Power Distribution to Multiple panels - OCPD requirements (Repost)

[jfabry]

Initially my thread got locked asking for clarification on my occupation. I am reposting as I was not able to reply to the question on my related occupation. I am an Electrical Engineering Panel Designer looking for clarification on a NEC code standard.

I have a NEC Code question on the tap rule in which I have read through the NEC240.21 multiple times trying to interpret what is being stated. Looking for some clarification on this forum.

I have an industrial piece of equipment that has multiple panels with one panel (Panel #1) servicing power to other panels on our equipment. In this instance I have 2 panels.

Panel #1 (Main Power Panel)- has 100Amp OCPD

This 100A OCPD is feeding another Panel (Panel #2) that has a Power Distribution Block on it. The conductors are sized appropriately based on the OCPD (100A) feeding this Power distribution block.

Is it a requirement that this panel (Panel #2) have a OCPD prior to the Power Distribution Block? Does the length of the conductors from Panel #1 to Panel #2 also having any outcome in determining the requirement?

Thanks in advance for your review.

 

[don_resqcapt19]

Your description is not clear to me. Can you show a one-line drawing.

 

[jfabry]

Circuit Example

Provided is a link to the image of the schematic circuit I am trying to describe.

 

[charlie b]

If I correctly understand your question, then as long as the distribution block is rated at 100 amps or higher, then it does not need an OCPD of its own. It will be protected by the upstream 100 amp OCPD, as will also the feeder conductors from Panel 1. And there is no length limit.

Part of your difficulty comes from the fact that you are not dealing with a "tap." Article 240.21 does not come into play for your project. Imagine a 100 amp breaker serving a 100 amp (non-continuous) load via conductors rated for 100 amps. Imagine a junction box somewhere along that circuit. Now go into that JB and attach wires rated only for 50 amps. Run those wires to a 50 amp load, and don't put a 50 amp breaker inside the JB. Neither that load nor those wires could be protected by a 100 amp breaker. That is a "tap," and it does involve some risk. We accept that risk because we minimize the opportunity for a short circuit along the 50 amp wires. We limit the length of the tap conductors, we protect them against physical damage, and we put a 50 amp breaker at the end of the tap conductors (i.e., before they hit the 50 amp load). That is what 240.21 is all about.

 

[charlie b]

Circuit Example

Provided is a link to the image of the schematic circuit I am trying to describe.

I can't view that example. I do not have an account with that site.

 

[jfabry]

Thanks for your explanation. Please let me know if your explanation still applies based on image below.

 

[charlie b]

Please let me know if your explanation still applies based on image below.

It does, but your initial description left out important information. Again, presuming Panel 2 itself is rated for 100 amps, you don't need an OCPD where your arrow is pointing.

However, you are showing #6 and #14 wires with no upstream protection other than the 100 amp fuses in Panel 1. As shown, you not only have a code violation, you also have a very dangerous situation. Everything carrying current must be protected against overcurrent at the limit of its ampacity, and that protection must be where the current first enters the item. The tap rules give us limited freedom to bypass that requirement, and your image goes beyond those limits.

But then again, none of this might matter at all. Is this "industrial equipment" listed by its manufacturer? Was it purchased with the interior wiring and connections as shown in your diagram? If so, the NEC does not apply, and the equipment can be used in whatever way its listing says it can.

So tell us, is this being put together on site or is it a listed set of components? It it is not listed, then the NEC does apply, and my comments at the beginning of this post also apply.

 

[jfabry]

Can you clarify please how the image goes beyond those limits and is in violation as you state the tap rules give us some limited freedom to bypass certain requirements? I would really like to understand and appreciate your comments greatly. Wouldn't the tap rule in this instance apply from the secondary of the distribution block to the fuses downstream allowing for the conductor to be sized per the fuses downstream. These connections would meet all the requirements of 240.21.B.1. Or is it no longer valid because we are connecting to another overcurrent protection device, therefore the conductors from the secondary of the DB need to be sized for 100A to each of the fuse blocks downstream?

 

[wwhitney]

You can apply the 240.21(B) tap rules to the #6 and #14 conductors. So they are compliant if short enough and the other conditions are met.

Cheers, Wayne

 

[charlie b]

Wayne, where the #6 wires connect to the distribution block, that is a tap. Where the #14 wires connect to the #6 wires, that is a tap of a tap. To me, the code language is not clear on whether this is allowed. Your thoughts?

 

[charlie b]

jfabry, two questions: 1. Is this stuff being assembled on site, as opposed to being a manufactured product? 2. What is downstream of the three fuse blocks at the bottom of the page?

 

[augie47]

As drawn you have have the #14s tapped off the #6 which, as charlieb notes, would be a violation. Do they actually connect in that manner or do the 14s connect to the distribution block ??

 

[wwhitney]

Wayne, where the #6 wires connect to the distribution block, that is a tap. Where the #14 wires connect to the #6 wires, that is a tap of a tap. To me, the code language is not clear on whether this is allowed. Your thoughts?

I assumed the distribution block had 3 sets of load side terminals, for the 3 separate taps. I agree that the #6 is a tap, and it may not then directly supply the #14.

Cheers, Wayne

P.S. As devil's advocate, suppose I run #14 less than 10' from the distribution block to the OCPD of 15A or less, compliant with the other requirements of the 10' tap rule. May I stick a junction box in the middle of that less than 10' run and splice my #14s (1 in, 1 out) there?

 

[jfabry]

Charlie,

All of the downstream fuses connect into the secondary of the distribution block (8 load connections), so it is not a tap of a tap.

1) Is this stuff being assembled on site, as opposed to being a manufactured product?

• This is assembled onsite from an OEM supplier.

2. What is downstream of the three fuse blocks at the bottom of the page?

• 60A is for a Servo Power Supply
• 6A is for a 480V to 24 Volt Power supply.
• 10A is for protection of a 3KVA XFMR

I would 100% agree that we would be in violation of a tap of a tap if this was the case.

 

[jfabry]

Thank you all for the dialogue on this topic.

 

[zbang]

1) Is this stuff being assembled on site, as opposed to being a manufactured product?

• This is assembled onsite from an OEM supplier.

To rephrase the question-
Does this stuff comprise a single manufactured unit, possibly shipped in sections, designed to be assembled only in certain way(s) as directed by the manufacturer?

An example could be a multi-section machine where the installation instructions specify a 1 1/2" conduit containing three 2awg THHN wires between panels #1 and #2 to be supplied and installed by the customer.

 

[jfabry]

In this instance, both panels are assembled by the OEM supplier. Yes, they are shipped in sections and the connections between both panels is done on site with provided materials and information from the OEM supplier. A #2AWG 3 Conductor Tray Rated cable is installed in a wire tray mounted on the machine between panel 1 and panel 2.

 

[charlie b]

In this instance, both panels are assembled by the OEM supplier. Yes, they are shipped in sections and the connections between both panels is done on site with provided materials and information.

Then I will say that the NEC does not apply, and the installation is acceptable.