Integrating a New 9kVA Load into Existing Electrical Setup: Seeking Guidance

[VENgineer]

Hello everyone,

I'm a new electrical engineer tasked with completing a project initiated by another engineer. I find myself in a specific situation regarding an installation in a warehouse that exclusively powers air conditioning equipment, including:

• A 180kVA Condenser Unit (CU)
• A 40kVA Air Reheat Unit (ARU)
• A 9kVA Air Curtain Unit

My question is as follows: We have a 600A 3-phase main disconnect that feeds into a junction box (gutter), from which power is distributed to the first two aforementioned loads, with a 500A disconnect for the CU and a 150A disconnect for the ARU.

I need to incorporate the third load, the 9kVA Air Curtain Unit, into the system. Is it possible to connect this load to the junction box (gutter)? We don't have a dedicated panel for connecting loads, so I am contemplating installing a disconnect near the load and routing power from the junction box.

Is this a feasible approach? Any advice on how to proceed correctly?

 

[petersonra]

It can be done. The devil is in the details, including the tap rules.

Junction boxes are not the same thing as gutters.

How are the existing loads connected inside the junction box?

 

[VENgineer]

There isn`t installed anything yet. I received this job yesterday because the previous engineer left the office, and I'm continuing his work.

A summary of the one-line diagram is as follows:

• From the meter, there's a 600A disconnect leading to two sets of 4 cables sized 350KCM each.
• From the disconnect, there's a 'gutter 8x8' (it appears to be more like a junction box, but that's what the plan says), with two sets of 4 cables sized 350KCM each.
• From the junction box, the power splits to two loads. One side goes to a 500A disconnect with two sets of 250KCM cables and one set of 1/0G cable.
• The other side goes to an ARU with a 150A disconnect. This side has 3 cables sized 1/0 and 1 cable sized 6G.

The third load only requires #6 wiring to its 40A disconnect.

Can I connect all three loads together using Polaris connectors? I've never done something similar before, which is why I have this question, especially considering the significant difference in cable sizes.

 

[winnie]

So your job is engineering a system which an electrician will install, and it sounds like you are kind of new to the requirements that need to be followed. One serious problem we have here is that we don't know what you don't know, and we don't know what you don't even know to ask. I urge you to have your plan reviewed by the electrician you will be working with, otherwise you can expect some expensive change orders.

1) Yes, you can 'split' the feed to several disconnects.

As @petersonra states, the most important issue here is the 'tap rules', which set the maximum length and the minimum ampacity of 'tap conductors'. A 'tap conductor' is protected from overcurrent at the end near the load, rather than the more easily understood approach of protecting the conductor at its supply end.

For example the 1/0 conductors feeding the ARU have a 75C ampacity of 150A, and are protected at the supply end by a 600A breaker, but are protected from overload by the 150A breaker at the load end.

See NEC 240.21 for the various tap rules.

2) Make sure you understand the requirements for parallel conductors. A parallel conductor must be joined at both ends to be considered a single conductor. So you will need connectors suitable for 6 conductors ranging in size from 350kcmil to #6. Polaris makes such, for example IPLD350-6.

There are probably other issues that need to be confirmed to insure a safe and code compliant installation.

-Jon

 

[don_resqcapt19]

In my opinion the 8" x 8" wireway is much too small. I would never even think about making those connections in that size wireway.

Don't forget that you have to have power distribution blocks or other type of connector that connects to both of the 350s on the line side and all of the load side conductors. I would be looking something like this.

If you are using the 25' tap rule, you will need conductors with an ampacity of 200 amps for the third load. You may need to use an oversized disconnect with fuse reducers, as 200 amps of conductor will not fit in a 60 amp disconnect. It might in a 100 amp one, but probably not.

 

[petersonra]

Don't forget that you have to have power distribution blocks or other type of connector that connects to both of the 350s on the line side and all of the load side conductors. I would be looking something like this.

My guess is by the time you get done with everything, a small 3 phase panelboard might end up being the most cost effective solution.

Incidentally, I noticed the SCCR of the distribution block Don suggested is only 10 kA. What is the available SCC at the junction box?

 

[don_resqcapt19]

My guess is by the time you get done with everything, a small 3 phase panelboard might end up being the most cost effective solution.

Incidentally, I noticed the SCCR of the distribution block Don suggested is only 10 kA. What is the available SCC at the junction box?

I never even looked at that...good catch.

 

[winnie]

My guess is by the time you get done with everything, a small 3 phase panelboard might end up being the most cost effective solution.

Incidentally, I noticed the SCCR of the distribution block Don suggested is only 10 kA. What is the available SCC at the junction box?

A good example of the many details that go into an electrical install.

-Jon

 

[VENgineer]

Thank you for your responses. I am the company's engineer-in-training. Everything I do goes through review and approval by senior engineers, who provides me with instructions for any necessary corrections. However, I'm someone who likes to research on my own and find solutions so that I can deliver a well-done job from the start and learn effectively.

Based on what was mentioned earlier, I understand the following: The third load (ACU 40A) is the farthest one (approximately 250 feet away), so complying with NEC 240.21 and sizing the cable for 200A would unnecessarily increase the installation cost.

Therefore, installing a 600A main circuit breaker panelboard to handle all three loads with their respective protections (500A, 150A, 60A) would likely be the best decision. I only had the junction box in mind because the previous person did it that way, and due to my inexperience, I continued with that approach. But thank you, and I'm very grateful for any comments or advice.