Article 450.3 Electrical Explanation

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earmandorobles

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Mexicali, Mexico
Hi everyone

This is my 1st post and i hope that you can help me with this little question that me and my buddies from work have been discusting for the past few days (in advance sorry for my english, im not from USA but we practically use the NEC standards for our wokr)

We need to feed a 150 KVA dry type transformer (480-208Y/277) which is almost 500 feet away from the main panel (we've been looking for a more short distance option but our client is very reluctant about it). This installation will be made in a "supervised location". This transformer will feed a panelboard but the secondary feeder is about 160 feet long, reason why we want to include a main breaker in the 208v panelboard.

Normally we consider 125% of the transformer FLA and use the next size up for the primary OCPD (which is a SQ-D breaker) and based in the size of the OCPD we select the conductor:

FLA (150KVA) = 150000 / (480 * 1.7321) = 181 amp
BKR = 181 x 1.25 = 225 amp
This will give us a 4/0 AWG conductor

Now here comes the discusion that we've had the past few days because for the secondary feeder we usually use the same criteria (125% for the main breaker and select the conductor based on the secondary OCPD).

The article 450.3 indicates that when using a primary AND secondary protection we need to consider a 250% for the primary protection. I have read a lot of articles in this website and also on the internet (EC&M, EEP, even schneider and bussman articles) where they make some examples and even when they say that the OCPD is for the transformer protection and not the conductors they still oversize the primary conductor according to the 250% OCPD.

My biggest question is if my 125% primary and 125% secondary criteria is correct and if not then what would be the right considerations and also the electrical explanation of why consider 250% primary OCPD just when we have a secondary OCPD and not when whe have just the primary OCPD.

I really hope i made myself clear.

Regards
 
There are two sets of rules that you need to look at for transformers. First the rules for the protection of the transformer as found in 450.3. As you have stated there are rules for primary only protection and for primary and secondary protection of the transformer. When the transformer has both primary and secondary protection, the primary protection is permitted to be 250% of the primary rated current. Note that this is a permission and is a maximum permitted rating of the primary OverCurrent Protective Device (OCPD)...you can use a lower rated one of you want to. Again this is just the rules for the protection of the transformer. Sometimes there are issues with a primary device rated at 125% tripping on the inrush when the transformer is energized.

Then we have to look at the rules in Article 240 for the protection of the conductors that will be connected to the transformer. The primary conductors will have to be protected at or below their ampacity, so the larger the primary overcurrent protective device the larger the conductors will have to be. The secondary conductors have to be protected by one of the rules in 240.21(C). I don't think any of the provisions of 240.21(C) will permit you to run 160' from the transformer before the OCPD, unless the conductors are outside.
 
don_resqcapt19 said:
There are two sets of rules that you need to look at for transformers. First the rules for the protection of the transformer as found in 450.3. As you have stated there are rules for primary only protection and for primary and secondary protection of the transformer. When the transformer has both primary and secondary protection, the primary protection is permitted to be 250% of the primary rated current. Note that this is a permission and is a maximum permitted rating of the primary OverCurrent Protective Device (OCPD)...you can use a lower rated one of you want to. Again this is just the rules for the protection of the transformer. Sometimes there are issues with a primary device rated at 125% tripping on the inrush when the transformer is energized.

Then we have to look at the rules in Article 240 for the protection of the conductors that will be connected to the transformer. The primary conductors will have to be protected at or below their ampacity, so the larger the primary overcurrent protective device the larger the conductors will have to be. The secondary conductors have to be protected by one of the rules in 240.21(C). I don't think any of the provisions of 240.21(C) will permit you to run 160' from the transformer before the OCPD, unless the conductors are outside.
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Wow that was quick, thanks a lot for the explanation. So this is a "permitted" consideration that we could use in case that we need it.

Thanks a lot
 
earmandorobles said:
Wow that was quick, thanks a lot for the explanation. So this is a "permitted" consideration that we could use in case that we need it.

Thanks a lot
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Almost all sizing for overcurrent protective devices in the NEC are maximums and you can use a smaller device, but with transformers, motors, and HVAC equipment the use of a smaller device can result in tripping on startup because of the inrush currents.
 
Every single 112.5 kva transformer I've done has gotten a 200 amp primary and a 400 amp secondary. Keeping it simple


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ADub said:
Every single 112.5 kva transformer I've done has gotten a 200 amp primary and a 400 amp secondary. Keeping it simple


Sent from my iPhone using Tapatalk
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Adub,

Wouldn't a primary of 175 be more appropriate?

112,500/830x1.25=169(about)?
 
Dexie123 said:
Adub,

Wouldn't a primary of 175 be more appropriate?

112,500/830x1.25=169(about)?
Click to expand...

Yes that would work. It's just been my experience to do 200. We have a keepfill stock of 3/0 so it makes the choice easier for us
 
ADub said:
It must be. There's 200 transformers in our plant I know about and not one of them is a 150 kva. I need to get out more
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Well that makes sense that one place with a lot of transformers would have standard size they use, it's easy that way.

On the other hand those of us that are always working in different buildings under different engineers end up installing every size made. Most common for us is a 45 or a 75 but we do others from 5 to 1000 kva.
 
Dexie123 said:
Adub,

Wouldn't a primary of 175 be more appropriate?

112,500/830x1.25=169(about)?
Click to expand...

The code allows up to 250% on many primarys and I find in this area the engineers seem to design close to that.

112,500/480/1.73x2.5=338 amp so a even a 300 amp breaker would not be out of the question
 
iwire said:
The code allows up to 250% on many primarys and I find in this area the engineers seem to design close to that.

112,500/480/1.73x2.5=338 amp so a even a 300 amp breaker would not be out of the question
Click to expand...
It's only half that if using the primary only protection schema.... but with primary and secondary OCP, may as well upsize at least little over 125%.... and I imagine that's where the 200A comes in vs. 175A. The thing is, the larger you go on primary OCP, the larger the conductors must be.
 
Smart $ said:
It's only half that if using the primary only protection schema.... but with primary and secondary OCP, may as well upsize at least little over 125%.... and I imagine that's where the 200A comes in vs. 175A.
Click to expand...

I agree 100%, or maybe even a little over 125%. ;)
 
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