Transformer Primary Conductors

[dnbob]

When sizing a transformer primary conductor of let say a 75 KVA 480D to 208Y/120 transformer, using the rules for a 25 foot secondary conductor, as found in 240.21C6, doesn't the primary conductor ampacity have to corrispond to the primary overcurrent device?

I was challenged on this, and have not found a whole lot to back myself up. The NEC handbook says this, but when reading 240.21 C6, it sure doesn't seem to say much about it.

Thoughts?

 

[david luchini]

Yes it does. The protection of the primary conductors would fall under 240.4 - "Conductors shall be protected against overcurrent in accordance with their ampacities specified in 310.15..."

 

[dnbob]

You are absolutely right, thanks for the help. I got stuck in 240.21 and should have broadened my horizons.

 

[dnbob]

So if using a 125 amp breaker on the primary side, I could get by with #2 copper, in lieu of #1, using 240.4B?

 

[kingpb]

Don't see why not. (#2 is good for 130A at 90 deg C, anyway)

 

[david luchini]

So if using a 125 amp breaker on the primary side, I could get by with #2 copper, in lieu of #1, using 240.4B?

Yes, as long as 125% of the continuous load plus 100% of the non-continuous load is 115 Amps or less (and the adjusted/corrected ampacity of the #2, if any adjustments/corrections are required, is greater than 110 and greater than the load) then you could use #2 to feed the transformer from the 125A c/b, per 240.4(B).

 

[templdl]

How would you guys address Art 450 table 450.3(B) where you are allowed to size the primary protection up to 250%? My thoughts have been that the secondary protection when properly sized will protect the transformer from overload and that the primary OCPD if sized at 125% would be redundant. As such I have always considered the primary protective device as providing short circuit protection the upstream distribution system from a transformer winding failure which would lead me to believe that is why 250% is allowed.
As such when would it be allowed to use the 250% and where is it allowed to be located? I have never been able to find a direct answer to this as I do understand that cable protection is also required.
In my former position ss a breaker and transformer applications engineer I was requested to do some coordination studies between transformer inrush currents anticipated and the magnetic pickup value of breakers and found that there can be some conflict leading to nuisance tripping when applied with energy efficient and K-rated transformers which customarily have higher inrushes which ar especially troublesome in the lower transformer KVA rating which can well exceed 13X the transformers FLA.
There's no reason to exceed the 125% primary protection except if there is an nuisance trip occurring upon energization of the transformer.


Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers
600 Volts and Less (as a Percentage of Transformer-Rated Current)

When both Primary and secondary protection are required:

Primary protection: Currents of 9 Amperes or More : 250% (See Note 3.)
Note 3. A transformer equipped with coordinated thermal overload protection by the manufacturer and arranged to interrupt the primary current shall be
permitted to have primary overcurrent protection rated or set at a current value that is not more than six times the rated current of the transformer
for transformers having not more than 6 percent impedance and not more than four times the rated current of the transformer for transformers
having more than 6 percent but not more than 10 percent impedance.

Secondary Protection:125% (See Note1.)
Note 1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does
not exceed the next higher standard rating shall be permitted.

 

[kingpb]

The HV side breaker protects the transformer. Just because it can be as high as 250%, often times this setting is above the transformer through fault damage curve.

 

[templdl]

Thanks kingpb ,
But you refered to a fault and as such wouldn't the transformer have already failed? Usually protecting the transformer would infer protecting it from an overload.
When one condidered a fault how can a trasformer be protected from a fault normally caused by a winding failure?
What I have always understood was you want to isolate the faulted transormer from the upstream distribution sustem so that wouldn't be affected.
My concern is when you do try to appy the 250% rule where can that breaker be located when one also must consider cable protection where the 125% rule applies.
Does the 25' tap rule apply when you land the conductor on a down stream breaker such a one for the transformer secondary protection where you may be able to apply the distance from the 250% primary to the 125% secondary breaker?

I see a conflict between being allowed to appy a 250% primary device while still needing to conblt with 125% for cable protection.

 

[david luchini]

My concern is when you do try to appy the 250% rule where can that breaker be located when one also must consider cable protection where the 125% rule applies.
Does the 25' tap rule apply when you land the conductor on a down stream breaker such a one for the transformer secondary protection where you may be able to apply the distance from the 250% primary to the 125% secondary breaker?

I see a conflict between being allowed to appy a 250% primary device while still needing to conblt with 125% for cable protection.

I'm not sure I understand what you are asking, but...you can apply the 250% rule for primary protection whenever the secondary is protected at not more than 125% (including next size up) per T450.3(B).

There are no rules for where that primary protection must be located.

The primary cable protection must be per 240.4. If your primary c/b is sized for 250%, the primary conductor must be protected by that (250%) c/b. If the primary c/b is sized for 125%, the primary conductor must be protected by that (125%) c/b. The secondary conductors must be protected per 240.21(C). I don't see what the confiict is.

 

[kingpb]

The fault could be on the LV side ahead of the downstream breaker

 

[templdl]

david luchini,
what I believe that you are saying is that I elect to go with the 250% then I must increase the size of the cable base upon that breakers ampacity.
What I was refering to as the 25' tap rule I may have found a referance to which leads me to believ that I can use the 25" tap rule as measured forn the pri OCPD to the Sec OCPD on the other side of the transformer:
Art 240.21 (B)
(3) Taps Supplying a Transformer [Primary Plus Secondary
Not Over 7.5 m (25 ft) Long]. Where the tap
conductors supply a transformer and comply with all the
following conditions:
(1) The conductors supplying the primary of a transformer
have an ampacity at least one-third the rating of the
overcurrent device protecting the feeder conductors.
(2) The conductors supplied by the secondary of the transformer
shall have an ampacity that is not less than the
value of the primary-to-secondary voltage ratio multiplied
by one-third of the rating of the overcurrent device
protecting the feeder conductors. [ROP 10?47]
(3) The total length of one primary plus one secondary
conductor, excluding any portion of the primary conductor
that is protected at its ampacity, is not over 7.5
m (25 ft).
(4) The primary and secondary conductors are protected
from physical damage in an approved manner, such as
enclosed in a raceway. [ROP 10?48]
(5) The secondary conductors terminate in a single circuit
breaker or set of fuses that limit the load current to not
more than the conductor ampacity that is permitted by
310.15.

 

[jim dungar]

Art 240.21 (B)

240.21(B) is not applicable to transformers with multiple output voltages. The OP said the unit was 480V - 208Y/120V, so, instead, the rules found in 240.21(C) must be followed.

 

[david luchini]

240.21(B) is not applicable to transformers with multiple output voltages. The OP said the unit was 480V - 208Y/120V, so, instead, the rules found in 240.21(C) must be followed.

I think Jim is confusing 240.21(C)(1) and 240.21(B). There is nothing in 240.21(B) about multiple output voltages.

What I was refering to as the 25' tap rule I may have found a referance to which leads me to believ that I can use the 25" tap rule as measured forn the pri OCPD to the Sec OCPD on the other side of the transformer:
Art 240.21 (B)

Yes, you can use the 25' Tap rule for supplying a transformer, but no, the 25' is not measured from the Primary OCPD to the Secondary OCPD. The 25' is measured from the point of the tap from the feeder to the Secondary OCPD, so there is still no rule for where the primary OCPD must be located.

Also, using the 25' Tap rule for supplying a transformer doesn't remove the requirements for Transformer protection in 450.3. If you had a 300A, 480V feeder and wanted to tap from it to supply a 75kVA xfmr, this would not be allowed because the 300A OCPD exceeds the 250% allowed for primary protection. A 112.5kVA xfmr could be tapped from the 300A feeder.

 

[templdl]

This is a great discussion as I have never been able to identify the correct application of the 250% rule for the pri. OCPD but still protect the primary cable.
One thing that would be very helpful would be if someone were to post a diagram showing a 75kva 480-208Y/120 transformer which included where the primary tap originates, the location of a 250% OCPD and the loacation of the 125% sec OCPD as well as the maximum allowable distance for the tpas that would be much appreciated.
Sometimes a picture is worth a thousand words. I can recall looking in my Stallcup's electrical design book which has a lot of good stuff but I can't remember it ever calrifying this code issue.

 

[david luchini]

This is a great discussion as I have never been able to identify the correct application of the 250% rule for the pri. OCPD but still protect the primary cable.
One thing that would be very helpful would be if someone were to post a diagram showing a 75kva 480-208Y/120 transformer which included where the primary tap originates, the location of a 250% OCPD and the loacation of the 125% sec OCPD as well as the maximum allowable distance for the tpas that would be much appreciated.

I don't think I have a diagram showing this, but let me see if I can explain it...

Let's assume that your 75kVA transformer is being tapped from a 225A feeder (#4/0 AWG Cu).

The location of the tap is irrelevant to the Tap to a Transformer rule in 240.21(B)(3). The tap can be anywhere on the 225A feeder, eg, 50' from the c/b, 125' from the c/b, 247' from the c/b, etc. It doesn't matter.

The 25' refers to the distance of both the primary and secondary conductors together. See 240.21(B)(3)(3). So on the primary, you could run the tap conductors 10' from the tap location to the xfmr, this would allow you 15' to run the secondary conductors to the secondary OCPD. Or you could run the primary (tap) conductors 19' to the xfmr, and this would leave 6' for the secondary conductors. Under 240.21(B)(3), the total length of the primary plus secondary conductor cannot exceed 25'.

Per 240.21(B)(3)(1), the primary (tap) conductors must have a minimum ampacity of at least 1/3 the rating of the primary OCPD. This would mean you would need tap conductors of at least 75A because of the feeders 225A c/b. However, the conductors must also be sized to carry the load per 215.2. If the load on the transformer was 72kVA non-continuous, then the primary load current is 86.6A, and the primary conductors would need an ampacity of at least 86.6A. #3 Awg Cu has an ampacity of 100, which is sufficient for the load served and complies with 240.21(B)(3)(1), so that would be the minimum primary (tap) conductor size.

Looking at the secondary conductor size, 240.21(B)(3)(2) requires the secondary conductor to have an ampacity of at least (480/208)*(225/73)= 173 Amps. But again, the feeder must be capable of carrying the load which is 199.9Amps at 208V. So the minimum size for the secondary conductors and c/b would be 200A in this example. #3/0 Awg Cu conductors would meet the requirements in 240.21(B)(3)(2) and (B)(3)(5).

 

[kingpb]

Transformer protection curve, as you can see that a breaker at 250% is above the transformer curve. The cable is still protected.

 

[jim dungar]

I think Jim is confusing 240.21(C)(1) and 240.21(B). There is nothing in 240.21(B) about multiple output voltages.

No I was commenting on the quote I refernced that mistakenly cites 240.21(B) but then gave the details associated with 240.21(C)(3).

The actual reference for my statement about multi-voltage transformers is found in both 240.4(F) and 240.21(C)(1).

 

[david luchini]

No I was commenting on the quote I refernced that mistakenly cites 240.21(B) but then gave the details associated with 240.21(C)(3).

But the quote doesn't mistakenly cite 240.21(B). Templdl is asking about Taps Supplying a Transformer from 240.21(B)(3). There's nothing in the section about Taps Supplying a Transformer that relates to multi-voltage transformers.

 

[jim dungar]

But the quote doesn't mistakenly cite 240.21(B).

240.4(F) says the secondary conductors of a wye transformer cannot be considered as being protected by a primary side overcurrent device. 240.21(B)(3) is about primary side conductor protection (i.e. not having a dedicated primary protective device).