How to Calculate Connected Loads

<...sum all the single phase loads and divide it by three to get a equivalent connected load in three phase?...>

It dpends upon the distribution of the single phase loads across the three phases.

In an ideal situation, the single phase loads will be distributed equally so as to minimise the feeder cable size and the ratings of the upstream circuit protection devices. In practice, equal distribution of single phase loads across the phases is not possible, as these are the loads more likely in some cases to be switched on and off. So some load imbalance is almost inevitable.

There may also be phase imbalance considerations as part of the utility supply organisation's tariff for the power consumed. The chances are that the red phase, or whatever the equivalent is today under BS7671(!), is more heavily loaded than the others, as ad-hoc additions to existing installations usually start at the top-left of the dis board, and the (old) red phase is usually at the top. It is therefore worth reviewing single phase load distribution from time to time to see whether economies in tariff charges can be made by redistributing single phase loads better across the (old) yellow and the (old) blue phases instead. Sometimes this can be more effective and more economical than uprating supply cables and protective devices elsewhere.

You should calculate separately three phase load and single phase load. If your service is 208/120V "Wye" , the single 120V should be added to this phase where is or will be connected, or 208V (often called single phase) should be added to both phases connected.

But If your service is 240/120V "delta", the single phase 120V (split-phase should balanced0 or 240V should be added to this phase where is or will be connected. Please remember about overcurrent protection.

Since the power source are three-phase from MDB. then at the sub-panels are mixed loads both single phase and three phase. The most important is look at your panel board shedules. You need to at least balance the load distribution per phase. At the maximum peak load considering that all load are energized, the maximum current draws on each phase shall not be exceed 80% of the maximum amp rating of the main breaker of panel board itself. Take note that unbalance load will instantly build up excessive current at one of the three-leg causes the unsymmetrical tripping at main.

DEAR,

CONNECTED LOAD IS NORMALLY GIVEN IN KW. KW IS A SCALAR QUANTITY OF POWER

SO, WETHER 3 PHASE OR SINGLE PHASE, TOTAL OF RATED KW OF ALL THE EQUIPMENTS IS TERMED AS A CONNECTED LOAD.

Its not true. it will give you wrong idea abt actual scenario. Toral 3 Phase kW of all single phase kW will be averge and not sum.

My dear

Don't get confused. Look at your power triangle formula. The KVA is the apparent power resulting from the quadratic quantities of Real Power(Kw) and Reactive power(KVar). It differs only because in your vector analysis @ phase reference @0 deg. @30 deg displacement then your power KVA and KW has something to do with Power Factor.KVA could only be cosidered equal to KW if Power Factor is unity. When sizing the acrual Gen. set use connected load express in KW or your total net load after application of demand factor. NEC adapt 80% power factor on load.

The only way to be absolutely certain that you have a true picture of the load distribution is to measure it. And, typically, it must be measured over time. The load balance will change as different loads are switched on and off. Without actually monitoring the system for a period of time, there is no way to predict accurately what you have.

Good day,

I would like to say that I agree with "Harry Potter".

The most important thing to take note of is the fact that three-phase loads can also be looked at as three (3) single-phase loads. Equation for calculating power consumption of a three-phase load is S_3phase = 3 x S_1phase = 3 xSingle_phase_power = 3 x ((V_line/sqrt_3)*I) = sqrt_3 x V_line x I. So, this is effectively the sum of ALL the three(3) single-phase power consumptions (which is always provided on the equipment nameplate).

Thus, since the load power_consumption (rated) for the single-phase and the three-phase loads is always known, you can just add ALL of their kW-ratings to calculate a connected-load. NB:- This is the total power-consumption as seen by the source i.e. with the assumption that every connected-load is switched on & that it consumes its rated/nominal power.

In answer to your question.

No.

Connected load is the total load "connected" to any phase and is considered the maximum load that phase will carry to date. You need to start by making out a panel schedule for the loads you are trying to balance. Refer to NEC Article 220 "Branch circuit, Feeder, and Service Connections. That should walk you through what you need to know to calculate connected load and balance your panelboard.

u havenot told whether delta or star connected, in the load side, sqroot(3)vi.p.f, or (1/sqroot(3)).v.i.p.f is power consumed at the load side.....manas sinha

Add the connected loads as follow:

1. Single phase load ( between line-neutral) 100% on each phase, trying to distribute as uniformly as possible between the 3 phases.

2. 3-phase load (irrespective if delta or Y) 33.33% of each on each phase.

3. (1-phase load (2-pole-load) line-line) : 50% of each on the 2 phases involved.

3. Add up loads for each phase

Example:

5 Loads ( Line-Neutral) (kVA): 10,50,40,20

3 Loads (3-phase)(kVA): 30,60,15

2 Loads (Line-Line) (kVA): 30,40

An example of load distribution between phases would be:

Phase A: 10 + 20 + 30/3 + 60/3 + 15/3 + 40/2 = 85 kVA

Phase B: 50 + 30/3 + 60/3 + 15/3 + 30/2 = 100 kVA

Phase C: 40 + 30/3 + 60/3 + 15/3 + 30/2 + 40/2= 110 kVA

The total connected load (for the entire 3-phase system) is 295 kVA. The connected load for each phase is as indicated above.

When dividing the total load by 3 (number of phases) (295/3 = 98.33) you get the average load.

Thanks every one for the reply. Perhaps I have not made myself clear in my initial querry. What I actually wanted to know is that while filling up the application for obtaining power supply from the state electricity distribution companies, there is a column space asking me to fill up the total connected load. Please note that I am in India and this question is country specific as I am not aware of the procedures in other countries. My question is what should I fill in this column against total connected load (though my application is seeking three phase power supply)

i) should i just sum up the kW rating as per name plate of all equipment (irrespective of whether they are three phase (star connected or delate connnected) or whether i should sum up the Kw rating of all the three phase equipment ( say A kw) and then sum up the single phase equipment (and divide by three) , say B kw and finally sum up these (A+B kW).

ii) What if my connected equipment is not expected to work all times but is a rotational standby equipment (say two pumps, one main and one standby). Should I add up the Kw rating of my stanby equipment also?

I would be very glad if any of engineers from India and who have handled such applications seeking power supply from EB authorities may answer my querry please.

What you are asking about then is called "Maximum Demand" and is not country specific.

This is a standard question and the local standards and codes will instruct you on how this is done with any special requirements for your location.

Regards,
Sapper