NEC 450.3 Transformer sizing

[mto12345]

Good morning,
This question has to do with cutoff secondary amperage I can have on a 75KVA transformer, before I must upsizing the transformer from the 75KVA to 112.5KVA understanding the threshold per NEC 450.3. Primary voltage is 277/480V and secondary voltage is 208V. I understand that per power formula the current on the secondary side is 208A but my question is how the 125% requirement is factored into this.

 

[augie47]

The 125% of the current (primary or secondary) is the allowance to provide transformer protection.
The transformer can safely deliver power at 125% of it's rating however prolonged overloading will likely shorten it's life.
You can search the Forum as there have been threads addressing this,.

Here is one:

What happens if a utility transformer exceeds its ratings temporarily?

I'm trying to get basic knowledge of what typically happens when a pad mounted utility transformer exceeds its ratings load temporarily. What is usually their ratings threshold. So for example lets assume we have a 500KVA transformer and the load it is feeding temporarily spikes (5-10minutes to...

forums.mikeholt.com

One of the wiser members of the Forum posted this:
But for "dry type" transformers that we typically see in distribution equipment, the rating is the rating. So unless it specifically STATES a rating higher than it does, you would be in violation of 110.3(B) if you tried to use it that way. However, transformers have a TEMPORARY OVERLOAD capability without being damaged based on NEMA design criteria as follows:

• 200% nameplate load for one-half hour
• 150% load for one hour
• 125% load for four hours

NEMA ALSO requires that there is a 50% load preceding and following the overload. So it's not a good idea to COUNT ON this overload capability unless you can control ALL aspects of the load.

 

[mto12345]

The 125% of the current (primary or secondary) is the allowance to provide transformer protection.
The transformer can safely deliver power at 125% of it's rating however prolonged overloading will likely shorten it's life.
You can search the Forum as there have been threads addressing this,.

Here is one:

What happens if a utility transformer exceeds its ratings temporarily?

I'm trying to get basic knowledge of what typically happens when a pad mounted utility transformer exceeds its ratings load temporarily. What is usually their ratings threshold. So for example lets assume we have a 500KVA transformer and the load it is feeding temporarily spikes (5-10minutes to...

forums.mikeholt.com

OK so my question is does the secondary amperage need to be @ 166.4A so the 208A (166.4* 1.25=208A) max current is not reached or is it 260A (208*1.25=260A). Also if i size the primary OCDP at 125% do I also need to size the secondary OCDP at 125%?

 

[augie47]

As shown above ideally you would not exceed the 208 amps for an extended period.
If the primary is protected at 125% the transformer has the required protection and the secondary protection would be 240.21(c) conductor protection.
I'll let folks smarter than me address the phase balancing issue.

 

[wwhitney]

This question has to do with cutoff secondary amperage I can have on a 75KVA transformer, before I must upsizing the transformer from the 75KVA to 112.5KVA understanding the threshold per NEC 450.3. Primary voltage is 277/480V and secondary voltage is 208V. I understand that per power formula the current on the secondary side is 208A but my question is how the 125% requirement is factored into this.

My understanding is that a 75 kVA transformer with a 208Y/120V secondary, barring high ambient temperature or blocked cooling arrangement, can deliver 208A on each of its secondary conductors all day long without any problems. That is, the rated secondary current of 208A is a continuous rating.

So as long as your calculated load (without any 125% continuous factors, as we already have a continuous rating) and actual current are at most 208A per leg, a 75 kVA transformer works. But if some of your loads are continuous, and you aren't use 100%-rated OCPD, then you will need to apply a 125% continuous use factor in sizing those OCPD. [This is a limitation of the OCPD only.] The result is that your secondary OCPD may need to be up to 125% of the rated secondary current (and rounded up to the next larger size if necessary.)

That just addresses the question of determining if your transformer is the correct size, and one reason why your secondary OCPD may need to exceed the transformer's rated secondary current. The rules in 450.3 are about protecting the transformer itself; the rules in 240.21(C) are about protecting the transformer secondary conductors.

Cheers, Wayne

 

[mto12345]

My understanding is that a 75 kVA transformer with a 208Y/120V secondary, barring high ambient temperature or blocked cooling arrangement, can deliver 208A on each of its secondary conductors all day long without any problems. That is, the rated secondary current of 208A is a continuous rating.

So as long as your calculated load (without any 125% continuous factors, as we already have a continuous rating) and actual current are at most 208A per leg, a 75 kVA transformer works. But if some of your loads are continuous, and you aren't use 100%-rated OCPD, then you will need to apply a 125% continuous use factor in sizing those OCPD. [This is a limitation of the OCPD only.] The result is that your secondary OCPD may need to be up to 125% of the rated secondary current (and rounded up to the next larger size if necessary.)

That just addresses the question of determining if your transformer is the correct size, and one reason why your secondary OCPD may need to exceed the transformer's rated secondary current. The rules in 450.3 are about protecting the transformer itself; the rules in 240.21(C) are about protecting the transformer secondary conductors.

Cheers, Wayne

Thanks Wayne so if i understand this right long as my secondary amp (load) is less than 208 Amps, i am compliant to the NEC code.

 

[d0nut]

I think you may be confusing yourself by converting between amps and VA a bunch of times and losing track of your demand factors. The load calculations shown in your panel schedules are done in kVA. Ignore the amps. Look at your demand load in kVA. Select a transformer larger than that number. Profit.

 

[augie47]

I think you may be confusing yourself by converting between amps and VA a bunch of times and losing track of your demand factors. The load calculations shown in your panel schedules are done in kVA. Ignore the amps. Look at your demand load in kVA. Select a transformer larger than that number. Profit.

agree...
according to your attachment the demand load is less than 75kva and the transformer protection and conductor sizing are correct.

 

[mto12345]

agree...
according to your attachment the demand load is less than 75kva and the transformer protection and conductor sizing are correct.

I think you may be confusing yourself by converting between amps and VA a bunch of times and losing track of your demand factors. The load calculations shown in your panel schedules are done in kVA. Ignore the amps. Look at your demand load in kVA. Select a transformer larger than that number. Profit.

Thanks!

 

[mto12345]

One more question, does the fact that I have two 200amp circuit breaker panels (CW and CW_B) have anything to conflict with the NEC requirement. Do I need to change my panels to 200 amp circuit breaker and 100 amp circuit breaker Panel. The reason why I am asking is the inspector is saying I am not NEC 450.3 complaint.

 

[augie47]

One more question, does the fact that I have two 200amp circuit breaker panels (CW and CW_B) have anything to conflict with the NEC requirement. Do I need to change my panels to 200 amp circuit breaker and 100 amp circuit breaker Panel. The reason why I am asking is the inspector is saying I am not NEC 450.3 complaint.

The 125 amp breaker on the primary complies with the 125% primary rule in 450.3 and as far as the transformer is concerned no secondary protection is required since you have adequate primary protection.
Ask the inspector what part of 450.3 he feels is in violation.

 

[d0nut]

You also have to make sure you meet the requirements for the transformer secondary conductors as required by 240.21(C), which your conductor sizes shown in the riser diagram meet. Those conductors get sized based on the overcurrent protective device upon which they land, not based on the nominal current from the transformer.

 

[mto12345]

The 125 amp breaker on the primary complies with the 125% primary rule in 450.3 and as far as the transformer is concerned no secondary protection is required since you have adequate primary protection.
Ask the inspector what part of 450.3 he feels is in violation.

You also have to make sure you meet the requirements for the transformer secondary conductors as required by 240.21(C), which your conductor sizes shown in the riser diagram meet. Those conductors get sized based on the overcurrent protective device upon which they land, not based on the nominal current from the transformer.

Thank You!