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- Retired Electrical Contractor
What do you all think the answer to this question is?
Ampacity Question
- Thread starter Dennis Alwon
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- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Well, since 42 is not one of the available choices, I will have to go with "B" for "Beck." The continuous lighting aspect has no bearing on the issue. I am curious as to how you came across this, and why you are asking. Is this one in a book, and does the book have the wrong answer?
I agree with mr beck i "B" don't know the light load either.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
You guys are confusing the "required ampacity" with the "available ampacity." When you calculate the ampacity of a conductor, you don't ask what the conductor will supply. It does not matter if it is a continuous load, or a harmonic load, or an intermittent load. The ampacity of a set of 4 current carrying THHN #4 copper will be 95*80%, or 76 amps, even if the actual total connected load is one amp of lighting.
jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
Ampacity = 76
Allowable continuous load = 60.8
Allowable continuous load = 60.8
Are you derating to 80% because of 4 CCCs, or because of continuous load? Why would not both apply?
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
I agree.
I agree with this as well. But it wasn't part of the original question.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
You never, Never, NEVER (sorry, I just love to emphasize things that way) "derate" for continuous loads. When you need to make an adjustment for the fact that a load is continuous, you may wind up with a larger wire. But that is not because the ampacity of the wire has been derated. Rather, it is because the load requirements have increased, and you need a wire with more ampacity, which means a larger wire.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
OK. I looked again. The question asks for ampacity. That is a characteristic of the wire, and has no relationship to the load. Try looking up the definition of "ampacity," and tell me where it says anything about the amount, or type, of load that you intend to supply with the wire. It is all about the wire itself, and the conditions under which it is used. The "conditions" refers to the ambient temperature, the number of current-carrying conductors, the ambient environment (i.e., free air versus in conduit versus underground).Look again.
So all those 80%s and 125%s in the Code don't mean anything? I guess I don't understand.
I'm thinking, in this case, you are calculating the ampacity of #4 THHN, not sizing it to a know load. If the lighting load in question turn out to be drawing , say, 70 amps, then yes you need a larger conductor.
But we're stuck with #4 by the question. So I figure 80% twice.... once for 4 CCCs and again for Cont. load.
If the question asked something like "What size THHN conductor is required for......?", then you'd size the wire to the load.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
The reason I posted it was because the answer was not only wrong but I am with Charlie in that the ballast continuous has no bearing. First off the author multiplied by 70%- obviously a mistake and then multiplied by 80% for the continuous load.
My answer was "B" based on 95 amps * .8= 76 amps. I don't see where the continuous load comes into play.
Authors answer was "D"
My answer was "B" based on 95 amps * .8= 76 amps. I don't see where the continuous load comes into play.
Authors answer was "D"
Yes, and two of the conditions are A. there are 4 CCCs, and B. it is a continuous load. Ambient temp is not mentioned, so you don't adjust anything for what is an unknown.
Given the question states there are 4 CCCs, you derate to 80% based on 310.15(B)(2)(a), . It also states it is a continuous load, so you need to derate again to 80% based on 210.19(A)(1).
There's no exception giving you an 'freebie' or 'out' that allows only one or the other. Both apply.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I look at it like this given the same scenario -- 4 CCC in the conduit and a load of lighting at 60.8 amps. If the lights are continuous then would the #4 Thwn work? I say yes because the wire ampacity is 76. 60.8 *1.25= 76
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Of course they have meaning. And we are bound to follow the rules they provide us.
But the amount of current you choose to run through a wire does not alter the ability of a wire to carry current. There are two very different and separate things going on here. Both are important. But they are different.
It is like saying you wish to buy a new TV, and your budget is $700. You go looking at stores, and you see many models at various prices. Question for you: if you go to a new store, and you see a different model, and it has different features than the others you have seen before, and if it is at a different price than the others you have see, does that fact cause your budget to become different?
I'd say the fact that the load is fluorescent lighting is irrelevant without a current draw. It could be 90 amps. It could be 50 amps. It could be 0.5 amps. That's not stated in the question, so what the lighting actually pulls cannot be taken into consideration. So we could easily substitute any load, call it continuous, and have the same question. Insert 'motors' or 'electric heat' or whatever your heart desires.
The question is about the ampacity of a conductor. We're not asked to size the wire..... that's been given. And the given conductor is #4 THHN.
So rewrite the question, taking out all the fluff and extraneous stuff, and you're left with; "What is the ampacity of four #4THHN with a continuous load?"
The question is about the ampacity of a conductor. We're not asked to size the wire..... that's been given. And the given conductor is #4 THHN.
So rewrite the question, taking out all the fluff and extraneous stuff, and you're left with; "What is the ampacity of four #4THHN with a continuous load?"
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