems for a living. Most vendors will be glad to help you select equipment. But as Augie said, What are you missing?

That is a great point. We kick out the guy who is trying to save a buck and not hire an electrician, we don't kick out the guy who is trying to save a buck and not hire an engineer.

 

[iwire]

Some people on the forum have conniptions over a non-electrictian installing a device faceplate, but it appears they have no problem with DIY engineering.

I see your point but at the same time in some areas engineering is part of the electricians job. Here we can, engineer what we install.

I am not saying we are good at it, just that it is legal for us and expected by some customers.

 

[jim dungar]

I see your point but at the same time in some areas engineering is part of the electricians job. Here we can, engineer what we install.

I am not saying we are good at it, just that it is legal for us and expected by some customers.

Most states probably say that you 'Design' what you install as they have separate licensing for Engineering.

 

[Jraef]

In Washington State (where the OP is from), a "Master Electrician" license allows for the type of design work he is doing here.

 

[jim dungar]

In Washington State (where the OP is from), a "Master Electrician" license allows for the type of design work he is doing here.

As is the situation in almost every state. BTW, the OP lists their occupation as Operations Manager.

It just concerns me when someone does not understand how equipment is labeled (maximum versus nominal) and how that information would be used in their design.
Equipment vendors and suppliers can provide quite a bit of valuable information and should not just be used as pricing services.

 

[Jraef]

As is the situation in almost every state. BTW, the OP lists their occupation as Operations Manager.

It just concerns me when someone does not understand how equipment is labeled (maximum versus nominal) and how that information would be used in their design.
Equipment vendors and suppliers can provide quite a bit of valuable information and should not just be used as pricing services.

My bad, I didn't look at his profile...

 

[Ingenieur]

is this the same OP asking about a data center?
wanted to feed his PDU's with 400/230

I agree
he needs to hire an engineer or a contractor that has one on staff
this is big power with dire (and expensive) consequences if not done correctly

the fact that he does not understand the function of the 600 vac vs 480 vac ratings ought to give fair warning

 

[Jraef]

I am closing this thread, in accordance with the Forum rules. This Forum is intended to assist professional electricians, inspectors, engineers, and other members of the electrical industry in the performance of their job-related tasks. However, if you are not an electrician or an electrical contractor, then we are not permitted to help you perform your own electrical installation work.


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[rbalex]

Some people on the forum have conniptions over a non-electrictian installing a device faceplate, but it appears they have no problem with DIY engineering.

People design electrical systems for a living. Most vendors will be glad to help you select equipment. But as Augie said, What are you missing?

I go through this often in the Hazardous (Classified) Locations forum. This is the reason I often refer the OP to the appropriate NEC Section(s) and suggest they discover the answer for themselves. I've told several I won't do their engineering for them. I might confirm or deny their (or someone else's) interpretation.