Transformer Secondary Calculation 10' Rule

[Icanwyre]

Hi Moderator,

I stumbled into the gray area when I was trying to figure out a design issue on how to incorporate an existing manual transfer switch for a generator into a new 600amp 480volt 3phase 4wire Y electrical service. Customer currently has 400amp 240/120volt 1phase 3 wire service, 2hots, 1neutral. I would be upgrading that to the 3phase service. I spoke to POCO about the new service having unbalanced loads coupled with the fact I would have to rewire the building and was reasonably told "it is what it is". Will wait and see what happens when I fill out work request. The new service rated MDP has a 600 amp MCB and 30 branch breakers. So I don't have to rewire the building I plan on refeeding the existing 400 amp 1phase "service" panel with 2poles of a 3pole breaker from MDP through a new 100kVA 480Volt to 240volt 1phase transformer via the existing manual transfer switch. The existing 400amp panel's MCB would be more than 10' from secondary side of new transformer. To comply with NEC 2023 I plan on installing a 400amp 2 pole fused disco.
So here's the gray area question… in black bold…regarding turns ratio and calculations.

Is there a mistake in NEC 2023 Hardcover Article 240.21(C)(2)(4) Calculation Example on Page 198?
Turns ratio in NEC Article says 480/208. Mike Holt Forum Article posted 10/2/2010 reply#4 says turns ratio is 480/120.
Question: Who is correct? If both, what don’t I understand?

10/2/2010 Forum is written as follows:

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Overcurrent Protection for Transformers and Automatic Transfer Switches

• Thread starterMachiavelli999
• Start dateOct 2, 2010

Not open for further replies.

M

Machiavelli999​

Member

• Oct 2, 2010

#1

Got into a pretty heated argument recently with a colleague and hoping that you guys can help clear this up.

First, a 480 - 208/120V delta-wye transformer is being installed and is being fed on the primary side from a switchboard breaker. On the secondary side, is another breaker necessary? I said yes and in the NEC Article 240.4(F) it says it pretty explicitly

"Single-phase and multiphase transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device"

There are a couple of exceptions given (a single-phase 2 wire or a delta-delta 3 phase transformer can be considered protected on the secondary side just by having a primary breaker) but it seems pretty clear that secondary breaker is needed.

The second question is for an Automatic Transfer Switches, if I have a breaker protecting the Normal feeder and the Emergency feeder, do I need a breaker to protect the Load feeder. My colleague says no. I think yes, but I am not as sure about this one. Couldn't find anything in the code about this.

iwire​

Moderator
Staff member
Location
Massachusetts

• Oct 2, 2010

#2

Machiavelli999 said:

First, a 480 - 208/120V delta-wye transformer is being installed and is being fed on the primary side from a switchboard breaker. On the secondary side, is another breaker necessary? I said yes and in the NEC Article 240.4(F) it says it pretty explicitly

I agree with you, the transformer may not require secondary protection but the conductors do require protection.

The second question is for an Automatic Transfer Switches, if I have a breaker protecting the Normal feeder and the Emergency feeder, do I need a breaker to protect the Load feeder. My colleague says no. I think yes, but I am not as sure about this one. Couldn't find anything in the code about this.

Here I disagree with you. Assuming that the 'load feeders' rating is equal to or exceeds the rating of the normal and emergency feeder breakers you do not need to add another breaker.

M

Machiavelli999

Member​

• Oct 2, 2010

#3
Thanks iwire.
What is the technical explanation though?
In the NEC handbook it delves deeper into this and says that a single phase 120V overload could cause sufficient current to cause problems for the secondary conductors but not trip the breaker on the primary (480V) side. However, I don't understand how that could happen.
In a balanced or unbalanced wye system, the phase (Ian) currents are the same as their respective line currents (Ia). So, lets say I had a 75KVA (480 - 208/120V) transformer. I would have a 100A breaker on the primary side. On the secondary side, my conductors would be rated to handle 250A. If I had an overload on one of my single phase loads and it started drawing 300A. The primary side, 100A breaker would see 300 * (208/480) = 130A on one of the phases and would trip.
That is what my colleague is thinking and I can't really disprove this. Any thoughts?

Smart $​

Esteemed Member
Location
Ohio

• Oct 2, 2010

#4
Machiavelli999 said:
... The primary side, 100A breaker would see 300 * (208/480) = 130A on one of the phases and would trip.
...
The voltage ratio is 120/480, i.e. secondary to primary winding. Two primary lines, with no other loads, would only see 75A.

D

DARUSA​

Senior Member
Location
New York City

• Oct 2, 2010

#5
Machiavelli999 said:
The second question is for an Automatic Transfer Switches, if I have a breaker protecting the Normal feeder and the Emergency feeder, do I need a breaker to protect the Load feeder. My colleague says no. I think yes, but I am not as sure about this one. Couldn't find anything in the code about this.
Asuming we are talking about the transfer switch, the load side will always be protracted for the breaker on the normal feeders or if the transfer switch is actived the EM load side breakers will protect your load side wire on the TS
Now I assume that the wire size of your load side of the transfer switch is the same size as your normal power and your EM power wire size!!!

Not open for further replies.

 

[David Castor]

I'm not sure exactly what your question is to be honest. But, for 480-208/120 V Delta-Wye transformer, the actual winding turns ratio is 480 V to 120 V or 4 to 1. The wye connection of three 120 V legs provides the 208 V phase to phase.

 

[Icanwyre]

Is there a mistake in NEC 2023 Hardcover Article 240.21(C)(2)(4) Calculation Example on Page 198?
Turns ratio in NEC Article says 480/208. Mike Holt Forum Article posted 10/2/2010 reply#4 says turns ratio is 480/120.
Question: Who is correct? If both, what don’t I understand?

 

[wwhitney]

This is question is apparently subject to some debate, see https://forums.mikeholt.com/threads/matching-transformer-primary-conductors-to-primary-ocpd.2578757/

To me, it is clear that the worst case or highest achievable voltage ratio should be used, which for a 480V : 120V/240V single phase transformer would be 480:120 or 4:1. If you apply the wording without consideration of the underlying physics, then you can reach the conclusion that the secondary voltage rating is 240V, so the ratio is 480:240 or 2:1.

Cheers, Wayne

 

[Icanwyre]

Thanks for response Wayne.
And to clarify question. Here goes.

My question is how to do the calculation when applying the 10' secondary conductor protection criteria of NEC 2023 Article 240.21(C)(2)(4)?

The code article states ... "secondary conductors shall have an ampacity that is not less than the value of the primary-to-secondary voltage ratio multiplied by one-tenth of the rating of the overcurrent device protecting the primary of the transformer." In the hardcover version of NEC 2023 on page 198 there is an example of how to do the calculation. In Step 2 of the calculation code book says on a 480V primary to 208Y/120 secondary. In Step 2 of the calculation it says to determine the line to line primary-to-secondary voltage ratio, and then states voltage ratio is 480/208=2.31.

I'm not sure, but it looks like the following forum article I referenced earlier and will here again, uses similar reasoning about voltage ratio;

Overcurrent Protection for Transformers and Automatic Transfer Switches​

• Thread starterMachiavelli999
• Start dateOct 2, 2010

Note: the voltage ratio stated in forum article by reply#3 was corrected in reply #4 by Smart $. Smart $ states the voltage ratio secondary to primary is 120/480 not 208/480.

It appears the forum is discussing delta/wye transformers, same as code article, but the forum determines the ratio is 1:4.
Thus, I think we should have a similar line of reasoning between the code article and the forum article and the voltage ratio in the code article calculation should be 4:1 not 2.31:1.

What is NEC 2023 Article 240.21(C)(2)(4) voltage ratio referring to?
Line to line as they state in the calculation example, or could it be line to ground (for lack of a better wording) as in the forum?

 

[Icanwyre]

I read the thread posted by wwhitney and I now understand the issue.
I will follow the code until further notice. Line to line when doing the calc. Thanks to everyone for your comments.

 

[Icanwyre]

I read the thread posted by wwhitney and I now understand the issue.
I will follow the code until further notice. Line to line when doing the calc. Thanks to everyone for your comments.

Here's the thread...
This is question is apparently subject to some debate, see https://forums.mikeholt.com/threads/matching-transformer-primary-conductors-to-primary-ocpd.2578757/

 

[wwhitney]

I will follow the code until further notice. Line to line when doing the calc. Thanks to everyone for your comments.

The code language is not definitive, as it does not use the full text of one of the Article 100 terms related to voltage. Using the worst case voltage ratio is conservative, and IMO using a lower ratio is non-conservative.

Cheers, Wayne

 

[Icanwyre]

I did try to find a definition of voltage ratio and couldn't.
Very often meanings come down to their definition.
Reviewing the calculation method and adding a definition IMO should be on a task list for the CMP. I considered sending them a message regarding the article, but I thought it would be easier to discuss in the forum. Ha Ha.
I agree with your opinion using the lower ratio is non-conservative, and would add could result in a problem under the right conditions.
I will certainly be thinking of this issue and the one's in your thread and will reconsider how I do the calculations.
If I understand you correctly, it's good to hear I can use a more conservative approach and still follow the code.

 

[david luchini]

Is there a mistake in NEC 2023 Hardcover Article 240.21(C)(2)(4) Calculation Example on Page 198?
Turns ratio in NEC Article says 480/208. Mike Holt Forum Article posted 10/2/2010 reply#4 says turns ratio is 480/120.
Question: Who is correct? If both, what don’t I understand?

The primary-to-secondary voltage ratio for a 480-208/120V transformer is 2.31:1.

 

[wwhitney]

The primary-to-secondary voltage ratio for a 480-208/120V transformer is 2.31:1.

Obviously we disagree, as per the earlier thread. The physics says the 4.0:1 case should control.

I agree that if you apply the Article 100 definition of "Voltage (of a circuit)" to both the primary and secondary, the ratio is 2.31:1. Yet the language in 240.21 is not "primary-to-secondary circuit voltage ratio," so that approach is not definitive.

Cheers, Wayne