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Hello everyone,

I work for an architectural firm and I am curious as to how electrical engineers compute the amp load on electric panels. I have even asked a couple of their employees about this and all they can tell me is that they enter the loads into a program and it tells them what the amp load is. Can someone explain it to me?

For example:

If a 120v single phase panel has, oh lets say, 5000 watts total load per leg, what would be the amp load on this panel?

How would the calculations differ on a three phase circuit?

Thanks.

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In a 120/240v panel you would double the load on eact phase, they add. In a 3 phase panel you multiply the line to line load by 1.73

If you are taliking about computing the load for a dwelling it is more complicated. It is based o the sqyuare footage of the home, to get the "general lighting load", then you add the dedicated circuits for laundry and kitchen plus all the fixed in place equipment. There are 2 ways to do this calculation that use different methods to account for diversity. If this is what you are looking for I can post the details ... but it has been done a number of times here. Article

220 in the NEC and the examples in the back explain this.
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| I work for an architectural firm and I am curious as to how electrical | engineers compute the amp load on electric panels. I have even asked a | couple of their employees about this and all they can tell me is that | they enter the loads into a program and it tells them what the amp load | is. Can someone explain it to me?

The software would likely be following the calculation methods described by the National Electrical Code. A good engineer should at least understand how to do that by hand, though certainly letting a computer do the work is a lot easier. It is rather complicated.

| If a 120v single phase panel has, oh lets say, 5000 watts total load | per leg, what would be the amp load on this panel?

For a specific wattage and voltage, it's easy. But coming up with that wattage actually needed for a given building is where all the "magic" is. It's complicated by having to consider things line concurrent usage and non-concurrent usage. For example, if a home has multiple stoves, they are allowed to factor in certain probability that not all of the stoves would be used at the same time, or if they are, other applicances won't be in use.

| How would the calculations differ on a three phase circuit?

If you convert all line-to-line loads to their equivalent line-to-neutral loads, the calculations go easier. On three phase, the ratio is the square root of 3 (1.732 is good enough for this purpose). If you have 3 100 amp loads connected line-to-line and evenly distributed among the phases, then each phase conductor will have 173.2 amps of current. If you have total building volt-amps (watts divided by the power factor), and know that it is evenly distributed on all three phases, just divide the VA by the L-N volts and you get the total amps. Divide that by 3 to spread those amps over the three phase conductors where evenly distributed.

5000 watts, divided by 120, then divided by 3, is 13.888 amps on each wire. That assumes even distribution. For small load amounts, that's less likely to be true. For a larger example, assume 1440 kVA in a large commercial building. That's 1440000/120, then divided by 3. That's 4000 amps. That's a huge and difficult to manage current. It's more likely to be provided as 480Y/277 at 1732.05 amps or 600Y/346 at 1385.64 amps. That still big but the benefit in lower current at that level overcomes the higher voltage. Some buildings may have even higher service voltages. Then a variety of step-down transformers, such as one per floor, can provide 208Y/120 in a more practical and safer way.
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I will back you on "it is complicated." Complicated is not the word for it. The NEC only contains design rules necessary for safety. Users too often attempt to use the NEC as THE design book of rules which it is not. The calculation of wire sizes and overload protection is a prime example. In the 2005 NEC Hand Book two distinct methods are used in 210.20(A) and 220.40 for finding the correct wire size and OCPD based on the load. One uses the terminal temperature and one does not. I have been working for ten years on trying to develop a satisfactory computer program that reads table 310.16 properly. I have reached the conclusion that the NEC writers themselves are not prepared to address the problem of ampacity. Dr Thomas Harman that sits on the Code making Panel for Article 210 and 220 publishes a book for learning the NEC called Guide to the National Electrical Code. In his book he avoids the problem of ampacity almost entirely by stating all problems in the book unless stated otherwise assume that the terminal and conductor insulation temperatures have the same rating (see bottom of page 35 of the 2005 edition.) How convenient. By the way my latest revision to my computer program with examples can be found at

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On 16 Nov 2006 18:48:17 -0800 snipped-for-privacy@electrician2.com wrote:

| I will back you on "it is complicated." Complicated is not the word | for it. The NEC only contains design rules necessary for safety. | Users too often attempt to use the NEC as THE design book of rules | which it is not. The calculation of wire sizes and overload protection | is a prime example. In the 2005 NEC Hand Book two distinct methods are | used in 210.20(A) and 220.40 for finding the correct wire size and OCPD | based on the load. One uses the terminal temperature and one does not. | I have been working for ten years on trying to develop a satisfactory | computer program that reads table 310.16 properly. I have reached the | conclusion that the NEC writers themselves are not prepared to address | the problem of ampacity. Dr Thomas Harman that sits on the Code making | Panel for Article 210 and 220 publishes a book for learning the NEC | called Guide to the National Electrical Code. In his book he avoids | the problem of ampacity almost entirely by stating all problems in the | book unless stated otherwise assume that the terminal and conductor | insulation temperatures have the same rating (see bottom of page 35 of | the 2005 edition.) How convenient. By the way my latest revision to | my computer program with examples can be found at |

Unfortunately, so many home builders do use the code as basis to do no more than is actually required. Then we end up with things like outlets spaced in the middle of walls where furniture tends to always block them because the shift might require yet another outlet to meet the 6 foot rule.

You should add a few more voltages to your voltage drop calculator, such as 600Y/346 (Canadians and one place I once worked at in Texas) and 480 single phase (extra long service drops where MV lines can't go). Or make a way to select the system type and enter any voltage.

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I visited a paper mill at International Falls MN in which some buildings had 600 V distribution, some had 480 V, and I'm sure some had both. Motor spares at that place must have been...interesting.

Bill Shymanski

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Its been done as a part of some specialized CAD systems.