AARRGHH!!! 2

[tulum]

Before I begin I am using the CE Code Book 2002...

We have a 1500kva 3 phase xfmr with 600v secondary. There is appropriate primary and secondary protection; 600A VCB and 1600A main breaker respectively.

The problem is the cables that were INSTALLED on the secondary are only 4 runs of 500mcm, which at 60deg is approximately 1260A. I say 60deg to be cautious...i.e. make sure temp is in line with breaker terminals. However, the cable insulation is good for 90deg which would be 1580A (to most people here this is close enough).

Rule 26-258 states that the conductor size on the secondary must be 125% of rated current or 1800A.

Rule 14-100 states that if the transformer was rated 750v or above the cables only need be rated for the secondary protection.

Also the breaker on the secondary has an electronic trip which may be set low, because the actual load is no where near 1600A. The problem with the cables is that there is only room for 4 cables (4 hubs and 4 terminals/phase) so I would have to remove all cables, I can't just add another run.

My question is what is the correct size cable, and disregarding that; is there anyway I can make this installation work without replacing all the cables?

I hate to be the messanger on this one...because I don't believe the saying "don't kill the messanger will apply!"

Thanks, I await your response...
TULUM

 

[jbartos]

Comment: Check the lug temp for the 1600A CB. It is normally rated 75degC. To correct the deficiency, there is a choice:
1. Increase conductor kCMIL, or
2. Reduce circuit breaker Amp Sensor to the existing correct ampacity rating, e.g. for 75degC, if lugs permit and if load permits, or
3. Add an additional subconductors run.
It would be prudent to test the current installation for proper parameters, e.g. conductor insulation since it may be damaged by now.

 

[rbulsara]

My practical and code compliant (imho) solution would be as follows:

I am going by my memory of NEC (USA).

FLA at 600V will be 1443A. Even at 75C, amapacity of 4-500kcmil is 1520A.

If you set the trip setting of the CB to 1520A or less you would be just fine, specially if the actual load is much below the 1600A.

The 125% conductor sizing rule is for 125% of the actual or calculated load. Do not confuse this with source (transformer) rating. As long as the breaker trip setting(overload protection) is equal or less than the condcutors ampacity, it meets the code.

If any theoratical short coming (if even so) is that the transformer will not be usable at full rating continuosly, which apparently is not a problem in your case.

There is not requirement in code to use every equipment at its full capacity, after all)

 

[gordonl]

If your cables are spaced in tray (which I suspect) you've got lots of capacity. From Table 1 and 5D the ampacity will be 4*660A*.74=1950A. Your transformer secondary is only rated at 1440A so that is where I would be looking to set the protection. If you know your never going to need this capacity (as you suggested) then set the trip at a more comfortable number like 1200A.

26-258(3) states that less than 125% is sufficient where your demand load won't exceed the rating and you have appropriate protection which it sounds like you do. (14-100d)

The concern of temperature rise at the cable terminations, I find it hard to imagine a switchgear assembly not constructed for use with 90C wire. (Otherwise what use is 90C wire)

 

[tulum]

rbulsara,

I hear what you are saying, and I agree totally with the logic. However, I have been over this with the inspector and his comment is that the wording is what he is bound to regardless...

26-258 - "The secondary conductors connected to the transformer shall have an ampacity rating not less than 125% of the rated secondary current of the transformer."

Any thoughts?

 

[jraef]

No help to offer since it appears the CEC is slightly different thant the NEC, but "been there, done that, got the T-shirt and the Hat". Unfortunately, the inspector is the final authority, at least here in the US. I imagine it is the same there in the Great White North. Any chance you can get the transformer manufacturer to "rerate" your transformer? Utilities do it all the time!

"Venditori de oleum-vipera non vigere excordis populi"

 

[gordonl]

I think the wording of 26-258.3 is equally clear that where the proper protection is in place less than 125% is "legal".

The 125% requirement is in response to 26-256 where 125% overcurrent protection is permitted, but as 26-258.3 states when less than 125% protection is used corresponding ampacity cables suffice.

If he insists that your not to code you could diplomatically suggest that you might like to refer to a "Technical Advisor" on this issue. They are generally not a part of the local inspection office, and are a more technical third party. (I'm in Ontario, I don't know if there are T.A.'s in other provinces)

 

[rbulsara]

darn!

unfortunately I have to go along with what jraef said. Can't help where the Inspector is adamant.

The re-rating of the tranformer sounds like a clever (new to me) idea, but I would think should have been done before the Inspector was brought in.

In the USA even tap rule could be applicable if the secondary CB is within 25 feet of the transformer and you can have a smaller breaker.

 

[gordonl]

I hate to sound rude but, are my posts even up on the board.

The very rule the inspector is quoting contradicts him. If you plainly layout for the inspector your system with transformer ratings, cable ratings, and protection settings and how it meets the code under rules 26-258.3 and 14-100.d then I am sure he will agree. If he still refuses I think you are remiss not to push it up over his head.

There are four parts to 26-258, don't be afraid to use section (3), it is printed in the same ink as (1) & (2).

Inspectors are the authority, but their authority only extends to the code. If an inspector, who comes from a background of inspecting 15A house wiring, is uncomfortable approving an installation beyond his experience, he should be willing to get advice from a more experienced inspector or from a "Technical Advisor". If he is so egotistical, I would be of a mind to go over his head. But I'm not a contractor relying on my relationship with these guys to keep my jobs on track, and hence my belly fed.

 

[peebee]

gordonl, did you say something?

Just kidding -- red star for you. Better?

 

[GOTWW]

PIPE IT WITH BUSS DUCT. Easy, not cheap.

 

[tulum]

Thanks everyone for the responses.

Gordonl,

I missed your first post...we must have been posting at the same time. To respond to your post...

It was my understanding that the cables could not be given a free air rating because they are armoured. Also the manufacturer lists the cable at 395A as per table 2. This puts me back at 1260A.

14-100d of the code only applies if the transformer secondary is rated 750V or more, which is not true in this instance. So I am back at 125% rating.

As Jbartos stated most CB lugs are rated for 75^c, therfore I must use the 75^c column of table 2, not the 90^c column even if my insulation can handle the temperature rise...the lugs are the weak link.

We are in the process of fixing the design whether we have to by code or not...

We are putting a extension box on the top of the MCC so we have room for 5 runs. Lugs that handle 6 connections were also purchased from the manufacturer.

Thanks for all the help.

Regards,
TULUM

 

[jbartos]

Comments: Normally, the electrical inspectors use very simple rules to the conductor ratings and ampacity, namely, the upstream protective device must protect the conductor. If there is a circuit breaker that is maxed to 125% of the transformer secondary, the downstream conductor should be rated accordingly. In some instances, the branch circuits having motor loads and motor starters are protected upstream by circuit breakers that are rated higher than their load side conductors; however, the starter overloads are meant to protect the conductors against overload. Therefore, the circuit breaker protects conductors against short circuit and heavy overloads.

 

[skiier]

I thought at first this was a simple 125% rule thing but now I am not so sure.
After reviewing the CEC 2002 I am somewhat confused. Here is how I interpret it.
Rule 26-258 (3) says that the secondary conductors of the TF only have to have an ampacity equal to the "demand load" provided they are protected in accordance to rules 14-100 and 14-104. The "notwithstanding Subrules (1) and (2)" wording at the beginning of the rule basically says "with the exception of.." and to me this means the 125% rule is not a fixed thing indeed. Nowhere in this rule does it say that the ampacity of the "demand load" must be a minimum of or greater than 125% of the transformer secondary current rating. Did any one see this? Or am I the only one? This rule tells me flat out that I can use a conductor with a rating of less than 125% current rating of the transformer.
Here is how I see it. You have a calculated demand load which gives you a 1600A breaker so you install a switchboard with a 1600A main breaker. The demand load would in most cases be 80% (1600 x 0.8 = 1280A)of the switchboard rating - this is how I would determine a 1600A breaker if I didn't know the actual connected load. Of course an actual load calculation is best. The 1280A figure might explain why your measured current readings are lower than you might expect.You feed the switch board with 4x500 MCM 90C rated copper in conduit for a combined rating of 4x395A = 1580A as required by code. No rule says you have to feed this breaker with a bigger wire any where. Rule 8-106 ( the 5% rule ) says I can use this ampacity with this breaker. So now I have to connect it to a source. Hmmm, do I make a transformer equal to the demand load? No. I buy a standard sized transformer. I use a standard 1500KVA because 1000KVA is too small. Why do we have to all of a sudden change our properly calculated figures just to come up to the ratings of the transformer? This is where the exceptions appear and they must be interpreted properly. The industry only makes a finite number of transformers. To rate the transformer to the load is almost impossible. So here is the exception and the people who write the codebook do not say it in plain English so I am going to.
"For a given code calculated demand load one must install a suitable distribution center containing a fuse or breaker that properly protects the demand load and distribution equipment. We realize that the conductor feeding the fuse or breaker in no case needs a rating of larger ampacity than the rating of the fuse or breaker simply because the rigid construction rules and the nature of the installation do not warrant it. So if you have a distribution panel fed from a transformer rated over 750V and all your feeders are "code distance" and heavily gaurded then we will allow exception to the 125% rule (26-258 (3))and let you connect your switchboard to an oversized transformer provided the feeders feed only the switchboard. The primary fusing protects the transformer properly, we know this and realize that secondary fusing is not always required but it's ok if you want to install it, and the switchboard main breaker protects the heavily gaurded switchboard feeder conductors from EVER carrying any more current than their rating. We also realize that sometimes we forget the origin of some rules and we must we be reminded of their original interpretation and meaning of their use ."
Is this not common friggin sense or what. Some times we have to allow "modifications" to rules or nothing would ever get done. I am so sick of inspectors who can't even interpret the rules of their own trade. Am I missing something or what? I found no rule in the code book that says you have add a conductor anywhere if your demand load is less than 1280A. You need to put down the tools and pick up a calculator and calculate your demand load. All the rules are there at your disposal it just a matter of interpretation.

 

[tulum]

Skiier, thanks for the input

As I said before I see the logic.. However, the inspector does not... he is stuck on the fact that rule 14-100 only applies to 750v or over...ON THE SECONDARY

Hmmm, Let me see,

A) Put down the tools - and tick off the inspector, vs.
B) Adding one run with a minor cost if we do it right - and stay in the inspectors good books for the future

...I think the calculations are clear.

Ya, Ya before someone replies I know - "Give him an inch he will take a mile"...well the way I see it is give him an inch this time and I will save a mile the next time I need some leeway.

Thanks for the help everyone. I think we all agree on the theory and calculations behind this one. However, we do not agree on a solution. That is neither here nor there... maybe if I had more time I would do it differently. However, the ball is rolling.

Regards,
Tulum

 

[jbartos]

Suggestion: Compare NFPA 70-2002 Art 240-3(C) Devices Rated over 800 Amperes. This Article states: Where the overcurrent device is rated over 800 Amperes, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device defined in 240.6.
This implies that small ampacity shortages to the protective device rating are not acceptable when it comes to over 800A rated circuits.

 

[skiier]

Jbartos. I don't understand your reply?
"equal to or greater than..." tells me I can use ampacity "equal to.." and I don't have use use an ampacity "greater than...".
Is a conductor with the same ampacity rating as the breaker it feeds not "equal to..".
This looks like the CEC rule that tells me I can't feed a fused disconnect with an ampacity less than it's rating. Feeding a fused disconnect with an ampacity less than it's rating is a most notable problem at splitters and distribution panels where cct modifications occur occasionally. At one time in Ontario you could feed a fused disconnect with less than it's rating as long as you fused for the conductor out of the disconnect. Proper fusing, proper protection. Everything is fine until someone removes a load from a disconnect and reconnects a greater load to that same disconnect without seeing what size conductor feeds that disconnect. The result was overloaded wiring and problems in general. So now we have to feed the disconnect with a conductor that has an ampacity equal to the maximum fusing of the disconnect. Only makes sense I suppose.
In circuits over 800A it would only make sense as well that we feed fuses and bkrs with a conductor of equal ampacity. Take for example a cct feeding a motor load requiring a 1000A bkr. The conductor feeding the motor load however is only rated at 400A. So we tap from the main splitter to our 1000A bkr with a 400A conductor as allowed at one time by code. Everything is fine until the motor load is removed because of machinery modifications and a new 800A heat load is reconnected to the 1000A bkr. You get the idea. This does happen in industry everyday. Believe ne I have seen it.

"This implies that small ampacity shortages to the protective device rating are not acceptable when it comes to over 800A rated circuits." Huh?

 

[jbartos]

Suggestion to skiier (Electrical) May 26, 2004 marked ///\\Jbartos. I don't understand your reply?
"equal to or greater than..." tells me I can use ampacity "equal to.." and I don't have use use an ampacity "greater than...".
///Yes, if the application results in such case.\\Is a conductor with the same ampacity rating as the breaker it feeds not "equal to..".
///Up to 800A and including 800A, the nearest higher rated circuit protector is allowed to use.\\This looks like the CEC rule that tells me I can't feed a fused disconnect with an ampacity less than it's rating.
///Not a bad rule. Stay on the safe side without cutting corners.\\
"This implies that small ampacity shortages to the protective device rating are not acceptable when it comes to over 800A rated circuits." Huh?
///That is what NEC indicates.\\\