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JasonMT

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Im having trouble with the calculation process of this problem. If the standard calculation method is used, the feeder demand for a dwelling with a 150 kilovolt-ampere general lighting load is?

Im using the table on 220.42, Lighting Load Demand Factors. I dont understand the mathmatical process for 100% to 35% to 25%. If someone could please show me the steps to get to the answer, I would appreciate it. Thanks.
 
you had a lighting load of 150,000 va
The calculation would be:
1st 3000 VA @ 100% = 3000
You now have 147,000 va to address
From 3001 to 120,000
that would be 117,000 @ 35% =40,950
The balance of 30,000 @ 25% =7,500

Total 51,450 va

You might take a look at Example D4(a) in Annex D
 
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JasonMT said:
Im having trouble with the calculation process of this problem. If the standard calculation method is used, the feeder demand for a dwelling with a 150 kilovolt-ampere general lighting load is?

Im using the table on 220.42, Lighting Load Demand Factors. I dont understand the mathmatical process for 100% to 35% to 25%. If someone could please show me the steps to get to the answer, I would appreciate it. Thanks.
Click to expand...
3,000VA ? 100% = 3,000VA
(120,000VA ? 3,000VA) ? 35% = 40,950VA
(150,000VA ? 120,000VA) ? 25% = 7,500VA

3kVA + 40.95kVA + 7.5kVA = 51.45kVA
 
Watch for your 125% continuous load demand factor that lighting is notorious for.

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vango said:
Watch for your 125% continuous load demand factor that lighting is notorious for.
Click to expand...
125% for continuous loads is not a demand factor, nor an Article 220 calculation.

125% factoring of continuous load(s) is conductor ampacity padding to prevent conductors from operating at terminal [and equipment] temperature limitations.

With that said, consideration must be taken during Article 220 calculations to keep continuous loads and noncontinuous loads separate so as to apply 125% continuous load factoring when determining conductor size.

Many believe 125% factoring for continuous loads do not apply to dwellings, on the theory it is pre-figured into the demand factors. I am of a contrary opinion. There is nothing in the Code which supports this theory. I have further surmised that demand factoring should apply to continuous loads first. For example, if the OP's load of 150kVA had 20kVA of continuous lighting load, this 10kVA would be in the first two levels of demand factoring: 100% ? 3,000VA + 35% ? 17,000VA = 8,950VA. The 130kVA balance is then figured using the remainder of the second level and third: 35% ? 100,000VA + 25% ? 30,000VA = 42,500VA. As you can see, the total is the same as I posted earlier (51.45kVA), but the continuous and noncontinuous loads are separate so 125% continuous load factoring can be applied to service conductor sizing. If the load is split at feeder level, additional separation would be required.
 
This is the standard method of calculating load demands, the optional method will change these values. However the NEC 2014 has brought even another method?

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Smart $ said:
125% for continuous loads is not a demand factor, nor an Article 220 calculation.

125% factoring of continuous load(s) is conductor ampacity padding to prevent conductors from operating at terminal [and equipment] temperature limitations.

With that said, consideration must be taken during Article 220 calculations to keep continuous loads and noncontinuous loads separate so as to apply 125% continuous load factoring when determining conductor size.

Many believe 125% factoring for continuous loads do not apply to dwellings, on the theory it is pre-figured into the demand factors. I am of a contrary opinion. There is nothing in the Code which supports this theory. I have further surmised that demand factoring should apply to continuous loads first. For example, if the OP's load of 150kVA had 20kVA of continuous lighting load, this 10kVA would be in the first two levels of demand factoring: 100% ? 3,000VA + 35% ? 17,000VA = 8,950VA. The 130kVA balance is then figured using the remainder of the second level and third: 35% ? 100,000VA + 25% ? 30,000VA = 42,500VA. As you can see, the total is the same as I posted earlier (51.45kVA), but the continuous and noncontinuous loads are separate so 125% continuous load factoring can be applied to service conductor sizing. If the load is split at feeder level, additional separation would be required.
Click to expand...

I am aware of the terminal temperature limitations. However I thought that the 125% is because your typical circuit breakers are a only designed for 80% of its nameplate rating when protecting a continuous load. If you had a 80 amp continuous lighting load the overcurrent device would need to be 80 * 1.25. So you would need a 100 amp breaker and would there fore have to upsize the conductors to 100 amps. So I don't believe the conductors are being padded for terminal ratings but because the The size of the breaker has been increased and therefore the conductors have to be increased also. Yes this will make the conductors cooler because of the increase in size. I just don't see that as the reason that you increased the conductor size. What are your thoughts.
 
HEYDOG said:
I am aware of the terminal temperature limitations. However I thought that the 125% is because your typical circuit breakers are a only designed for 80% of its nameplate rating when protecting a continuous load. If you had a 80 amp continuous lighting load the overcurrent device would need to be 80 * 1.25. So you would need a 100 amp breaker and would there fore have to upsize the conductors to 100 amps. So I don't believe the conductors are being padded for terminal ratings but because the The size of the breaker has been increased and therefore the conductors have to be increased also. Yes this will make the conductors cooler because of the increase in size. I just don't see that as the reason that you increased the conductor size. What are your thoughts.
Click to expand...
Your belief is a mix of fact and myth. Circuit breakers are designed to handle their rating in amperes continuously. What is at issue is the dissipation of heat generated by the current through the breaker and both supply and load conductors. Mounting in a non-ventilated enclosure inhibits dissipation of that heat. Add to that the heat generated by adjacent breakers in a panelboard, for example, but that is somewhat offset by the larger enclosure. Operating ambient temperature affects the trip level... and long term integrity. Padding for the continuous loads is just a means to keep temperatures at a moderate level and ensure long term integrity.
 
Smart $ said:
Your belief is a mix of fact and myth. Circuit breakers are designed to handle their rating in amperes continuously. What is at issue is the dissipation of heat generated by the current through the breaker and both supply and load conductors. Mounting in a non-ventilated enclosure inhibits dissipation of that heat. Add to that the heat generated by adjacent breakers in a panelboard, for example, but that is somewhat offset by the larger enclosure. Operating ambient temperature affects the trip level... and long term integrity. Padding for the continuous loads is just a means to keep temperatures at a moderate level and ensure long term integrity.
Click to expand...

I agree with this in part. Circuit breakers are designed to handle their handle rating just like conductors are designed to handle their listed ratings within the scope of 310.15b16. When you get outside of the parameters of the table you have to apply adjustment factors for number of current carrying conductors and ambient temperature. So not taking voltage drop in consideration I believe the reason that you increase the wire size on a continuous load is because of increasing the overcurrent device. The load has not changed. I agree that the conductors act as a heat sink and am aware about the extra heat being generated by adjacent overcurrent devices.
You are absolutely correct about padding the breaker because it could cause nuisance tripping on a continuous load on the thermal side from heat being produced by adjacent breakers.

I called a manufacturer a couple years back to talk to him about the trip currents of breakers. He told me that a breaker loaded beyond 80% is going to trip based on how much above 80%. He was suppose to be an engineer and worked in the tech department. I forget the whole conversation but told him I didn't really buy off on his statement. I asked him to explain it to me and he said that he knew what he was talking about and that he didn't need to explain it to me.
I thought about it overnight and decided to call him again the next day. I didn't get his name only his extension. When I called back they said that extension was no longer available. I wanted to clarify that he was not talking about continuous loads.
 
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For the most part, I agree with your first paragraph. However, conductor ampacity is not increased to match the breaker rating. In fact, the conductor ampacity only has to equal or exceed the load... not factored 125% for continuous loads. The conductor's adjusted and corrected ampacity can be less than the breaker rating (800A or less), but not equal or less than the next lesser standard rating.

125% factoring is only applied to the size required for terminal [and equipment] temperature limitation... at the corresponding allowable ampacity for that limitation. Conductor insulation rating really has nothing to do with this part of the determination. It's entirely based on the heat generated by the current the conductor may possibly handle. This is why sizing for this part is based on Table 310.15(B)(16) and not whatever table's ampacity applies to the conductor's condition of use beyond the enclosure.

Regarding the manufacturer rep', he may well have been an entry-level engineer and only knew what he had been instructed to say. Arrogance likely prevented him from appearing ill-equipped. ;)
 
I am not being arrogant but am always trying to seek out the where's and why's. I have taught code classes for 16 years and take pride in not only explaining the code requirements but also the why. So I am always interested in learning.I am also aware of 240.4b.
Are you basing it on u.l. Or the code. Could I please get more detail. As I mentioned I an aware of not exceeding the weakest link temperature and the conductors acting as heat sinks.
Here is what Code sections that I am looking at and can not see your explanation totally.
Article 110.14c. 210.19a1. 210.20a. 215.2a. 215.3. 230.42 a.
 
I do believe that you are correct after reading the link below. This article was written in 1996 and Without looking do not know exactly if the code in these particular articles are the same as then.

It is hard to come to that conclusion by just reading the code articles that I quoted. I remember some of what you said from years ago. Somewhere I know I read from a book that you increase the wire size because you increased the overcurrent protection. I am thinking that this is the simple explanation without getting into the heating effects. Thanks for the info. Take a look at the link below.


http://m.ecmweb.com/content/sizing-continuously-loaded-conductors-made-simple
 
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