Power Calculations For An Industrial Control Panel

rustydud · Oct 26, 2018

Hello everyone, and thanks in advance for viewing.

I have an industrial control panel that is powering a VFD, pilot lights, relays, digital panel meters, and some other devices. There is one(1) power feed powering the panel: it is 460VAC, 3-phase from a motor control center (MCC) bucket.

The panel contains the following:

(1) VFD. It is 460VAC, 3-phase and is running a 20HP pump.
(1) 500VA transformer for transforming 460VAC, 3-phase to 120VAC, 1-phase. A few loads in the panel are 120VAC, 1-phase.
(1) 24VDC power supply that is powered by the aforementioned 120VAC, 1-phase. Several loads in the panel are 24VDC. 24VDC loads include pilot lights and digital panel meters.

I know how much current / power each device draws - I have this information readily available.

Suppose you are asked to calculate the power usage of this panel. I am looking for the proper method to do this. What is the proper method for calculating the power usage of this panel?

Rson · Oct 26, 2018

With a panel FLA - you can calculate the Watts/kwh/etc.

IMO, ignore all the 120VAC and 24VDC - they are powered by a branch of 480VAC in your application. You do need to calculate them to get the transformer size - and you should oversize the transformer to make sure you have room to add if necessary.

Just calculate the 480 loads - which for you are:
20HP motor - 27A FLA (per NEC chart)
500VA Transformer - about 1/2A at 480VAC

Generally, you want the largest load (20HP motor - so 27A) listed on your main panel tag along with the total load ( ~ 27.6A). I don't typically build UL panels - so I'm a little grey on the UL portion - I believe they want the actual NEC chart value and not the actual calculated value.

Personally, I would size this panel with a 100A disconnect, fused at 70A, (60A for the motor) in case any additions happen in the future. If you are sure there will be no additions, you could fuse at 60A with a 60A disconnect.

(my values are from Bussman's SPD - if using other manufacturers or breakers, these numbers don't work)

jraef · Oct 26, 2018

Rson said:
With a panel FLA - you can calculate the Watts/kwh/etc.

IMO, ignore all the 120VAC and 24VDC - they are powered by a branch of 480VAC in your application. You do need to calculate them to get the transformer size - and you should oversize the transformer to make sure you have room to add if necessary.

Just calculate the 480 loads - which for you are:
20HP motor - 27A FLA (per NEC chart)
500VA Transformer - about 1/2A at 480VAC

Generally, you want the largest load (20HP motor - so 27A) listed on your main panel tag along with the total load ( ~ 27.6A). I don't typically build UL panels - so I'm a little grey on the UL portion - I believe they want the actual NEC chart value and not the actual calculated value.

Personally, I would size this panel with a 100A disconnect, fused at 70A, (60A for the motor) in case any additions happen in the future. If you are sure there will be no additions, you could fuse at 60A with a 60A disconnect.

(my values are from Bussman's SPD - if using other manufacturers or breakers, these numbers don't work)

I agree, but I’ll just point out that this only tells you what it MIGHT consume, worst case scenario. The motor will only pull FLA if DEMANDED by the load, but the load is king here. If someone wants to know the ACTUAL power being consumed then the answer can ONLY be “it cannot be calculated, it can only be measured”. Many VFDs will actually tell you the kW it is putting out, so add about 3-5% to that value as input losses within the VFD and CPT and you will have a workable number that means something.

Power Calculations For An Industrial Control Panel

rustydud

Member

Rson

Member

jraef

Member

Similar Topics