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AssuredElectrical, Master Electrician
Category: Electrical
Satisfied Customers: 4241
Experience:  Contractor-42+ Years in the ElectricalTrade
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Electrical build out question - allowed amps wired up vs incoming

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Electrical build out question - allowed amps wired up vs incoming amps

Hi, I'm working on a project that involves a lot of power and a lot of drops of electrical wiring for servers, etc.

If we have say 600 amps(208 three-phase) coming into a building, are we limited on how much we can build out in outlets?

I was told we wouldn't be allowed to have more than 10% overage of 30-amp 208v drops because of the incoming amps. So if we get a bunch of 30-amp 208v three phase drops in place, but we're only planning to use 21 of those amps on each, we can't actually max out our 600 amps coming into the building because (by code) we won't be allowed to put in the extra drops necessary to make up the difference. Is this correct?

(Another way to explain my question... 600 amps 3-phase is 215 kW coming in. 35 drops of 30a-208v @ 6.2kW each would use all that power. Would we be allowed by code to put in 60 drops if our plan was to under-utilize those circuits?)

We are located in central Washington state if that changes anything.

Welcome. My name is XXXXX XXXXX would be glad to assist.

To determine if the service is adequate for any additional build out, one must first know the complete electrical as it is now.

The buildings purpose and square footage, the heating and air conditioning loads(sizes and amperage), any other existing equipment in the building (sizes and amperage).
Once you have the total need determined, then you can see what is available for future expansion.

Is all this information available?

Customer: replied 2 years ago.

For now assume the other stuff is negligible. We have less than 2 kW of lights running, heating will be disconnected, A/C will be run by us(and we can account for those loads, but just hypothetically assume we have 2 60a 208v circuits dedicated to A/C. There's nothing else in the building except a garage door opener that might pull 2kW at max(but I doubt it).


What level of 30amp circuits would we be allowed to add?


Thanks, Jared

Ok, thanks.

On these "drops" at 6.2kw, how long are they operating per day?
What is the actual load being served?

Are all the drops operating the same amount of time?
Customer: replied 2 years ago.

24/7, actual load is somewhere between 12 and 26 amps.


Yes, most but not all drops are active at the same time.


I can easily calculate out what we can run on our incoming 600 amps. What I'm trying to figure out is what code allows us to build because we may be in a situation where we need more drops but put less load on each one as time goes on, but if we don't build for it up-front it is more expensive to add it later.



Ok, thanks.

When you have equipment such as this operating in an almost continuous manner, you calculate it at 125%.

So, the number for the service load calculation goes up at 6.2 x 125%= 7.75kw

With just guesstimate of AC and lighting, you probably have 20kw.

Leaves you with 195kw at most (not counting anything else in the building)

So, you could have 25 drops of your maximum.

You have to calculate at the maximum usage point that the equipment is capable of operating at any point in time.

Just like on a stove, the entire stove is 6000 watts even though you rarely use it all, only a small portion is normally operated.

But it is capable of the total, so it must be accounted for in the calculation..

Any other electrical will take away from that number as well. Like water heating, outdoor lighting, convenience receptacles, other lighting or more AC needs for heat dissipation etc.

Customer: replied 2 years ago.

Ok, but what does the code allow? What if the maximum load we expected from one device was 12 amps / 2.5kW, would we be able to run more and still meet WA electrical code?

It is based on what is in place, not what is expected, but what it is capable of consuming

So, if you have half of your posted 6.2kw in place for operation, then you would have double the amount available, once all existing electrical is accounted for and calculated

That is why it is critical to know what the equipment nameplates state as the wattage.
That is what must be used for the determination.


If there is no equipment in place, you can place the drops for possible future movement of equipment. If there is nothing there to operate, there is no load at the time.

Just specify each drop at 2.5kw for your installation.


That is the NEC.

Customer: replied 2 years ago.

So so long as we have nothing there at the time of inspection, we can run whatever we want?


The reason I am asking is we want to run extra drops to accommodate future possible needs, with full knowledge that plugging in all of them could run us over limits and cause the main breaker to blow. We just can't perfectly anticipate our future needs, but we were told (by a non-electrician) that code only allows for a 10% net overage.




Ok, I understand a bit more on the drop situation now. Not all in use and unsure exact needs in the future etc. Makes sense.

The key will be what the drops are specified for, in the permitting portion.

Specify the lowest KW per drop you anticipate in order to get the drops in your various locations. That will keep you from encroaching any limitations.

Or, if you have them calculate the feeder to the various locations in anticipation of future drops, the proper wire size will be installed to all the locations and drops can be added at any point along the way.
Similar to what is done on an assembly line, the feeder runs the entire length but drops are only installed at various points as needed until a later time.

Remember, The entire building will have to be calculated as it is now before any definitive numbers can be determined. So it can change for better or worse on what is available for the drop installations.

Not sure what this 10% overage is about.
You calculate the total needed in loads, existing and new installations added together.
The total KW.
AssuredElectrical, Master Electrician
Category: Electrical
Satisfied Customers: 4241
Experience: Contractor-42+ Years in the ElectricalTrade
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