Why balance loads on electrical panel if we dont know which loads will be used on site?

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roscoe0

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Hello,

I have a question regarding electrical panel load balancing.

Say you're doing a panel schedule and youre balancing the loads to get same wattage on each phase and your panel schedule ends up perfectly balanced with 10kW phase A, 10kW phase B, 10kW phase C. My question is why balance loads so perfectly on paper if we don't actually know which loads will be turned on at the site?

For example in the morning the building might have 4 kW worth turned on in phase A, 8kW on phase B and then 1kW on phase C.
In the afternoon it might change again say 9kW phase A, 2kW phase B and then 6kW phase C.

I feel like the panel will be always unbalanced despite having it balanced on ''paper'' because we dont know how the building will use their loads.

I hope my question makes sense..

Thank you!
 
There's no way to constantly achieve perfect balance. But you can certainly arrange the loads within the panel to aim towards not overloading any one or two phases.
 
I understand what you're saying.

So let's say I initially balanced the panel schedule on paper with 9.6kW phase A, 10.2kW phase B, 11.1 kW phase C. I would still have the same question..
 
roscoe0 said:
I understand what you're saying.

So let's say I initially balanced the panel schedule on paper with 9.6kW phase A, 10.2kW phase B, 11.1 kW phase C. I would still have the same question..
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If you have that kind of balance, what's the problem?
 
roscoe0 said:
I understand what you're saying.

So let's say I initially balanced the panel schedule on paper with 9.6kW phase A, 10.2kW phase B, 11.1 kW phase C. I would still have the same question..
Click to expand...
Balance for worse case, all loads on. What if can be endless. If after everything set and there is some issue you may need to make some adjustments, but if you do the calculations with known continuous loads balanced and best possible guess on the rest of what will be simultaneous, you should be close. Object would be to not overload a single phase while leaving another underloaded.
 
I admit I have never even bothered to "balance a panel on paper" for a new build. It just seems pointless due to changing loading, real life differing from NEC calcs, and in most cases due to the bloated nec calcs feeders and service will have plenty of extra capacity. IMO better results would be obtained by taking numerous clamps after the place is up and running and moving a few things around if something is consistently way off.
 
You can't really know how much occasional use equipment might run at once.

Copy machines, garbage disposals, meat slicers, curling irons, electric weed eaters....

Whether it's residential or commercial, you don't have any way of knowing how to anticipate.

My best guess on residential work is to group 15s all on one side and group 20s all on the other side.

That way lighting (more continuous) has the best chance to be balanced, and small appliance stuff (more occasional) has the best chance to be balanced.

Follow the panel schedule from the print on commercial. I've changed it a few times, but not often.
 
Well, put 80% of all your wires on one side, and 20% on the other, see what happens. Or put all the lights on one side, the rest on the other.

And as noted by others, it's good to know what type of BC's the wires are for, then 50/50 each in the panel = best chance of evenly loading the supply when the supply is tapped or multi-phase.
 
FionaZuppa said:
Well, put 80% of all your wires on one side, and 20% on the other, see what happens. Or put all the lights on one side, the rest on the other.

And as noted by others, it's good to know what type of BC's the wires are for, then 50/50 each in the panel = best chance of evenly loading the supply when the supply is tapped or multi-phase.
Click to expand...

Many moons ago, I took over a 168-room hotel build about halfway through construction. Each room had two circuits.... a 120v 20a for all the receps and lights, and a 30/208 circuit for the PTAC. The original electrician had wired every (42-space) panel so the two breakers for each room would be on top of each other. In other words, all the 120v loads would be on phase A, and all the PTACs on B-C.

That was a disaster waiting to happen. What's the first thing people do when they get into their room? Turn on the PTAC, drop off their luggage, use the facilities.... then leave to go have something to eat. So B and C would be heavily loaded, and A would hardly be used. So I rearranged the circuits so the fourteen 120v circuits were on numbers 1-14, and all the PTACs on 15-42.
 
FionaZuppa said:
Well, put 80% of all your wires on one side, and 20% on the other, see what happens. Or put all the lights on one side, the rest on the other, see what happens.

And as noted by others, it's good to know what type of BC's the wires are for, then 50/50 balance each in the panel = best chance of evenly loading the supply when the supply is tapped or multi-phase.
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just for clarity, it should read as bolded.
 
220.61(A) has the neutral calculation based on the maximum calculated single phase load on any one leg, thereby assuming 100% imbalance.
 
roscoe0 said:
Say you're doing a panel schedule and youre balancing the loads to get same wattage on each phase and your panel schedule ends up perfectly balanced with 10kW phase A, 10kW phase B, 10kW phase C. My question is why balance loads so perfectly on paper if we don't actually know which loads will be turned on at the site?

For example in the morning the building might have 4 kW worth turned on in phase A, 8kW on phase B and then 1kW on phase C.
In the afternoon it might change again say 9kW phase A, 2kW phase B and then 6kW phase C.

I feel like the panel will be always unbalanced despite having it balanced on ''paper'' because we dont know how the building will use their loads.
Click to expand...

10 kW at 120V = 83 Amps
10 kW balanced on all three phases will fit on a 125A panelboard, after applying the 125% continuous load factor.

If you don't balance these loads, it may require your panelboard and supply to the panelboard to be much larger. Suppose 60% of the total 30 kW load were on Phase A, and the remaining phases had 20% of the load. This would mean you'll have 150A on phase A, and 50A on each of phases B and C. This now requires your panelboard to be 200A and have a 200A feeder.

It doesn't stop there. If this load is supplied by a transformer, the KVA rating of a transformer assumes loads are balanced. A 30 kVA transformer, is really 3x 10 kVA transformers, with the secondaries wired together in one network, and the primaries wired together in another network. In the above example, you have 18kVA loading the phase A transformer, which is in excess of its 10kVA rating, had you used a 30 kVA transformer. This means, an imbalance like that would govern a 54 kVA transformer, which in practice would be 75 kVA.

Long story short, if you balance your loads among the phases at their theoretical maximum capacity, you optimize your equipment for its amp and kVA rating. You could be unlucky enough to only load one phase the way the end-user turns on loads, and it is OK. That doesn't affect how you need to design your distribution system on that particular site. In the aggregate, it will end up being close to balanced when added up with everything else on the substation anyway. There is a known bias for loading the A-phase disproportionately, and staggered phase assignments at the service point will correct for this.
 
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