Multiple 120 V circuits and derating for ampacity, condu

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bjp_ne_elec

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I want to make sure I'm clear on derating (with temperature impact) and then considering multiple conductors - Table 310.15(B)(2)(a).

Let's say you have six (6) 120 volt cicuits - so twelve conductors - six ungrounded/six grounded. The run of conduit is 300 feet, but for a section it travels through a bolier room where it's going to experience temperatures of 125 degrees Farenheit. What's the final derated current for this scenario?

Thanks

Brett

[ January 22, 2006, 08:19 AM: Message edited by: bjp_ne_elec ]
 
Re: Multiple 120 V circuits and derating for ampacity, condu

Great question. :)

How long is the section that passes through the boiler room?

It makes a difference. (310.15(A)(2) Exception)
 
Re: Multiple 120 V circuits and derating for ampacity, condu

OK as the section through the boiler room is longer than 10' we have to treat the entire run as being exposed to 125 F.

Let's say you have six (6) 120 volt cicuits - so eight conductors - six ungrounded/six grounded.
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I will assume six two wire circuits, we can get into multiwire circuits and their neutrals later as that adds complexity we do not want yet.

I will also assume you are talking about 20 amp circuits and THHN/THWN run above ground.

12 current carrying conductors would get a 50% derate and the 125 F also gets .76 derate.


So here is what I come up with.

8 AWG THHN CU is rated 55 amps @ 90 C.

55 amps x .5 = 27.5 amps due to 12 current carrying conductors.

27.5 amps x .76 = 20.9 amps due to temp.

So for 6 two-wire circuits run in the same raceway through an ambient temp of 125 F using THHN you would need 8 AWG copper.

[ January 22, 2006, 07:41 AM: Message edited by: iwire ]
 
Re: Multiple 120 V circuits and derating for ampacity, condu

Bob - so back to the 120 V two-wire - how would the neutrals be dealt with? I'm looking at the last sentence of 300.15 (B)(2) - "each current-carrying conductor of a parralleled set of conductors shall be counted as a current-carrying conductor". This is the part I'm having trouble with. I get from this, that I would treat the 120V circuits as a parralleled set - but that wouldn't consider if any of the neutrals were carrying an imbalance.

Thanks

Brett

[ January 22, 2006, 07:49 AM: Message edited by: bjp_ne_elec ]
 
Re: Multiple 120 V circuits and derating for ampacity, condu

Let's say you have six (6) 120 volt cicuits - so eight conductors - six ungrounded/six grounded.
Click to expand...
Wait, do you have 8 conductors or 12? Is it 6 circuits with separate neutrals or 6 circuits with 2 neutrals?
 
Re: Multiple 120 V circuits and derating for ampacity, condu

Just a comment - maybe I should have said an odd number - like five (5). Six may simplify the problem as you could have two from each phase, and then have a balanced load on the neutral. So one thing I'm really looking for here is clarification on a "parralleled set" - and how it impacts the overal derating.

THanks
 
Re: Multiple 120 V circuits and derating for ampacity, condu

Brett with 6 hots and six neutrals you have 12 current carrying conductors.

Once you start sharing neutrals it gets a bit more complicated.

A shared neutral may or may not be counted as a current carrying conductor, it depends on many factors.

If the supply system is 240/120 a shared neutral would not count.

So six circuits from a 240/120 system consisting of 6 hots, 3 neutrals would be a total of 9 conductors but only the 6 hots would be counted as current carrying conductors.

Six circuits from a 208Y/120 system consisting of 6 hots and 2 neutrals would be a total of 8 conductors but only the 6 hots would be counted as current carrying conductors if the load was linear.

If the loads where non-linear the neutrals would count.

If you only use two phases of a three phase system the neutral will also count.
 
Re: Multiple 120 V circuits and derating for ampacity, condu

Originally posted by bjp_ne_elec: So one thing I'm really looking for here is clarification on a "paralleled set" - and how it impacts the overall derating.
Click to expand...
You are not dealing with a "paralleled set." You are dealing with a set of circuits that happen to share a conduit. What they mean by "parallel" comes into play with a single circuit.

Suppose you need two conductors to serve a single phase load. Suppose the load is 300 amps. Suppose you don't have a temperature derating to deal with (for this example).

Looking at Table 310.16, you see that a 350 MCM conductor (rated at 75C) is good for 310 amps, so you could use that size. But you have another choice. You could use a pair of 1/0 conductors, connected to each other at the beginning and at the end, as though they were a single "ungrounded conductor." You use another pair of 1/0 conductors for the "grounded conductor." This gives you an ampacity of 150 plus 150, and thus is sufficient for your 300 amp load.

The point of the statement you are asking about is this: If you put all four of these conductors (two parallel 1/0 ungrounded, and two parallel 1/0 grounded) in the same conduit, it counts as four conductors, not as two conductors. Therefore, it would need to be derated by 80%. The ampacity would then be 80% of 300, or 240 amps. This would no longer be enough for your 300 amp load. On the other hand, if you ran one set of the parallel conductors (one ungrounded, one grounded) in one conduit, and the other set in a second conduit, then you would not have "more than three current-carrying conductors" in the same raceway. Therefore, you would not have to derate, and the pair of parallel 1/0 would serve your needs.
 
Re: Multiple 120 V circuits and derating for ampacity, condu

charlie b - thanks for that clarification. I was thinking they were possibly also covering when you might group a neutral with each "hot". Not sure why someone would to that, as you would would probably run a three conductor grouping (2-hot, 1 neutral) for 120/240 system and a four conductor grouping (3-hots-1 per phase, and 1-neutral) for 3 phase systems.

Brett
 
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