NEC 220-35 ?

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I need to find the available amperes in an existing manufacturing facility so they can add more machinery. So I am using NEC 220-35.
I have received a report from the local utility company here with the following information:
Total Usage
12 month total = 958,080.
Average = 79,840.
I now apply Article 220-35 with my voltage as 480v 3 phase. My amperes = 120.04. This seems low?
Is this the correct way to use Article 220-35?
 
charlie b

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
Re: NEC 220-35 ?

It sounds to me like those figures represent the energy usage, in units of kilowatt-hours. What you need is the maximum demand observed during the year, in units of kilowatts. You might be able to get that information from the same utility report. But here?s a ?sanity check? on your results:
Total energy usage over a year = 958,080 KW-Hr. Divide this by 365 days and again by 24 hours a day, and you get an average (not peak) demand of 109.4 KW. Divide this by 831 (i.e., 480 times the square root of 3) and you get an average current of 132 amps. The peak demand will be higher than this, but probably not by a vast great amount.
 
G

Guest

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Re: NEC 220-35 ?

Thank you for your prompt reply. I think you are right about the kilowatt-hours, the utility companies report is not clear. I have posted the question back to the utility company.
 
dereckbc

dereckbc

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Re: NEC 220-35 ?

If you have the time rent yourself a recorder and monitor the current being used over several days. The KW usage is only going to give you an average, you need to know peak demand.

For example lets say it is a 400-amp service, from the bills you figure 140 amps. But if you measured, you might notice a peak demand of say 375 amps. Could be trouble.
 
G

Guest

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Re: NEC 220-35 ?

I agree with your style and may also do this recording test. I am still hopeful the utility company can provide me information of ?true power? watts, rather than kilowatt-hour. Article 220-35 applies a demand factor of 125% onto the maximum demand data from a 1-year period. A recorder will give me a current use reading for a stretch of time, which would help to support my evaluation. I am really looking for the simplest way, thank you for your input.
 
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peter

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Location
San Diego
Re: NEC 220-35 ?

I thought of that 8 hour day thing too.
One thing I remember is that the utility meter maid would clip off a seal on the meter globe each month and reset a dial. Look at your meter and see if it has one of these demand functions.
Also, if you have the time, you could visit the meter and note the reading and the time. Then, after a while, you repeat the process and do some elementary calculations.
Do you use electric heat or air conditioning in your factory. :eek:
~Peter
 
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bennie

Esteemed Member
Re: NEC 220-35 ?

Have you asked the utility to give you the demand figures? They will have them.
 
G

Guest

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Re: NEC 220-35 ?

Again I am hopeful to remain simple and normal. I would think the utility company can provide me the correct unit of measure, they should be the most accurate as they are the agency measuring the usage. I am not going to make unnecessary risk here by crowding the available amperes, so if the results are close to the ?full mark? I will upgrade the main/delete the items they want to add. At least the plant will know the available amperes both for now and the future.

Article 220-35 does have an exception, which allows regular 15-minute recordings of demand continually for 30 days minimum, this recording shall reflect the maximum demand and has to include the larger air conditioning/heat or other loads that may be periodic.

I do not know what they heat or cool with. At this stage my company (a C-10 contractor and electrical designer) was hired by this manufacturer to provide the installation of their new machinery and with this exposed the need to apply Article 220-35.

I did actually send the utility company this article and specifically requested ?The maximum demand data for a 1-year period.? I am sure they will provide this early next week if I do not already have it with the info they sent.
 
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Ed MacLaren

Senior Member
Re: NEC 220-35 ?

darren@eatonelectric.com,

Do you know if the facility in question has power factor correction equipment installed at present?

If they do not, have you/they considered the installation of power factor correction equipment in order to free up system capacity for the new loads?

Ed
 
charlie b

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: NEC 220-35 ?

Originally posted by stamcon: Charlie, if the plant was in operation only 8 hours a day, 5 days a week, would you still calculate 365 days/24 hours?
Click to expand...
I would not. But this would give you a much higher value for ?average demand,? as follows:

Total energy usage over a year = 958,080 KW-Hr. Divide this by 52 weeks, and again by 5 days per week, and again by 8 hours per day, and you get an average (not peak) demand of 460.6 KW. Divide this by 831 (i.e., 480 times the square root of 3) and you get an average current of 554 amps.

But the key point is that you need to know peak demand, not average.
 
G

Guest

Guest
Re: NEC 220-35 ?

From Ed MacLaren

Do you know if the facility in question has power factor correction equipment installed at present?

If they do not, have you/they considered the installation of power factor correction equipment in order to free up system capacity for the new loads?

Ed

They do not have power factor correction equipment. I wonder were I can get more information on this equipment? They may be wise to install this.

The Utility Company is still working on getting me the demand figures either in a unit of measure of kw or the best formula to use to convert kwh to kw usage. I think Charlie is right; I?ll have to find out how many day?s and hours per day the plant operates.
 
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