Standard Basis for Power Distribution Load Study 1

[edaughters]

Are there any national standards, codes or recommended practices for performing a load study on an existing electrical distribution system in a petro-chem process plant? We have a fairly typical power distribution architecture: power feeders, transformers, multiple MCC distribution – motors, lighting, panels, welding receptacles, etc. and have been asked to determine and document the available capacity.

We have two previous studies. One uses its set of assumptions and ran the calculations in SKM (we don't have the output files that I can find). The other was 10 months later using a different set of “experienced based practical real world” assumptions used for other plant designs. Neither study report indicated an NEC, IEC, or any other standard or code as a basis for the assumptions made.

So, here are my questions:
1. Is there a common standard used to base a load study assumptions? If so, what?
2. Does using a modeling program like SKM or ETAP inherently incorporate commonly accepted assumptions?

My appreciation for your response.

 

[wroggent]

If you don't adhere to any standard of how a power system should be designed, you could claim you have infinite capacity.

1.) The NEC, IEEE 141 Red Book, and IEEE 242 Buff Book are standards I use frequently in the line of work I do- this might be a good start.

2.) Using ETAP (and probably SKM) will allow you more quickly see where you might have Code violations, but it won't tell you everything.

There are companies that can be hired to do this sort of work; I recommend you use one if you or your co-workers don't know how to do this. An improperly designed electrical system can kill. If you don't know much about your power system I suggest you first see if it is up to Code and then only if it is ok or after you correct violations, you can make a determination of any extra capacity you might have.

 

[edaughters]

Thank you for replying to this post. For clarity, the plant power distribution system was designed and installed in accordance with all applicable codes and standards. The issue being posted here is; what standards apply to making the assumptions necessary to determine the remaining capacity? For example, should all current loads be determined at 100% of rating based on NEC tables (FLA)? How about “spare” equipment motors? How about lighting panels (normally on)? How about welding receptacles (typically not in use)? How about panelboards?

Perhaps these assumptions are made on a company-by-company or contractor-by-contractor basis but the question is; are there national or international standards, codes, etc. that provide a generally acceptable set of assumptions to use for determining spare capacity?

Sorry if I was unclear about the purpose of the post. Thank you again for your answer.

 

[mastergate]

I found this comparison NEC / IEC. I hope it will be useful

National Electrical Code, NFPA 70
Art. 220 Branch-Circuit, Feeder and Service Calculations
The NEC contains specific rules for calculating the size of electrical service, feeders, and branch circuits, and how much
load can be safety connected to each. General lighting loads are based on volt-amperes per square foot and the volt-ampere
values vary between 0.25 and 3.5 VA per square foot.
There are various demand factors for multiple loads where all of the loads are not expected to be energized at the same time.
For feeder and service loads, there are optional calculations which are permitted to be used. Farms have a different load
composition, therefore, separate rules are specified for computing farm loads.

IEC 60364
133.2.4 Power equipment is to be selected to be suitable for the load
Sec. 311 Maximum demand and diversity
Maximum demand and diversity are two factors that need to be considered in sizing electrical circuits, and the power equipment is to be selected to be suitable for the load.
No guidance is provided for determining diversity factors and conditions under which they can be applied. Each country has to determine the minimum safe electrical service that can be provided for premises and how much load can be applied to each circuit.

 

[rcw retired EE]

In an existing facility that is operating, use real world data, not calculations. Look for demand meters, maximum demand metering or other records of actual load. A demand meter with a 15 minute or 30 minute demand reading will approximate the actual load on the electrical equipment. From that, use engineering judgment to account for spare motors, unused welding outlets, unusual operating conditions, temporary power needs during shutdowns, cold weather operation with lots of heat tracing, etc. The NEC paragraph 220.87 "Determining Existing Loads" recognizes this as a valid method.

Calculations are great, but real world data and currents are what determine the real equipment capacity.

I am not aware of a standard that describes this engineering judgment. I don't think the ETAP style programs include knowledge based adjustments except as a demand or load factor that can be entered for individual loads or groups/types of loads. Typical demand or load factors for a refinery are in the 60-80% range (actual load is = 60-80% of connected or calculated load).


As an example, we sized transformers = 75% of total connected load. During full plant operation the transformers were running 75% loaded. Actual loading was 75% x 75% = 56% +/- of connected load.

Another facility where the client required us to design for 100% connected load had 1500 and 2500 kVA unit substations. The maximum demand metering after two years of operation showed max load on any transformer was 450 kVA. Good for the client’s future capacity but a waste of our money.