Manual Disconnect spring loaded finger blade type

Status
Not open for further replies.
jcassity

jcassity

Senior Member
Location
24941
Hello everyone, random newbie here and I think I will be on often.

I sure hope someone here can help cause i cant find the answer in my NEC 2005 or the Ugly's or online or for that matter,, I just simply at random searching for any company willing to talk about my issue.

Pls site your answer with an NEC ref, there is a poorly served customer out there IMHO with several sites wired goofy.


We all know that circuit breakers, EEE Disconnects and fused disconnects must be cabled with the correct ampacity of wire size.
so, here is the question,,,,,,

Does this rule apply for manual disconnects with spring loaded finger blades as well?

Its getting really bad with your local power company says " sir, your way over my head on that one" , same holds true with Telcordia but i await a call back from one of the editors of the NEC 2005 that works for Allegheney Power.



A little background,,, and this happens to be a DC Power Plant application.

I have noticed on about two dozen Time Warner sites and a few PacBell sites that the 1520AH battery string will have a 1200A manual two finger blade disconnect on the output (not a breaker, no fuses, no current limiting at all even in a thermal runaway situation). However,, I only see one 350kcmil cable on the disconnect and one 350Kcmil cable on the return. The ampacity of 350 at 75degC "in raceway is only 310A.

This battery cable travels a one way distance of 60' to the DC rectifier plant collector buss.

If i were to engineer that situation treating the disconnect as a breaker (which it is not), the formula would be...

11.1 x Loop Lenght x 80% of max breaker size / .25 voltage drop.

so..... 11.1 x 120' x 960A / .25vd = 5,144,880cmil

This setup would require at worse case 3 cables of 750Kcmil per polairity or 6 total runs.

I have enjoyed my never ending frustration with the NEC and its glorious three or 4 paragraph reference to DC power only to cover such silly topics isolated to grounding. Even in the Emergency power section of the nec, we get a smidge of tech speak with reguard to batteries.

As a side note,, I asked Allegheney power the following question as to help me reach my goal.
I said,,,,,,,,"If an employee of yours arrived at a site and was reading the meter of a 400A single phase service and while doing so, glanced over and noticed a manual spring loaded finger blade 600A disconnect, What would you think?"

My answer directly from two engineers was the following:
"the NEC strictly states the disconnect must not be rated less than the service rating".
Yep, they are right, i would be an idiot to disagree.
I asked if this site needs to match the disconnect to the service size an they said NO. The customer has obviously planned on room for upsize service in the future. Yep, Ill agree to that,,(although the only obvious growth would be 120/208 3phase).
I asked if its a violation, they said no.

So back to my original issue,,I have a 1520AH battery stack wired for -48vdc and the disconnect is 1200A but the cables are too damn small IMHO, and ,,, to beat all,,, the customers dc power load currently is 400A. For now, the site is not in trouble,, but as soon as they add more DC power telephony gear, the dc power draw will increase. They do not even have a current limiting device in line with the hot leg of the battery stack.

Hope someone here and answer the above question,, ignore the rest of the rant.
All i need to know is what ref is there to rules on wiring non fused blade disconnect and if there is a ref to treaing them the same as breakers with respect to ampacity rating.
 
anyone??????????:grin:

I spoke with Tim Kroacher (sp?) who was an editor for the NEC 2002. That converstation whet well but he did share information that I was not ready for.
HE insisted that conductors need only sized to the "application"
 
jcassity said:
... Hope someone here and answer the above question,, ignore the rest of the rant. ...
Click to expand...
I couldn't pick a specific question out of this. Perhaps you could narrow it down to the installation specifics and the all improtant question.

jcassity said:
... All i need to know is what ref is there to rules on wiring non fused blade disconnect and if there is a ref to treaing them the same as breakers with respect to ampacity rating.
Click to expand...
I have no solid clues if I am even close to your concern. Given that:

A 1200A rating on a disconnect has no connection to the size wire that must be used. (other than the load and the wire better be less than the disconnect rating) That specific size was probably used because it is common in the industry. Your engineer was right - size the wire to the load. The disconnect may not even have a DC rating - does it? It may not even be rated for breaking a DC inductive load - is it?

jcassity said:
... If i were to engineer that situation treating the disconnect as a breaker ...
Click to expand...
It's a switch, not OCP. Why would you to do this?

carl
 
You didnt read my post,, and also,, i never said i had an "engineer" anywhere. That person would be me. Your not helping by talking down to me.
 
The only question I can find is:

jcassity said:
.....Does this rule apply for manual disconnects with spring loaded finger blades as well? .....
Click to expand...

What rule are you referring to?

I assume by "spring loaded finger blades" you mean quick-make quick-break....
 
Your problem is you dont realize the disconnect needs to be a circuit breaker,,yet the NEC does not point it out (because no on associated with its creation has a clue about DC) and while i am at it, Im not sure anyone in the AC industry could tell me what size wire I would need for the following spec......

200A at 300' with a vd of .25.

Im so glad I have background in both AC and DC.

My question isnt that difficult,,,,,,,, and your asking questions with questions is rude since i put a lot of effort in giving details. Dont ask me why I think the cable is too small when you have no clue on how flexible the load could be. Today the load on site X could be 480A, next year it could be more and up to the quantified amount of rectifiers the customer has been supplied with by me which is 3.6kw modules x 18,,, Yes,,thats a 1200A plant ***CAPABLE**.

I have pics of examples for your view pleasure but if you dont understand nor have hands on experience with DC power plant,, then stay off this thread.
 
480sparky said:
The only question I can find is:



What rule are you referring to?

I assume by "spring loaded finger blades" you mean quick-make quick-break....
Click to expand...


Yes,, after reviewing some threads prior to joining, I noticed a lot of people getting sacastic remarks for lack of detail. SO,, if a spring loaded finger blade manual disconnect does not nail it down to a quick make due to various ways people may call it by name, then I cant help on any more detail.

And,, the rule(s) i refer to are realting to wire sizing to breaker.

I almost feel like i need to move on, no one on this site seems to have the experience i thought might be here. Its pretty bad when even one of the authors of the NEC2002 tells me Im over his head. I think Ill move on and kill this thread.

I think I will lay it out to the customer by starting off with the following NEC08 ref 310.15.
"Preservation of safety benefits of established insdustry practices and standardized procedures"

next I will go to 240.4 and then 240.21-h.

I can see Im on my 0wn here.
 
Last edited:
jcassity said:
Yes,, after reviewing some threads prior to joining, I noticed a lot of people getting sacastic remarks for lack of detail. SO,, if a spring loaded finger blade manual disconnect does not nail it down to a quick make due to various ways people may call it by name, then I cant help on any more detail.
Click to expand...

I'm not being sarcastic. I'm only requesting that you ask your question.

If you 'can't help with any more detail', how about posting the pix you have?
 
j:

It's going to be pretty difficult for others to follow this thread if you modify a post after it's been responded to.

It's not that there's no one here that can answer your question, it's the fact that you haven't asked one.

If you do, then maybe someone here can reply. Or someone who can will answer when they're online later.
 
480sparky said:
OK, let's try again.

What is the question?
Click to expand...


I guess i need to start off like this.

Customer X buys two DC power plants.
Customer can draw 1200A off each plant
The customer does not have to draw 1200A for many reasons.
There is not enough equipment in the facility to draw that much
or they planned for unforseen futhre growth (meaning they have the power if ever the time comes they need to use it.

The customers load could be anything from 0 on up to what ever figure you decide to fit in, but we will say for general speaking, thier load is 400A.

What this means is they can add DC equpment up to about 1100A and maintain N+1 on thier rectifiers.

This also means that if that day comes when the Power plant reaches the N+1 rating due to customer adding equppment, there could also be an outtage on the 480/277 Wye service. As a secondary backup when the generater fails to start, the batteries are discharging upon the customers load.
There is not current limit in my original post explaination, nor is this disconnect thermal or fused.

Two things.....
when the batteries age, there are a host of things that happen that we all know and eventually failure. I account for the thremal runaway as an option because it can happen. I also account for the following situation,
the utility comes back on and the rectifiers are now the load bearing supplier to both the customers equipment and the discharged batteries.

I can see my rectifiers running at 125% each (18 modules 3.6kw ea).

DC power plants are different, you have to plan for the application, and the application happens to be the size plant they purchased ,, which was a 1200A plant.
the size disconnect does not bother me, its the wire size compared to worse case batttery discharge AH rate and the problems associated with the plants capablity to produce and deliver 1200A across 1/350 per polarity.

Today this is not a problem,, but the customers DC load will grow, upon installation, a system should be cabled to its application.

SO my question was slightly modifed so as to apply it in the world of AC.

Where in the NEC does it say wires must be sized to a quick make disconnect amapcity rating?

Allegheney power said exactly what i found in my 2002 manual,, A disconnect must not be sized lower than its service size.
My primary focus is the wire size i explained earlier.,, not the disconnect size.

My secondary issue is the fact that this disconnect needs to be of a breaker type for protection such as those made by EEE.


My goal is to correct the customers sites (its my job to do so) and also to present them with references that illustrate the potential for fire hazards and equpment damage if these systems are not properly cables.

A generator company installed these DC plants, they had not idea what they were doing. The jotted down notes that thier anticpated load back then would be about 300A but they would like more DC power growth for future.

SO,, what did the AC guys do? they cabled for the customers load back then, now the load continues to grow.

Imagine the batteries discharging 1000A across one 350 per pole on that system.
 
Last edited:
480sparky said:
j:

It's going to be pretty difficult for others to follow this thread if you modify a post after it's been responded to.
.
Click to expand...

I am sorry, I have recently learned that this site has time limits on responses and or edits. I timed out more than once already.

again, sorry to confuzzel you and thank you for setting the record.
 
brian john said:
I have read and reread his post and cannot figure out the issue.
Click to expand...

ok,,maybe i type too much, and i should not provide detail and let you guys ask a million questions.:smile:


Customer has a 1200A DC power plant and a 1520AH UnigyII battery stack is cabled with 350mcm to the the DC plant.

The customers draw can be from 0 - 1200A but is currently 400A and growing.

A DC plant with a battery stack residing 120 loop feet away from the plant at a .25 voltage drop comes out to be 4/750 per pole.
Derating the breaker to 80% would get you by with less cable. this is the second time i have explained this part.

That is my issue in a nutshell, but since the disconnect is not a fused or breaker disconnect, my question still remains,

Do we size our cables to a quick make/break disconnects ampacity or not?
 
Last edited:
brian john said:
I have read and reread his post and cannot figure out the issue.
Click to expand...

ok,,maybe i type too much, and i should not provide detail and let you guys ask a million questions.:smile:


Customer has a 1200A DC power plant and a 1520AH UnigyII battery stack is cabled with 350mcm to the the DC plant.

The customers draw can be from 0 - 1200A but is currently 400A and growing.

That is my issue in a nutshell.
Currently there is a 1200A quick make/break disc wired with 1/350 per pole.
 
JC, my contribution:

Missing from your descriptions is a means of over-current protection. Are there any fuses or breakers in the lines? There should be over-current protection to prevernt damage to the conductors themselves - where is that?

The cables should obviously be sized to the load, and the disconnect should be rated at least that much. The battery rating in amp-hours does not tell you their current capability into a direct short - that could be thousands.
 
LarryFine said:
JC, my contribution:

Missing from your descriptions is a means of over-current protection. Are there any fuses or breakers in the lines? There should be over-current protection to prevernt damage to the conductors themselves - where is that?

The cables should obviously be sized to the load, and the disconnect should be rated at least that much. The battery rating in amp-hours does not tell you their current capability into a direct short - that could be thousands.
Click to expand...

wow,,

Your telling me that I am missing information:D
You are the first one to illude to the direction I know to be fact ,, but your answer to your question resides in post 1
 
LarryFine said:
Okay, so no over-current protection. How was it decided that a 1200a disco was adequate?
Click to expand...

Yes sir,, and now you know why i am here.

If it were a EEE breaker, I would have cabled it at worse case, 3/750 per pole.

The load can change well after the installation is complete. Two years from now, the customer could have tripple that load.,, seen it many times where they are treading beyond N+1 redundancy.

Now to thicken the plot since your with me,,,

there are actually two 1200DC plants which are load sharing and are cabled to the same collector bar system. There are actually 3 battery stacks per plant.

What this means is the customers point of Max ampacity draw is 600A on each plant. The reason is if plant "LOAD A" were to fail, Load B plant would have the rectifier capacity to take on an additional 600a.

what this has to do with the topic >> nothing.:cool:


I hope you can see my point now and earlier I referenced an NEC code which nails the issue as this does deviate from the industry standard.
***however****,
The cables are compliant with the customers load at this time. IT was not cabled to the system applicaton but to the customers application I would assume. At the time, the customers expected load was low but is growing.

SO,, the definition of "applicaton" is where this issue resides unless i can find out in the NEC where it says a quick disc must be cabled to its ampacity.

From there, I have to use a DC power specific forumula in order to calculate the correct size wire. There is a specific and industry standard "allowable" voltage drop within all DC power plant layouts.
.25v (from the battery to the Rectifier Power plant)
.75v to .875 (rectifier bay to the first main distribution bay called a BDFB)
.5v(from the BDFB or battery distribution fuse bay out to the gear that needs power)

When you measure the voltage at the far end equipment, you should see a voltage loss of no greater than 1.5vdc + - a smidg
 
Last edited:
Status
Not open for further replies.
Top