No primary protection required?

[electrofelon]

I wonder what the transformer loss will cost over the long term? Any ideas?

I have a pair of transformers for a step up step down to my house. They are 15 KVA each, each one is about 60 watts 24/7. Thats about $150 per year. That is only no load losses of course, with load losses it will be more.

 

[kwired]

I may go along, though a little reluctantly, with the need for a disconnect on second transformer. This only for a two wire secondary/three phase three wire application though.

240.21(C)(1) says the conductors on a two wire secondary can be protected by the primary device - if you have done this, adding additional secondary protection (at same primary to secondary ratio and same trip curve type) doesn't increase or decrease protection level at all, it will be a race to see which one will trip first when there is overcurrent conditions.

Makes all the sense in the world - except the code doesnt allow it. Might be a great suggestion for a future code cycle.

Sent from my LM-X212(G) using Tapatalk

What code would you cite is in violation? If you had a secondary conductor that is protected by a primary device in combination with primary to secondary voltage ratio, why wouldn't anything connected to that conductor not be considered to have same level of protection? Shouldn't matter what is connected to the conductor.

Example you want a 20 amp 120 volt circuit at the load- you have a 480 x 120 two wire to two wire transformer. You put 5 amp fuse in the primary. Secondary can't draw more than 20 amps or it will blow primary fuse. It shouldn't matter if that 20 amps is direct load, via a receptacle, or a primary of another transformer, it still has 20 amps (@120 volts) of protection.

 

[Isaiah]

What code would you cite is in violation? If you had a secondary conductor that is protected by a primary device in combination with primary to secondary voltage ratio, why wouldn't anything connected to that conductor not be considered to have same level of protection? Shouldn't matter what is connected to the conductor.

Example you want a 20 amp 120 volt circuit at the load- you have a 480 x 120 two wire to two wire transformer. You put 5 amp fuse in the primary. Secondary can't draw more than 20 amps or it will blow primary fuse. It shouldn't matter if that 20 amps is direct load, via a receptacle, or a primary of another transformer, it still has 20 amps (@120 volts) of protection.

A lot can go wrong over a distance of 2000+ feet - that's why 450.3(B) is applicable. A disconnecting means near the second tranny (primary) makes it safer for operations - 450.14 is applicable. I spoke with the NFPA technical staff regarding this issue and they are in agreement with this interpretation.

 

[jumper]

A lot can go wrong over a distance of 2000+ feet - that's why 450.3(B) is applicable. A disconnecting means near the second tranny (primary) makes it safer for operations - 450.14 is applicable. I spoke with the NFPA technical staff regarding this issue and they are in agreement with this interpretation.

Yep. No where in 450 do I find where primary protection and a disconnecting means is not required for every transformer.

 

[electrofelon]

Yep. No where in 450 do I find where primary protection and a disconnecting means is not required for every transformer.

No where in 450 do I find where it is allowed to paint my transformer pink

Not to keep going around in circles, but kwired and I arent saying primary protection Isn't required. We just don't see any requirement about where that protection is located. Disconnect can be remote , although a structure needs a disconnect . If it's just "equipment" you prolly could have a remote disco.

 

[jumper]

No where in 450 do I find where it is allowed to paint my transformer pink

Not to keep going around in circles, but kwired and I arent saying primary protection Isn't required. We just don't see any requirement about where that protection is located. Disconnect can be remote , although a structure needs a disconnect . If it's just "equipment" you prolly could have a remote disco.

A remote disco is fine, I am saying that one disco and OCPD cannot protect two transformers in series, step up to step down for example.

 

[Isaiah]

Yep. No where in 450 do I find where primary protection and a disconnecting means is not required for every transformer.

Over the last few years I've noticed a growing trend in major industry of manipulating the code to say what it really doesn't -- all in the name of saving money. Among the popular excuses I usually hear: "NEC is not a design guide" OR "that's just your interpretation"; blah blah blah. The code is pretty straight forward in my opinion.

 

[iceworm]

You hit a nerve - but not a particularly painful one. Following mild rant is not directed at anyone here. It is directed at the code panels.

In this case, (step-up/step-down transformers) I absolutely agree with you (and the others). The NEC is clear. Good design and the physics don't require a primary OCP on the second transformer, but the NEC sure does. So, tough luck - put it in.

However:
[RANT-ON]

... Among the popular excuses I usually hear: "NEC is not a design guide" ....

As all know, per 90.1.A, the NEC self-proclaims - It is not a design guide. Excuse or not, it is not.

However, it is true that most of the design criteria deficiencies show up when one is building Grandma's A-1 Abrams factory as opposed to Grandpa's cottage by the beach.

... The code is pretty straight forward in my opinion.

And it is, mostly - but not always. There are sections that leave me wondering what were they smoking. And sometimes that requires interpretation*. Generally speaking, if one is not trying to value-engineer the last nickel, and the intent is to have a job that is safe, reliable, and meets customer's operational spec, the work will exceed the NEC. Luckily for me, the AHJ agents generally agree with me.
[RANT-OFF]

If you got this far, thank you for listening

(*oh-oh, I used the "I" word)


the worm

 

[electrofelon]

I absolutely use the code to reduce costs whenever possible for me and/or my customers. I am very proud of that.

 

[iceworm]

I absolutely use the code to reduce costs whenever possible for me and/or my customers. I am very proud of that.

As well you should be. There is an unlimited amount of work and a limited amount of money. If you can save them money on one job, then they can do more jobs.

Now, the question is:
Are you striving for minimum installed cost?
Are you striving for minimum Life Cycle Costs?

Some of my clients want minimum installed costs. Their bonus is dependent on the bottom line at the end of the project. As soon as the equipment passes commissioning, they are done. If the maintenance eats the company lunch next year, not their problem.

Some, usually the operations branch, wants minimum Life Cycle costs. Their bonus is dependent on the equipment up-time. They want the equipment running reliably for the next 20 years.

And a few don't know the difference.

And I do my best to get them all what they want.

 

[JFletcher]

Question here, if a ocpd is required at the second Transformer, the step down, could it be placed on the secondary side? I ask because a 240-volt rated fused disco is a lot cheaper than a 600 volt rated one, and a fault on the secondary side of the second Transformer would be stopped before it could damage the primary side.

Ballpark price, what would a 600 volt rated fused disco cost for a 3kva xfmr? The 480 volt primary would only be rated for little over 6 amps correct?

 

[electrofelon]

Question here, if a ocpd is required at the second Transformer, the step down, could it be placed on the secondary side? I ask because a 240-volt rated fused disco is a lot cheaper than a 600 volt rated one, and a fault on the secondary side of the second Transformer would be stopped before it could damage the primary side.

Ballpark price, what would a 600 volt rated fused disco cost for a 3kva xfmr? The 480 volt primary would only be rated for little over 6 amps correct?

That is one reason someone may want to avoid a high side disconnect and OCPD. It's not so bad for 600 volts or less, but if you get into MV territory, it would be nice to avoid mv gear. But Padmounts typically have fusing provisions on the primary so one can get your 450.3 protection from that. Then if you can meet the requirements for the remote disco, you'd be good.

 

[kwired]

A remote disco is fine, I am saying that one disco and OCPD cannot protect two transformers in series, step up to step down for example.

Why can primary device on first transformer protect the secondary conductors as well as other secondary equipment but not another transformer on the secondary? IMO protection is already there, need for or the location of additional disconnecting means is all that is questionable here. If second transformer would be at a separate building or structure then that disconnect becomes more necessary than if at same building as the first transformer, but that is more of a art 225 and separate building issue than it is an art 450 issue.

 

[Dale001289]

if its not listed as an exception, it is non-compliant and requires the AHJ for arbitration.

 

[kwired]

If I have a 50 amp fuse at the beginning of a circuit, they have a 100 amp fused switch with 100 amp fuses in it as local disconnect near the load - that load is still protected by a 50 amp overcurrent device even though there is a 100 amp fuse in the series. I don't see the transformer situation of this thread being any different.

If the first device in the circuit is sized per maximum allowed VA then you could put in two or one hundred transformers in series, that first device still respond when VA exceeds what it was sized for - how is that not protecting any of the others assuming they are at least equal or greater to the VA rating allowed by that first device? A multiwire secondary somewhere in there changes the rules - it is possible to overload a portion of the secondary in those applications, but two wire in/two wire out VA in is going to be equal to VA out, with relatively negligible losses.

 

[jap]

I was originally going to side with the team that were saying a primary OCPD wasn't needed for the line side of the 2nd transformer myself, and felt the conductors to the 2nd transformer would be protected by 1st one that was protected by it's primary, and they are.

But then I read 450.4 (A).

Putting aside the mention of any OCP, 450.4(A) indicates 600v nominal or less shall be protected by and individual overcurrent device be installed in series with each ungrounded "Input Conductor".

That alone would lead me to believe that the "input conductors" to the 2nd transformer (although already protected by the first transformer) would still require an OCPD installed in the conductors to the 2nd transformer.

JAP>

 

[Dale001289]

If I have a 50 amp fuse at the beginning of a circuit, they have a 100 amp fused switch with 100 amp fuses in it as local disconnect near the load - that load is still protected by a 50 amp overcurrent device even though there is a 100 amp fuse in the series. I don't see the transformer situation of this thread being any different.

If the first device in the circuit is sized per maximum allowed VA then you could put in two or one hundred transformers in series, that first device still respond when VA exceeds what it was sized for - how is that not protecting any of the others assuming they are at least equal or greater to the VA rating allowed by that first device? A multiwire secondary somewhere in there changes the rules - it is possible to overload a portion of the secondary in those applications, but two wire in/two wire out VA in is going to be equal to VA out, with relatively negligible losses.

Taking it through the steps:
Per 450.3(B) you may omit secondary protection of the transformer windings where the primary OCPD is set at 125%. Per 240.21(C)(1) you may omit secondary protection of the conductors where the system is two-wire. However, in no case may the primary OCPD be omitted for ANY transformer – 450.3(B) does not differentiate between single or three phase transformers. Since the NEC doesn't directly address a step-up and step-down scenario, one has no choice other than default to the basic protection requirements.

 

[jap]

With the output of a transformer being an "induced voltage" from the primary to the secondary, it would be a stretch to say that the primary OCPD was actually in series with the output conductors seeing as how there is not a mechanical connection between the input and output winding of the transformer.

But that's just me thinking out loud.

JAP>

 

[Dale001289]

With the output of a transformer being an "induced voltage" from the primary to the secondary, it would be a stretch to say that the primary OCPD was actually in series with the output conductors seeing as how there is not a mechanical connection between the input and output winding of the transformer.

But that's just me thinking out loud.

JAP>

Agreed, each transformer is separately derived.

 

[kwired]

Taking it through the steps:
Per 450.3(B) you may omit secondary protection of the transformer windings where the primary OCPD is set at 125%. Per 240.21(C)(1) you may omit secondary protection of the conductors where the system is two-wire. However, in no case may the primary OCPD be omitted for ANY transformer – 450.3(B) does not differentiate between single or three phase transformers. Since the NEC doesn't directly address a step-up and step-down scenario, one has no choice other than default to the basic protection requirements.

If primary conductor is protected so is the primary - at same current level, I don't see why that is so hard for some to grasp. Putting in a second device - say 100 amp fuse on a 240 primary then a 50 amp fuse on the second primary - it will just be a race to see which one blows first if the final secondary is loaded to say 150 amps and may not be predictable at all which one will go first.

With the output of a transformer being an "induced voltage" from the primary to the secondary, it would be a stretch to say that the primary OCPD was actually in series with the output conductors seeing as how there is not a mechanical connection between the input and output winding of the transformer.

But that's just me thinking out loud.

JAP>

I agree it is not in electrical series, but is still magnetically coupled at a set ratio. If output side draws x amps then input side draws current at same ratio as primary to secondary voltage, you don't pick up additional power via magnetic coupling of a simple transformer, and losses that are there are for the most part negligible.

I was originally going to side with the team that were saying a primary OCPD wasn't needed for the line side of the 2nd transformer myself, and felt the conductors to the 2nd transformer would be protected by 1st one that was protected by it's primary, and they are.

But then I read 450.4 (A).

Putting aside the mention of any OCP, 450.4(A) indicates 600v nominal or less shall be protected by and individual overcurrent device be installed in series with each ungrounded "Input Conductor".

That alone would lead me to believe that the "input conductors" to the 2nd transformer (although already protected by the first transformer) would still require an OCPD installed in the conductors to the 2nd transformer.

JAP>

450.4 is for autotransformers.