Transformer calcs fed from MCC

[AmjadIzz]

Hey all,

I have a 3ph 75kVA transformer that is fed from MCC bucket with breaker of 200A from 2 phases only. So I have 3/0 from each phase (2 only) to the transformer. Now I know the ground wire is the EGC and I sized it to be #6. The confusion I have is because I'm running 2 sets of 2/0 in the secondary to ATS and then to multiple panels.
The SSBJ sized to be #2, the SBJ is sized to be #2, and the GEC for the transformer is sized to be #2. It feels weird kinda "tapping" #6 with two #2.
Am I missing something? I know it's not carrying current and I would argue the fault from the secondary is greater than the primary and the transformer GEC cover both. But want to make sure.

Second question:
I consider all the cable from the transformer to ATS to the junction box to panels to be SSBJ. Is that correct? The cable to each panel is smaller in size and so is the SSBJ for the panels.

 

[Elect117]

I wouldn't consider the bonding jumpers to be "Tapping" the equipment grounding conductor. When a fault occurs on the secondary of the transformer, it should go back to it's source, which is the transformer through the SBJ (where you bond the GEC to the grounded conductor/neutral). That low impedance path back to it's source will allow for the current to rise and trip the Overcurrent on the secondary side of the transformer. The primary side protection only protects the transformer and the primary conductor feeding the transformer. The equipment grounding conductor is sized based on the overcurrent device for carrying the fault current.

For supply side bonding jumpers, system bonding jumpers, etc., the code is written to take the largest ungrounded conductors in to consideration. It doesn't specify which side of the transformer, so if the primary has larger conductors then I would use those when selecting.

Inside of the transformer, you still bond the primary side's EGC to the same "bus" as the secondary side and the metal housing of the transformer so that a fault on the equipment clears.

Your supply side bonding jumper is ran to the first OCPD (overcurrent protective device) on the secondary side of the transformer. I am not entirely understanding the set up with the reduction in size since your wire on the secondary of the transformer must terminate in a single OCPD prior to changing the wire (ampacity of the circuit).

 

[AmjadIzz]

So my reasoning is correct. Different side of transformer require different GEC and ECG based on the fault size.
My secondary of the transformer is going to a junction box where from there goes to 3 different panels ( with smaller conductors now) .
so it is not single OCPD.

 

[david luchini]

So my reasoning is correct. Different side of transformer require different GEC and ECG based on the fault size.
My secondary of the transformer is going to a junction box where from there goes to 3 different panels ( with smaller conductors now) .
so it is not single OCPD.

That's a Code violation (240.21(C))

 

[AmjadIzz]

What is exactly ? and How come?

I looked at 240.21(C) but couldn't figure out what you mean? Could you elaborate please?

 

[Elect117]

What is exactly ? and How come?

I looked at 240.21(C) but couldn't figure out what you mean? Could you elaborate please?

There are multiple sections that require compliance with overcurrent protection in this install. You need to comply with panelboards, transformers, and conductors.

Transformers is the easiest to understand. Read 450.

Panelboards are required to have overcurrent protection in compliance with 408.36. So I would first ensure that they each have their own main breaker. If they are not installed, you need to read the times in which Overcurrent protection in the primary side of the transformer is allowable for the secondary side of the transformer.

Overcurrent protection of conductors in section 240 allows for taps (240.21).

The subsections of 240.21 describes where that overcurrent protection is required to be installed to protect a set of conductor.

So if you go to 240.21(C) and read that paragraph you will notice that you MUST meet the rules specified in (C)(1) to (C)(6). There is no rule from 1 to 6 that allows you to tap the secondary and protect it based on the smaller size.

From more of a technical standpoint, lets say you have a secondary conductor rating of 100A. You install 3 taps of 50A conductor and protect them at 50A.

What happens if all 3 taps pull 39A continuously? Well that secondary conductor is overloaded and is expected to fail. The primary overcurrent will not trip because all that heat will be eaten up in the transformer winding with no issue. Most transformers can be overloaded without any problems. Conductors on the other hand, can not.

So to ensure that the secondary conductors are properly protected you need to have an overcurrent protection at the secondary conductor.

 

[Elect117]

To make your install compliant, just install a fused disconnect ahead of the tapped secondary conductors.

You can also put each set of the tap conductors directly to the secondary landings inside of the transformer. You can have more than one set of conductor run from the transformer itself. Each of those runs, individually, must comply with 240.21(C).

 

[wwhitney]

I looked at 240.21(C) but couldn't figure out what you mean? Could you elaborate please?

It's actually the second sentence of 240.21: "Conductors supplied under the provisions of 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4."

So when you have multiple sets of secondary conductors, each set must originate at the transformer secondary terminal as per 240.21(C).

Cheers, Wayne

 

[AmjadIzz]

I been looking at the code and you're right.
So the ATS wouldn't be considered a OCPD in any way? I'm just asking if I'm missing something?

 

[Elect117]

So the ATS wouldn't be considered a OCPD in any way? I'm just asking if I'm missing something?

Does it provide overcurrent protection for the conductors on the secondary side of the transformer?

I am sure there are some brands of ATS that might but not every ATS does.