Adding additional loads to MCC 1

[rockman7892]

I am trying to determine weather or not I can add additional loads to a 480V MCC. The MCC has a 2000A bus and is protected by a 2000A breaker.

I added the loads using 1hp = 1kVA. I took 25% of largest motor and 125% of combined continuous non-motor loads. When adding in the additional loads I come up with a total connected load of 2009A.

This 2009A is obviously greater than the 2000A bus but does not account for any diversity factor. I'm sure others here have been in similar situations in the past and wanted to see what the rule of thumb was for this type of situation.

How are breakers for welding receptacles which typically aren't connected all the time figured?

 

[davidbeach]

The NEC has a provision that allows you to determine loading through a month's worth of data from a recording meter.

 

[dpc]

I agree with David - determine what the actual load is. The 1 kVA/hp is conservative and assumes every motor is fully loaded, which is almost never the case. The actual demand load is probably closer to 70% of the connected load.

You can also take credit for motors which do not run at the same time.

If the 2000 A breaker protecting the MCC is in a 480 V switchgear, it may have a 100% rating and this could come in handy. If the 2000 A breaker is in the MCC, it is only 80% rated so you do need to apply the 125% factor to continuous loads as you have done.

 

[rbulsara]

There is no formula for diversity factor. You need to talk to the people who designed the systems of which these motors are part of. They would know as to how many motors are expected to run simultaneously. Combine that with what dpc said those running motors may not be fully loaded. Wuth all that info you can come up with a good expected demand on the MCC.

In most cases, connected load exeeds the rated capacity of an MCC.

However, if this project is to add new loads, as you imply, I would not kill myself making the existing MCC work. If the case is borderline add new MCC for new loads.

 

[burnt2x]

Try NEC 430.26. As mentioned by posters above me, you must validate your loading with historical loading data of this particular MCC.

 

[waross]

When using a recorder to determine loading, be aware of and consider seasonal changes in loading and possible occasional changes in operating procedures.
For instance during a maintenance shut down some large machines may be off line but many welding plugs may be in use.
The load will probably be less than normal but you should evaluate rather than assume.
You may have better luck looking for demand factors rather than diversity factors.
Both the CEC and the NEC have rules and tables for "Demand Factors".
Neither the CEC nor my old copy of the NEC show "Diversity" in their respective indexes.

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

 

[rockman7892]

dpc

Good point about the possible 100% rating of the breaker. The referenced breaker is a 480V Siemens RL Power Circuit breaker and is located on switchgear which is remote from the MCC. I'm currently trying to find weather or not this breaker is 80% or 100% rated.

I see what everyone else is saying about demand factors and measuring some actual values. I'll look in the NEC to see if there are any references to demand factors for this application as well as begin to take actual measurements.

Thanks for the help.

 

[rockman7892]

I looked into this situation a little bit more and found that this MCC is fed from a 2000kVA Transformer along with another 1200A bus MCC. So on the secondary of the transformr there is a 3000A main breaker, feeding a 300A bus and then a 1200A and 2000A feeder breakers to feed the two MCC's.

Looking at this I'm realizing that there must have been some demand factors, factored into this design, because a 2000kVA transformer has a rated 2408A secondary output and the connected busses on this secondary are a 1200A and 2000A bus totaling 3200A. Both the main breaker and two feeder breakers are set with LTPU's of 1 meaning that thier trip units are set at 3000A, 2000A, and 1200A.

Seeing this I begin to question weather or not this secondary main breaker and feeder breakers are adequately protecting the transformer. The transformer is fused with E rated fuses on the primary so I guess that takes care of the protection there. But with a 2408A rated transformer output and a bus load of 3200 is there a demand factor that must be assumed. Does this change the situation any?

 

[davidbeach]

What is your governing code? What are the applicable transformer protection requirements? What is the top rating of the transformer? Frankly none of it sounds unreasonable.

 

[dpc]

The load cannot be determined by adding up the breaker sizes. Take at look at the panelboard in your house.

A 3000 A breaker on a 2000 kVA 480 V secondary is perfectly reasonable. It's within 125% of the transformer rating.