Sizing Primary and Secondary Transformer Load

[electricalintern]

How do I determine the correct amperage for sizing a Transformer
The major load on the secondary of a transformer will be a Roof Top Unit

Since for example, if there was a 75kVA transformer
Pri: 480/3P = 90.2A
Sec: 208/3P = 208.2A

Would I use a 100A breaker for the Primary and a 200 AMP breaker for the Secondary.

Or should I size the Primary as 125A breaker and the secondary as 225 AMP
Since the RTU will be a continuous load
(Based on the roundup rule)

 

[electricalintern]

Based on Table 450.3(A) [Location used was any location]
Looks like I can use either 100A or 125A for the primary and 200 AMP and 225AMP for the secondary.

Let me know if I am reading this table incorrectly?

 

[electrofelon]

Based on Table 450.3(A) [Location used was any location]
Looks like I can use either 100A or 125A for the primary and 200 AMP and 225AMP for the secondary.

Let me know if I am reading this table incorrectly?

You want table 450.3(B).

Your big concern with sizing the primary OCPD is to have it not trip when energizing the transformer. It's a trade off of cost vs likelihood of tripping on inrush I would go 125, if at all possible.

 

[electricalintern]

You want table 450.3(B).

Your big concern with sizing the primary OCPD is to have it not trip when energizing the transformer. It's a trade off of cost vs likelihood of tripping on inrush I would go 125, if at all possible.

Thank you for the help.

So based on 450.3(B), for the secondary protection, should go with 225 AMP or 260AMP (208.2*1.25) when connecting after the transformer.

I know Secondary protection is not required if the primary has protection only. But I want to see what size protection is recommended on the secondary side.

 

[augie47]

If you protect your primary at 125%, I would likely size my secondary protection to match the MOCP on the RTU,
You will need the correct OCP for the unit so there is no need to have a larger transformer OCP device feeding a smaller RTU OCP>
Note you still need to comply with 240.21(C) rules.

 

[jim dungar]

I like to start at the load and work my way to the transformer primary. I find I do less backtracking/tweaking of previous choices.

Once you know the OCPD for the combined loading you can pick a transformer size, then move to 240.21(C), and finally to 450.3 for your transformer primary protection.

 

[electricalintern]

After reading Section 240.21(C), I would need to use section 240.21(C)(6), however I am still unclear how to size the secondary size

FYI: I have the NEC 2017 Handbook and when I was looking at the example calculation. I was confused why they have 203V in the voltage ratio calculation

 

[david luchini]

After reading Section 240.21(C), I would need to use section 240.21(C)(6), however I am still unclear how to size the secondary size

Are you installing a panelboard?

FYI: I have the NEC 2017 Handbook and when I was looking at the example calculation. I was confused why they have 203V in the voltage ratio calculation

It's a typo.

 

[augie47]

1. Is the transformer primary protected at 125% of the kva rating (next size up allowed) ??
2. What is the load ? (only the RTU ?)
3. What is the nameplate data for the RTU ??

If the answer to #1 is "Yes" then the size of the secondary protection is almost not relative to the transformer and can be selected based on the load and 240.21 tap rules.

Determine what protection you need for the load and size your transformer secondary protection by that keeping in mind the requirements of 240.210C) as far as the length of the conductors to your protection device (normally 10 or 25 ft)

 

[electricalintern]

Are you installing a panelboard?

Yes, they are installing a panel board with a MCB feeding from the 75kvA transformer

1. Is the transformer primary protected at 125% of the kva rating (next size up allowed) ??
2. What is the load ? (only the RTU ?)
3. What is the nameplate data for the RTU ??

If the answer to #1 is "Yes" then the size of the secondary protection is almost not relative to the transformer and can be selected based on the load and 240.21 tap rules.

Determine what protection you need for the load and size your transformer secondary protection by that keeping in mind the requirements of 240.210C) as far as the length of the conductors to your protection device (normally 10 or 25 ft)

There will be a Panel feeding the RTU

If it was option #1: Still unclear how i use 240.21 to size the secondary

 

[augie47]

240.21(C) basically gives you the requirements for the secondary conductors in terms of size and length,.
In your case, I would start by selecting a conductor with an ampacity equal to or greater than the rating of you panel main breaker and determine the length and see which tap rule applies (C-1 thru C-6) and see if you comply.
If the panel is beyond the length limit of the secondary rule you will need to add an overcurrent device that meets the rule as far as distance.

If you have the transformer protected at 125% on the primary there are almost no restrictions on the secondary size/length except those set forth in the C-1 rules..

What size main is in the panel being fed and how far is it from the transformer ?

 

[electricalintern]

240.21(C) basically gives you the requirements for the secondary conductors in terms of size and length,.
In your case, I would start by selecting a conductor with an ampacity equal to or greater than the rating of you panel main breaker and determine the length and see which tap rule applies (C-1 thru C-6) and see if you comply.
If the panel is beyond the length limit of the secondary rule you will need to add an overcurrent device that meets the rule as far as distance.

If you have the transformer protected at 125% on the primary there are almost no restrictions on the secondary size/length except those set forth in the C-1 rules..

What size main is in the panel being fed and how far is it from the transformer ?

There is no Main Panel.
We are feeding the 75kVA transformer directly from the 100 AMP 480V meter in the utility room.

This distance will be around 100ft to the transformer

 

[augie47]

I was referencing the panel on the secondary side. How far is it from the transformer to the secondary panel ?

 

[augie47]

I may have confused the situation.
Back to the basics.
You have to take two Code Sections into account, Art 450 for transformer protection and Art 24-0 for conductor sizing/protection.

You gave the basic information:
Since for example, if there was a 75kVA transformer
Pri: 480/3P = 90.2A
Sec: 208/3P = 208.2A

To protect the transformer most folks apply the 125% rule to the primary so 90.2 X 1.25 = 113 so you can use a 125 amp breaker and a primary conductor with an ampacity of 125 amps.
Once you have protected the transformer primary at 125%, from an Art 450 standpoint , it really makes little difference what you do on the secondary side as the transformer is protected,
The secondary conductors do have to be protected and that's where Section 240.21(C) takes over.
The secondary conductors are protected by the overcurrent device they fee so they need to be sized relative to that device.
If you choose to supply a 225 amp breaker on the secondary then you need to use a conductor with a 225 amp rating.
Since the conductors are not protected right at the source (transformer) the Code limits the length to the point they do receive protection.
Normally this is 10ft but there are 25 ft and longer allowances (240.21(C)(1) - (C)(6) with specific requirements for each,

Mu suggestion was, if your were not feeding a panel, to select a breaker that matched the RTU requirement so it could serve two ends, providing protection required by 240.21(C) and provide protection for the RTU.
(This would only work if the conductors to the breaker met the length, etc. requirement t of the tap rule)





,.

 

[electricalintern]

I may have confused the situation.
Back to the basics.
You have to take two Code Sections into account, Art 450 for transformer protection and Art 24-0 for conductor sizing/protection.

You gave the basic information:
Since for example, if there was a 75kVA transformer
Pri: 480/3P = 90.2A
Sec: 208/3P = 208.2A

To protect the transformer most folks apply the 125% rule to the primary so 90.2 X 1.25 = 113 so you can use a 125 amp breaker and a primary conductor with an ampacity of 125 amps.
Once you have protected the transformer primary at 125%, from an Art 450 standpoint , it really makes little difference what you do on the secondary side as the transformer is protected,
The secondary conductors do have to be protected and that's where Section 240.21(C) takes over.
The secondary conductors are protected by the overcurrent device they fee so they need to be sized relative to that device.
If you choose to supply a 225 amp breaker on the secondary then you need to use a conductor with a 225 amp rating.
Since the conductors are not protected right at the source (transformer) the Code limits the length to the point they do receive protection.
Normally this is 10ft but there are 25 ft and longer allowances (240.21(C)(1) - (C)(6) with specific requirements for each,

Mu suggestion was, if your were not feeding a panel, to select a breaker that matched the RTU requirement so it could serve two ends, providing protection required by 240.21(C) and provide protection for the RTU.
(This would only work if the conductors to the breaker met the length, etc. requirement t of the tap rule)





,.

Thank you for this clear explanation

I was referencing the panel on the secondary side. How far is it from the transformer to the secondary panel ?

It is about 25ft. Hence why I was looking at code section 240.21(C)(6).
But I was curious how big the secondary panel can be based on the size of the 75kva Transformer.
For example the Secondary came out to be 208/3P = 208.2A. Based on Table 450.3(B), you can size the secondary with primary protection at 208.2*1.25 = 260.3A.
Can you size it up to 300 AMP or should keep it at 225AMP?

I trying to determine the maximum load a 75 kVA transformer can be sized on the secondary side.

 

[augie47]

Take another look at Table 4503(B) and you will note that there is no restriction on secondary conductors when you have 125% primary protection.
So with your 125% primary you can ignore your "225 amp" calculation and install whatever secondary protection you desire as long as you follow the rules in 240.21
Assume for a moment you project a future load of 600 amps at 208 so you install a 600 amp panel and parallel 350s feeding it but at present your load is only 100 amps so you install a 75kva transformer and will upgrade later. That would be compliant.

 

[Elect117]

Thank you for this clear explanation


It is about 25ft. Hence why I was looking at code section 240.21(C)(6).
But I was curious how big the secondary panel can be based on the size of the 75kva Transformer.
For example the Secondary came out to be 208/3P = 208.2A. Based on Table 450.3(B), you can size the secondary with primary protection at 208.2*1.25 = 260.3A.
Can you size it up to 300 AMP or should keep it at 225AMP?

I trying to determine the maximum load a 75 kVA transformer can be sized on the secondary side.

When using a transformer to feed a panelboard, do not forget to protect the panelboard in accordance with 408.36.

All the equipment (transformers, AC units, Panelboards, motors, etc.) can have their own requirements you need to meet at a minimum. It can be confusing in the beginning.

Most of the time, if you meet the requirements of Art.240, design for FLA, and obey the manufacturer, you will be okay. You are allowed to round up, doesn't mean you have to. You can size protection lower than the ampacity of the conductors as long as it is still larger than the load to be served and there are no mechanical/physical issues (max wire size landing on breakers, space in the panelboard, etc.).

A 75KVA transformer should only feed a continuous load of 208A unless otherwise noted by the manufacturer on allowable overloading.