Intermittent Loads

[timm33333]

In load flow, we designate loads as continuous (100% demand) and intermittent (50% demand). So if we have four loads of 100kVA, one of them is continuous and other three are intermittent: then the general practice is to size the transformer for 100+50+50+50=250kVA.

Is this a correct practice to size the transformer? I think even if three loads are intermittent, still there is possibility that all four loads will be turned on together at some point, and at that point the total load will be 400 kVA; and so the 250 kVA transformer will not be enough. What do you guys think? Thanks

 

[Skogsgurra]

It depends on the timing.

If the on period is short in comparison to the transformer's thermal time constant, then you should be fine with the rule you are using.

What is a typical (representative) load cycle in your case?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[crshears]

Such a tight number doesn't seem to allow for across-the-line motor starting characteristics; some additional margin to allow for simultaneous starts might be in order, not necessarily because of any violation of transformer time constant but because of possibly adverse effects on motor [and load] run-up due to voltage decline.

How to quantify it is an art...

Also, might there eventually be additional loads connected to the transformer? Allowing for load growth now could save you some future headaches.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Have you checked your code? How does your code define intermittent loads?
I work under the Canadian Electrical Code.
The CEC allows the use of demand factors but rather than one rule, there is a section devoted to demand factors depending on the type of occupancy and the type of load.
We don't have any rule allowing a blanket reduction in capacity for all intermittent loads such as you quote.
That said, the code does not apply to utility transformers. A utility may overload their transformers if they wish.
A contractor or design engineer is expected to provide an installation that will give satisfactory service indefinitely.
A utility has the crews and transformers available to up-size any transformer that becomes badly overloaded under service.
Although a utility may use a demand factor such as you describe, in the event that the type of load is known, based on the utilities experience, to result in serious overloading then a a more conservative demand factor may be used.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[timm33333]

It is for an industrial plant for a Canadian project. CEC section 8 addresses the demand factor, but these are mostly for sizing of conductor/receptacle (not for sizing of transformer).

The overloading of the transformer would not be allowed, which means that we cannot depend on the thermal time constant of the transformer. In the past I have worked on the industrial plants where there were lots of intermittent loads. I used the continuous loads as 100%, intermittent as 50%, and stand-by loads as 0%. And this criteria worked fine.

We have to add 25% future loads. But again this depends on the demand. If we take the connected load, then the total load would be 400 kVA and 25% future load would be 100kVA. But if we take one load as 100% and three loads as 50%, then total load would be 250 kVA and the 25% future load would be 62.5 kVA.

So for an industrial plant, if not much information is given and only available information is that which loads are continuous, intermittent, and stand-by: then I can take the continuous demand as 100% and stand-by demand as 0%. My question is that what is the general practice for the selection of intermittent demand in this situation, should it be 50%?

 

[X49]

Section 8 of the CEC covers "conductor ampacities and equipment ratings required for consumer's services, feeders, and branch circuits". This would include transformers (unless they are utility-owned).

Section 8-106 deals with demand factors. There are not many loads that you are allowed to apply demand factors to, however 8-106 (4) states:

"Where a feeder supplies loads of a cyclic or similar nature such that the maximum connected load will not be supplied at the same time, the ampacity of the feeder conductors shall be permitted to be based on the maximum load that may be connected at any one time."

The CEC Handbook notes on this rule states:

"Subrule (5) is problematic because it pertains to loads that are cyclic. Although these loads are physically connected so that it is possible to operate them at the same time, the operation of the system is such that this should not happen in normal circumstances. In this situation, the ampacity of the feeder conductors is allowed to be based on the maximum load that might be connected at any one time (called the operational diversity of the system). For example, in a custom machine shop or fabricating plant, many machines are connected, but usually only a few operate at one time. Because operation of the machines at the same time depends on the business cycle, any lessening of the load requirements requires careful discussion between the parties involved. In such cases, it is better to calculate on the larger side; otherwise, if the installed electrical equipment is too small for the load, an expensive change may be required."

In other words, you are allowed to apply a conservative demand factor for cyclic loads based on good engineering judgement. This demand factor will be different depending on the type and number of loads.

 

[timm33333]

Thanks. So if in an industrial plant we have Six 60A welding receptacles, will it be Ok that I apply a demand factor of 50% to One welding receptacle, and a demand factor of 0% to the remaining Five welding receptacles?

 

[davidbeach]

It's one thing to have six receptacles for one welder and something else again to have six receptacles for six welders.

 

[waross]

I realize that demand factors are used, but other than the factors allowed by the code, I can't see any rule that allows a designer to arbitrarily assign a demand factor.

Thanks. So if in an industrial plant we have Six 60A welding receptacles, will it be Ok that I apply a demand factor of 50% to One welding receptacle, and a demand factor of 0% to the remaining Five welding receptacles?

If the owner gives direction in writing that the receptacles are for flexibility of location of one welding machine that will be used occasionally for maintenance you are probably safe. However, in the absence of direction and assurance from the owner, suppose that there is a peak in workload and six welders are connected and operated at high duty cycles joining heavy plate?
We can make suggestions but we can't give you blanket permission to assign arbitrary demand factors, that are not supported by the code. And certainly not based on the sparse information given.
Call it tough love.
26-240 Transformers — General

26-258 Conductor size for transformers
(1) The conductors supplying transformers shall have an ampacity rating
(a) not less than 125% of the rated primary current of the transformer for a single transformer; or
(b) not less than the sum of the rated primary currents of all transformers plus 25% of the rated primary
current of the largest transformer for a group of transformers operated in parallel or on a common
feeder.
(2) The secondary conductors connected to transformers shall have an ampacity rating
(a) not less than 125% of the rated secondary current of the transformer for a single transformer; or
(b) not less than 125% of the sum of the rated secondary currents of all the transformers operated in
parallel.
(3) Notwithstanding Subrules (1) and (2), primary and secondary conductors shall be permitted to have an
ampacity rating not less than that required by the demand load, provided that they are protected in
accordance with Rules 14-100 and 14-104.



26-260 Transformer continuous load (see Appendix B)
For the purpose of transformer overcurrent protection and conductor sizes selected in accordance with
Rules 26-252 to 26-258, the continuous load as determined from the calculated load connected to the
transformer secondary shall not exceed the values specified in Rule 8-104(4) or (5).

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Looking at the code again:

(3) The calculated load in a consumer’s service, feeder, or branch circuit shall be considered a continuous load
unless it can be shown that in normal operation it will not persist for
(a) a total of more than 1 h in any two-hour period if the load does not exceed 225 A; or
(b) a total of more than 3 h in any six-hour period if the load exceeds 225 A.

I would interpret this to mean that you must be able to justify any demand factor that you choose to use for intermittent loads.

Bill
--------------------
"Why not the best?"
Jimmy Carter