Verifying ambient temperature adjustment and OCPD.

[notmyspace]

Ok. I just want to verify I am correct on this.

Someone installed a single phase 240/480 to 120/240 50KVA transformer on rooftop. The feed is a 2 wire + 1 ground (2) #2 and (1) #6 gnd

This is what they put in. I think it?s incorrect


Source (3 phase 200A switch gear) to a 200A Meter then to a 100A Service disconnect 600V switch to a single phase 50KVA transformer with (2) #2 & (1) #6 gnd wire ( approximately 50 ft). The secondary conductor is (3) #3/0 &(1) 4 gnd to a 200A main breaker panel. The total load is 36.88KVA which is about 154A.

I think they should be using a 150A disconnect and (2) #1/0 & (1) #6 gnd conductors on the primary side and a 150A meter. The primary side OCPD based on 450.3(B)

Transformer rated at 50KVA/480V = 104A
With 125% it?s at 130A. So the next size up based on 240.6(A) is 150A.

Since this is rooftop, I check make sure the ambient temperature adjustment is ok.
Ambient temp = 34C - so factor is 0.96 based on 310.15(B)(2)(a). Conductor #1/0 is rated at 170A @ 90C
170X0.96 = 163.2 A
@75C #1/0 is rated at 150A. So the ampacity is 150A

Am I correct on the OCPD and ambient temperature adjustment ?

 

[jim dungar]

Am I correct on the OCPD ...

The NEC does not require you to use 125% of the transformer's primary current, you are allowed to use any value below that amount. The transformer inrush characteristics and protective device time-current curve should really be the starting point.
Be aware that 'upsizing' a transformer's primary protective device should not be done without consideration of its effect on other sizing requirements (there may be unexpected consequences).

 

[notmyspace]

The NEC does not require you to use 125% of the transformer's primary current, you are allowed to use any value below that amount. The transformer inrush characteristics and protective device time-current curve should really be the starting point.
Be aware that 'upsizing' a transformer's primary protective device should not be done without consideration of its effect on other sizing requirements (there may be unexpected consequences).

This is confusing as heck...do I need OCPD on the primary winding on the transformer or does not required? I thought somewhere in article 450 mentioned about OCPD too..also, I look at the link below as example

http://ecmweb.com/contractor/transformer-installation-made-easysort

Between step 1 and 2, i need protection on the primary side based on the link

 

[david luchini]

This is confusing as heck...do I need OCPD on the primary winding on the transformer or does not required? I thought somewhere in article 450 mentioned about OCPD too..also, I look at the link below as example

http://ecmweb.com/contractor/transformer-installation-made-easysort

Between step 1 and 2, i need protection on the primary side based on the link

Yes, you need protection on the primary side of the transformer. You HAVE protection on the primary side of the transformer.

Per 450.3(B), the primary protection cannot be greater than 250% of the transformer primary rated current (or 125% if you were using "primary only protection.") In your case, the 100A primary OCPD is NOT greater than allowed. As such, it meets the Code requirements.

 

[notmyspace]

Yes, you need protection on the primary side of the transformer. You HAVE protection on the primary side of the transformer.

Per 450.3(B), the primary protection cannot be greater than 250% of the transformer primary rated current (or 125% if you were using "primary only protection.") In your case, the 100A primary OCPD is NOT greater than allowed. As such, it meets the Code requirements.

So, since I am only using primary only protection which i need 125% then I need to have a 150A on the disconnect since 100A is less than 125%.

The number is based on the transformer rating right? So, if the transformer rated at 104A (primary side) then I need higher than 100A?

 

[Gregg Harris]

This is confusing as heck...do I need OCPD on the primary winding on the transformer or does not required? I thought somewhere in article 450 mentioned about OCPD too..also, I look at the link below as example

http://ecmweb.com/contractor/transformer-installation-made-easysort

Between step 1 and 2, i need protection on the primary side based on the link

Any location	Not more than 6%	600% (See Note 1.)	300% (See Note 1.)	300% (See Note 1.)	250% (See Note 1.)	125% (See Note 1.)
More than 6% and not more than 10%	400% (See Note 1.)	300% (See Note 1.)	250% (See Note 1.)	225% (See Note 1.)	125% (See Note 1.)
Supervised locations only (See Note 3.)	Any	300% (See Note 1.)	250% (See Note 1.)	Not required	Not required	Not required
Not more than 6%	600%	300%	300% (See Note 5.)	250% (See Note 5.)	250% (See Note 5.)
More than 6% and not more than 10%	400%	300%	250% (See Note 5.)	225% (See Note 5.)	250% (See Note 5.)
Notes: 1. Where the required fuse rating or circuit breaker setting does not correspond to a standard rating or setting, a higher rating or setting that does not exceed the next higher standard rating or setting shall be permitted. 2. Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device. If both circuit breakers and fuses are used as the overcurrent device, the total of the device ratings shall not exceed that allowed for fuses. 3. A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons monitor and service the transformer installation. 4. Electronically actuated fuses that may be set to open at a specific current shall be set in accordance with settings for circuit breakers. 5. A transformer equipped with a coordinated thermal overload protection by the manufacturer shall be permitted to have separate secondary protection omitted.

Table 450.3(A) Maximum Rating or Setting of Overcurrent Protection for Transformers Over 600 Volts (as a Percentage of Transformer-Rated Current)
Location Limitations Transformer Rated Impedance Primary Protection over 600 Volts Secondary Protection (See Note 2.)
Over 600 Volts 600 Volts or Less
Circuit Breaker
(See Note 4.) Fuse Rating Circuit Breaker
(See Note 4.) Fuse Rating Circuit Breaker or Fuse Rating

 

[notmyspace]

Any location	Not more than 6%	600% (See Note 1.)	300% (See Note 1.)	300% (See Note 1.)	250% (See Note 1.)	125% (See Note 1.)
More than 6% and not more than 10%	400% (See Note 1.)	300% (See Note 1.)	250% (See Note 1.)	225% (See Note 1.)	125% (See Note 1.)
Supervised locations only (See Note 3.)	Any	300% (See Note 1.)	250% (See Note 1.)	Not required	Not required	Not required
Not more than 6%	600%	300%	300% (See Note 5.)	250% (See Note 5.)	250% (See Note 5.)
More than 6% and not more than 10%	400%	300%	250% (See Note 5.)	225% (See Note 5.)	250% (See Note 5.)
Notes: 1. Where the required fuse rating or circuit breaker setting does not correspond to a standard rating or setting, a higher rating or setting that does not exceed the next higher standard rating or setting shall be permitted. 2. Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device. If both circuit breakers and fuses are used as the overcurrent device, the total of the device ratings shall not exceed that allowed for fuses. 3. A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons monitor and service the transformer installation. 4. Electronically actuated fuses that may be set to open at a specific current shall be set in accordance with settings for circuit breakers. 5. A transformer equipped with a coordinated thermal overload protection by the manufacturer shall be permitted to have separate secondary protection omitted.

Still confusing me. Do you based on the initial rated number based on the transformer rating right? if the transformer rated at 104A, how can I use a 100A disconnect to protect the primary winding on the transformer? sorry for keep asking questions...just don't make sense..if I follow the link and based on the book and table..

 

[kwired]

So, since I am only using primary only protection which i need 125% then I need to have a 150A on the disconnect since 100A is less than 125%.

The number is based on the transformer rating right? So, if the transformer rated at 104A (primary side) then I need higher than 100A?

You can have 60, 70 or 80 amp OCPD on this transformer if you wish, and if the OCPD will hold upon energization, but you will not get full 104 amp rated load out of it. There is no minimum value needed other than to be able to supply the calculated load connected to it.

The transformer does need overcurrent protection, but the max level allowed on primary or secondary does depend on if there is protection on the other side, what level, and other factors.

ETA: read OP again, looks like load on transformer is 36.88 KVA which is about 77 amps on the primary. Looks like you would want at least 80 amps primary device no matter what else applies. We can only assume 36.88 KVA is final load calc and already includes demand factors, continuous duty, etc. if it applies.

 

[notmyspace]

You can have 60, 70 or 80 amp OCPD on this transformer if you wish, and if the OCPD will hold upon energization, but you will not get full 104 amp rated load out of it. There is no minimum value needed other than to be able to supply the calculated load connected to it.

The transformer does need overcurrent protection, but the max level allowed on primary or secondary does depend on if there is protection on the other side, what level, and other factors.

ETA: read OP again, looks like load on transformer is 36.88 KVA which is about 77 amps on the primary. Looks like you would want at least 80 amps primary device no matter what else applies. We can only assume 36.88 KVA is final load calc and already includes demand factors, continuous duty, etc. if it applies.

Yes, the load on the primary side is about 77A. The whole OCPD is so confusing. The language is messed up to me...

 

[jim dungar]

Yes, the load on the primary side is about 77A. The whole OCPD is so confusing. The language is messed up to me...

This is absolutely one time the NEC is not a design manual.
You get to choose any primary side protective device you want to use. It can have any rating you think will support the inrush of the transformer and provide the proper current for your load.

However, you need to see if your choice conforms to the requirements of the NEC.
Effectively Table 450.3(B) has two conditions for the rating of the primary: 1) a restrictive value when the secondary is ignored, or 2) a looser value when you consider the secondary. It is you choice which condition you want to use. Regardless which condition you choose to investigate, the result is the maximum value of the protective device not a minimum value.

Some people like to 'teach' protective device selection by using examples that always use the maximum values allowed by the NEC and then work backwards. While I agree this is a quick and dirty method, it does not teach someone how to design a system 'from the load back towards the source'.


Anyway.
Assuming someone designed this so everything will actually work together.

You have a load (about 77A) that is adequately protect by a 100A fuse.
You have a transformer that can be protected up to a maximum of 130A (before other considerations).
Your 100A fuse is less than 130A, therefore you meet the conditions in Table 450/3(B) and your transformer is considered as being protected.
You are done as far as the NEC is concerned.


Starting at the maximum and working backwards will often require equipment that is over sized in ampacity and physically, resulting in a much more costly installation.

 

[kwired]

Yes, the load on the primary side is about 77A. The whole OCPD is so confusing. The language is messed up to me...

There are two things that come into play here.

Table 450.3(B) applies to protection of the transformer.

Then comes 240.4(F), which applies to protection of the secondary conductors.

In your case you can protect the transformer with 125% or 250%, but if you are supplying secondary system that is 120/240 volts, 240.4(F) says the secondary conductors are not protected by the primary device. So you will need to protect the secondary conductors to their ampacity, and then apply table 450.3(B), which if the secondary conductors are at/near rated secondary current of the transformer kind of makes anything between 125 and 250% acceptable for max primary protection.

Now throw in your ambient temp problems in determining what the ampacity of your secondary conductors actually is. Primary conductors - also need ampacity adjustments with final ampacity needing to be at least 77 amps, but protective device can be 250% of rated 104 amps, or 260 amps, pretty sure you would have to go down and not up here so 250 amps.

 

[notmyspace]

So, here is the question. I am a big fan of future capacity growth etc. I am sizing it to 150A, that would sizing the 50KVA for future capacity not to the load right? As for code, 100A is suffice.

 

[jim dungar]

So, here is the question. I am a big fan of future capacity growth etc. I am sizing it to 150A, that would sizing the 50KVA for future capacity not to the load right? As for code, 100A is suffice.

You are correct.

 

[notmyspace]

You are correct.

thanks!

Does Mike Holt's a has quick reference book for sale the one I can use to find like wiring sizing, grounding, conduit support etc...like a tab ?

 

[MarineTech]

Would it be possible to get a picture or lisitng of the transformer nameplate?

After the basics are done for sizing OCP devices and conductors per ambient correction (includes rooftop) and current. I would apply the same temp rise to the transformer to findout if it requires derate. May have to call the manufacturer to understand effects of greatly elevated ambient on kVA capacity.

I always double check the loads for apparent power rating (VA) and non-linear operation.