# Electrical demand reading and voltage



## oldsparky52 (Feb 25, 2020)

Are you saying the peak demand is 175KW?

At what PF?


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## just the cowboy (Sep 4, 2013)

Short answer is both plus more.
Your demand is 175KW that is roughly 291 amps at 600 volts. If you are looking at single phase loads at 347 it is 504 amps. The thing to remember is KW is power and that is voltage times current plus some other minor things such as power factor and efficacy


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## joe-nwt (Mar 28, 2019)

just the cowboy said:


> Short answer is both plus more.
> Your demand is 175KW that is roughly 291 amps at 600 volts. If you are looking at single phase loads at 347 it is 504 amps. The thing to remember is KW is power and that is voltage times current plus some other minor things such as power factor and efficacy


What kind of new fangled math did you use to come up with 291A?


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## just the cowboy (Sep 4, 2013)

joe-nwt said:


> What kind of new fangled math did you use to come up with 291A?


kw convertor, it is on the web so it must be true. Yea I knew something was wrong when I tried to check my numbers but did not have time to find out why. I'll sit in the corner now.


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## joe-nwt (Mar 28, 2019)

175000/600*1.73 = 168.6A @unity


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## joe-nwt (Mar 28, 2019)

Maybe you though he had a .58 power factor.


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## just the cowboy (Sep 4, 2013)

joe-nwt said:


> Maybe you though he had a .58 power factor.


Funny you say that, that was what I came up with when checking, so I knew I was off.


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## Slay301 (Apr 23, 2018)

You should really put a power logger on it for a week


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## SWDweller (Dec 9, 2020)

The information is collected at the POCO meter. Each POCO has its own why of doing demand.
Most that I have dealt with is a 15 minute window with a internal 5 minutes sliding window. Time to ask the POCO for the way they do it.
Call the POCO and get the max demand from them last year. Cant argue with them. Unless you spend a LOT of money on better equipment. 

Sounds like commercial or industrial location. Do you know your electric rate structure? Do you know what POCO charges per KW of demand plus the KWH? Is there a PF competent to the billing?
Where is the next break in the demand billing structure? Adding load could double your electric bill.


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## paulengr (Oct 8, 2017)

The metering cabinet has CTs and PTs. So no matter what the actual voltage and current is, the meter sees 208/120 voltage (or 240/120 open delta) and 0-5 A. The ratios are set in the software for conversion.

In the US everyone gets the same power contracts. The utility has several rate schedules which are how they charge you. These are set by a combination of a public utility board (political appointees) and the utility. You can get whichever rate schedules you qualify for. They are all public information.

A typical demand formula will be that they continuously measure your kVA. They average it over say 10 minutes. The take the three highest 10 minute averages in the month and average those. Then multiply by so many dollars per kVA. They might also do it where they use kilowatts instead based on a minimum 0.85 power factor. If you are under 0.85 they “gross you up”. So if you are say 0.75, they will multiply by the ratio of 0.85 / 0.75. All of these combinations of doing things are programmable in any typical revenue meter. Some plants install their own to verify the utility. An example of a nice cheap one is the Shark 200 from Electro Industries.

Demand used to be a small part of the building ll. But public utility boards are political. So you hear stupid statements like no rate increases even as they close cheap coal plants for gas plants that use a fuel that costs 50% more never mind solar which is insanely expensive. But demand has shot up so much everyone pays more. It’s like the car dealer that gives you a below market price on a new car by hitting you with a super low trader in price.

There are all kinds of scams with power: in most cases the effect on the power bill is small.


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## katwalatapan (Jul 7, 2012)

Thank you very much for your insights.


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## katwalatapan (Jul 7, 2012)

Is it possible to estimate available capacity in an electrical system based on only the peak demand information available? For e.g. would it be safe to assume that a 800A, 347/600V, 3-phase electrical service, should have 472A available capacity, if the peak demand is 175kW. Following is my calculation based on unity power factor:

800 x 0.8 = 640A (max. loading)
175kW = 600V x A x 1.732 i.e. A = 168A
Available capacity at 600V = 640-168 = 472A

Would appreciate some thoughts on this process. Also, would the available capacity change, if there are 600V - 208V step-down transformers downstream of the electrical system?


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## oldsparky52 (Feb 25, 2020)

I believe your approach is valid, but ... what are you trying to accomplish? 

Typically you would need to do a load calculation and show your loads. Has anyone attempted this? 

The step down transformers should already be in the peak demand loads. 

Around my part of the world, I would need an EE to do the calculations in order to get a permit to add loads if I could not show the NEC calculations to support what I wanted to do. So, if you have those restrictions, you may be money ahead to just consult with one if one is available.


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## katwalatapan (Jul 7, 2012)

The intention is to identify available capacity in an existing electrical system to ensure that any additional load which is added to the system is within permissible limits and also to help in selecting the equipment based on available capacity. For permit purposes, the load calculations would be conducted. My intention was to see if the peak demand information on the bill could assist in identifying available capacity in the system.


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## oldsparky52 (Feb 25, 2020)

katwalatapan said:


> The intention is to identify available capacity in an existing electrical system to ensure that any additional load which is added to the system is within permissible limits and also to help in selecting the equipment based on available capacity. For permit purposes, the load calculations would be conducted. My intention was to see if the peak demand information on the bill could assist in identifying available capacity in the system.


I think everything you are saying is accurate, BUT w/out load calculations you can't move forward. (IMO)


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## emtnut (Mar 1, 2015)

katwalatapan said:


> The intention is to identify available capacity in an existing electrical system to ensure that any additional load which is added to the system is within permissible limits and also to help in selecting the equipment based on available capacity. For permit purposes, the load calculations would be conducted. My intention was to see if the peak demand information on the bill could assist in identifying available capacity in the system.


8-106(8)

The demand info isn't on the bill. You have to request it from Hydro.


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## SWDweller (Dec 9, 2020)

katwalatapan
"would it be safe to assume that a 800A ,,,,," 
No where I have ever been has the POCO and AHJ allowed additional load to a service over a certain amount without a recording meter installed on the service and a third party certifying that the recording is accurate. Along with the POCOs numbers.

Never liked the word "assume" when it comes to loading numbers. Sure a spit ball back of an envelope approach helps some times. I like hard numbers, just me the way I was taught.


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## oldsparky52 (Feb 25, 2020)

SWDweller said:


> Never liked the word "assume" when it comes to loading numbers. Sure a spit ball back of an envelope approach helps some times. I like hard numbers, just me the way I was taught.


EXACTLY! 

If you want to move forward, you will have to do it anyway.


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## paulengr (Oct 8, 2017)

katwalatapan said:


> Is it possible to estimate available capacity in an electrical system based on only the peak demand information available? For e.g. would it be safe to assume that a 800A, 347/600V, 3-phase electrical service, should have 472A available capacity, if the peak demand is 175kW. Following is my calculation based on unity power factor:
> 
> 800 x 0.8 = 640A (max. loading)
> 175kW = 600V x A x 1.732 i.e. A = 168A
> ...


No.

When we estimate for transformer sizing we need two pieces of information. We need to know load as per NEC. That is for overload purposes. You can certainly estimate this from demand data if it is given in terms of kVA. If not you need power factor. Guessing at power factor can be a huge error (been there, done that). Still converting to amps and multiplying by 1.25 (assume everything is continuous or a motor load) can be used to size cables and/or a service.

But you also have to estimate voltage drop with transformers. Say I have two 100 HP motors, 0.85 power factor, running at 100% load during production. This gives a demand loaf of 200 kVA. So with your method this is acceptable. But if we have say 5.5%Z (ANSI standard) with 1 motor running when the second starts, we get approximately 700 KVA of load. VD will be 19% assuming infinite bus on the primary side. You are in danger of the starter dropping out and starting torque assuming 150% has been reduced to 98%…so it is no longer possible to reach full speed and the system will stall out.

Need both the largest starting load AND highest average load to size transformers.

NEC commercial/residential works similar. That is why we use heating and cooling ratings directly but we can estimate lights and receptacles with square footage under the alternative method.


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## katwalatapan (Jul 7, 2012)

Thank you very much for your inputs. My intention was not to move away from performing load calculations, but to inquire if the peak demand value available from the utility bills could be used to estimate maximum available capacity in the existing electrical system which may help in selecting equipment within a certain amperage range to ensure we stay within permissible limits and avoid tripping of the main breaker.


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## oldsparky52 (Feb 25, 2020)

katwalatapan said:


> Thank you very much for your inputs. My intention was not to move away from performing load calculations, but to inquire if the peak demand value available from the utility bills could be used to estimate maximum available capacity in the existing electrical system which may help in selecting equipment within a certain amperage range to ensure we stay within permissible limits and avoid tripping of the main breaker.


Yes, but you would still need to do load calculations for a permit.


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## Viggmundir (Sep 13, 2019)

katwalatapan said:


> Thank you very much for your inputs. My intention was not to move away from performing load calculations, but to inquire if the peak demand value available from the utility bills could be used to estimate maximum available capacity in the existing electrical system which may help in selecting equipment within a certain amperage range to ensure we stay within permissible limits and avoid tripping of the main breaker.





oldsparky52 said:


> Yes, but you would still need to do load calculations for a permit.


I am in the middle of a school building project, a new building being fed from the existing school building. I did a load calculation for the building being built, but I did not do a load calculation for the entire existing school. Talked to my inspector, and we based it off of kW load and demand from previous electrical bills, so it can be done.


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## emtnut (Mar 1, 2015)

Viggmundir said:


> I am in the middle of a school building project, a new building being fed from the existing school building. I did a load calculation for the building being built, but I did not do a load calculation for the entire existing school. Talked to my inspector, and we based it off of kW load and demand from previous electrical bills, so it can be done.


Kinda like what the good lookin' guy in post #17 said 😂


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## Viggmundir (Sep 13, 2019)

emtnut said:


> Kinda like what the good lookin' guy in post #17 said 😂


Kinda like that 

Also the not necessarily doing a full load calc.


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