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 Post subject: Conductor Size Estimation
PostPosted: Mon Jul 20, 2015 12:03 pm 
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Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
We are making a proposal for a large protective coordination and arc hazard analysis (1600 points). We have been advised that in most cases, there is no record of the conductor sizes, either by cable schedule or shown on one-line diagrams. Physically determining the conductor size would involve opening cable entrance compartments, removing covers from switchgear, etc. We will not be able to de-energize most of the equipment for the data gathering. Even after gaining access to the cable terminations, I expect that we won't be able to see the cable markings in many cases without manipulating the cables. Measuring the size of cables might be possible with insulated calipers in some cases. In any case, to do this safely will be very time consuming or impossible.

Do you believe that it would be acceptable to assume that the cable size is the minimum that would be protected, according to NEC, by the overcurrent protective device? Do you know of any reference that could justify such an assumption? Do you know of any reference that would prohibit such an assumption?


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 Post subject: Re: Conductor Size Estimation
PostPosted: Mon Jul 20, 2015 3:16 pm 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 523
Location: Wisconsin
jghrist wrote:
Do you believe that it would be acceptable to assume that the cable size is the minimum that would be protected, according to NEC, by the overcurrent protective device? Do you know of any reference that could justify such an assumption? Do you know of any reference that would prohibit such an assumption?


Cable insulation type has very little impact on arc flash results.
Conductor size means everything.
If you simply estimate the input data, you might as well just simply estimate the results.

Some people use a micrometer to measure the outside of the conductor insulation. This can give a reasonable approximation of the conductor size.
Some studies will be run using a range of NEC sized and reasonable larger and smaller conductors, with the intent of finding the worst case result. Some installations used 90C wire sizes, in which case you might need to look at conductors 2-3 sizes smaller than the 75C tables.


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 Post subject: Re: Conductor Size Estimation
PostPosted: Tue Jul 21, 2015 10:26 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
Most of the time, the cable markings are so faded that I use them by exception, not the rule. And even if you just go by the breaker sizes, it is inaccurate at best, and we can't forget that NEC has not one but THREE different methods of doing a simple ampacity measurement. The most intriguing is the one where actual measurements are taken over a period of time. I frequently use this in industrial plants when doing modifications because the alternative could mean replacing a substantial amount of switchgear and wiring to upsize everything when in actual reality the loads never approach their theoretical estimates. Quite often careful data logging over a period of time reveals that there is more than enough ampacity to add at least a reasonable amount of additional load without upsizing cables, breakers, etc. And poor past maintenance practices in the past have resulted in jacked up breaker settings so that it becomes nearly impossible to tell what the correct cable size is based on breaker settings alone. And we haven't even touched on the "fully rated" breaker issue that crops up from time to time either.

Suffice to say, you get insulated micrometers and you measure it. You first look for markings and hope for the best but if you can't find it, it takes some digging and some ingenuity. This is one of the parts of doing the field survey that are aggravating.


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 Post subject: Re: Conductor Size Estimation
PostPosted: Mon Jul 27, 2015 10:46 am 
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Joined: Fri Apr 15, 2011 7:43 am
Posts: 177
Location: Colorado
When I cannot read the cable size or get the cable information I use the breaker size and calculate the wire size based on 75 degC. Most importantly, I notate exactly what has been done on the one-line I provide in some fashion as well as the input data in the report. I do the same with the cable length as well.

My theory is:
1. Be as accurate as Possible.
2. Use good engineering judgement/practice where necessary.
3. All electrical calculations are a reasonable guess based on the best information available, experience, and education!
4. Explain, clearly, anything that is not obvious.


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 Post subject: Re: Conductor Size Estimation
PostPosted: Wed Jul 29, 2015 4:52 pm 
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Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
Thanks for the responses.

I think we should propose using breaker or fuse sizes for fixed trip devices, at least where the lengths are not long. Cable size makes less difference in the fault current for short lengths. Also, the cable size may be increased on long runs for voltage drop reasons.

The load doesn't really make that much difference, as long as the breakers are sized based on the conductor size selected.

Where there are relays or settable trip devices, we have a problem. The scope includes protection coordination. Part of what goes into the selection of settings is protecting the cable. If you don't know the cable size, that's a problem. We could use the existing settings to estimate cable size, but relay settings are usually governed by factors other than cable protection.

I doubt that we will get the project anyway. It is a fixed fee contract, with adjustments if the number of points labeled changes from the estimated number. We have been advised that there are no cable schedules or one-lines showing cable sizes. There are too many unknowns to make a reasonable firm cost estimate; what is really needed is a cost plus contract.

Even if we assume wire sizes for fixed trip devices, there will be enough cases where we need to open up and measure cable sizes that the cost will be very high to do it safely. De-energizing will not be an option in most cases. I imagine that a safe estimate for determining cable sizes alone will exceed the total amount that the client has budgeted for the study.


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 Post subject: Re: Conductor Size Estimation
PostPosted: Fri Jul 31, 2015 10:01 pm 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
With that many unknkwns and the fact that it should be cost-plus or T&M, the only remaining way to provide a bid is either to provide a bid to do an estimate to a given class, or else to specify a specific, narrow scope of your own and bid against the narrow scope. Anything beyond scope comes out in a change order. This is common in government, maintenance, and FEED contracts. As an example a maintenance contract may specify a specific scope of work to say inspect and clean a gearbox, replace seals, and repaint. It can then specify that any further needed repairs found during inspection are not included in the price. I have also as a customer given a detailed scope of work and specified that rate sheets must be included. The scope is purely fictitious, and part of the scope is defining the scope. As long as meaningful payments/draws are in the agreements, this allows for meaningful means to begin work and determine the real scope at a later date. This approach allows for a fixed bid process on what is essentially an unknown/undefined scope of work, and is somewhat more acceptable than say straight cost-plus or T&M, or bidding on doing the detailed scope of work. Having dealt with an unscrupulous contractor that abused T&M's, this approach avoided the issues with T&M's by providing a legitimate basis for comparison. The biggest pitfall is that a vendor knowing that the "scope" is a straw man, may underbid and make it up in a change order. The only way to combat this is with staged draws, and cancelling the PO after partial work and rebidding on the revised scope. T&M is best when the customer manages the job, when it is for a certain amount of labor/crafts when the customer runs the job.


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