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-   -   Conduit Fill & Ampacity (http://www.diychatroom.com/f18/conduit-fill-ampacity-18980/)

capndave 03-25-2008 12:03 PM

Conduit Fill & Ampacity
 
I am hoping a licensed electrician can provide answers, given the following scenario.

- A single 75 foot run of 1/2" EMT Steel Conduit.
- 9 Wires Total (Three 8ga., Three 12ga, Three 14 ga.)
- The 8 ga wires feeds the residential air conditioner.
- The 12 ga wires feed the jacuzzi tub's blower motor.
- The 14 ga wires feed the jacuzzi tub's heater.

Question 1:
What is the percentage of conduit fill for that run of conduit?
According to some basic calculations, it is significantly
over-wired.

Question 2:
If the conduit is indeed over-wired, what are the practical risks or drawbacks?

Question 3:
I have come across the term "ampacity", but I am not digesting
how it applies to over-wiring of the conduit. Can you please
explain the correlation (if any)?

Thank you

handyman78 03-25-2008 12:15 PM

Not a licenced electrician but by experience it appears overwired to me. There are tables in the NEC that pertain to conduit fill. A regular electrician will probably look them up for you.

To answer #2 -fire, shorts, insulation breakdown, overheating

#3 The conductors are normally identified by ampacity per gauge of wire. In certain situations that ampacity needs to be reduced or "derated" due to certain conditions like excessive length. For example, 12g copper has a 20a rating under standard conditions, non-standard conditions would require the amperage to be limited to 15a for the same 12g conductor.

InPhase277 03-25-2008 12:39 PM

Quote:

Originally Posted by capndave (Post 110722)
I am hoping a licensed electrician can provide answers, given the following scenario.

- A single 75 foot run of 1/2" EMT Steel Conduit.
- 9 Wires Total (Three 8ga., Three 12ga, Three 14 ga.)
- The 8 ga wires feeds the residential air conditioner.
- The 12 ga wires feed the jacuzzi tub's blower motor.
- The 14 ga wires feed the jacuzzi tub's heater.

Question 1:
What is the percentage of conduit fill for that run of conduit?
According to some basic calculations, it is significantly
over-wired.

Question 2:
If the conduit is indeed over-wired, what are the practical risks or drawbacks?

Question 3:
I have come across the term "ampacity", but I am not digesting
how it applies to over-wiring of the conduit. Can you please
explain the correlation (if any)?

Thank you

Let me first say that I'm impressed that you were able to fit thos wires in a 1/2" conduit! Then let me ask why you have three wires for each circuit? Does each circuit have a ground? If so, you can eliminate all but the #8 ground. You only need one ground, and that has to be sized to the largest circuit in the conduit.

Then, we have to know what kind of insulation the wire has on it. Is it THHN, XHHW, TW, what? This is important because the overall size of the conductor is dependent on not only on the conductor itself, but the thickness of the insulation as well.

You can only fill an EMT conduit to 40% of it's cross sectional area when you have more than two wires. I'll leave the math to you, because it is a good practice exercise.

1/2 EMT has a cross sectional area of 0.304 sq. inches.
#8 THHN has an area of 0.0366 sq. in.
#12 THHN has an area of 0.0133 sq. in.
#14 THHN has an area of 0.0097 sq. in.

InPhase277

CowboyAndy 03-25-2008 12:52 PM

Quote:

Originally Posted by InPhase277 (Post 110734)
Let me first say that I'm impressed that you were able to fit thos wires in a 1/2" conduit! Then let me ask why you have three wires for each circuit? Does each circuit have a ground? If so, you can eliminate all but the #8 ground. You only need one ground, and that has to be sized to the largest circuit in the conduit.

Then, we have to know what kind of insulation the wire has on it. Is it THHN, XHHW, TW, what? This is important because the overall size of the conductor is dependent on not only on the conductor itself, but the thickness of the insulation as well.

You can only fill an EMT conduit to 40% of it's cross sectional area when you have more than two wires. I'll leave the math to you, because it is a good practice exercise.

1/2 EMT has a cross sectional area of 0.304 sq. inches.
#8 THHN has an area of 0.0366 sq. in.
#12 THHN has an area of 0.0133 sq. in.
#14 THHN has an area of 0.0097 sq. in.

InPhase277


Just because I need the math practice, did i get these right...


3 #8
9 #12
12 #14

Stubbie 03-25-2008 01:09 PM

Andy

Yes you have it right if your speaking each calculation being the max wires in a 1/2 EMT conduit. Remember though that a ground wire if used must also be calculated in the fill. In this case I guess the emt is acting as equipment ground.

I'm not sure why the three 8's to the a/c.

capndave 03-25-2008 01:18 PM

Correction:

There are two 8 ga., not three. There are two 14 ga, not three.

The entire home (Illinois) is grounded via the steel conduit. We don't use Romex.

I didn't wire this, the electrician did. I am a homeowner, in the middle of a spirited 3-way debate with my builder and the electrician. The jacuzzi's GFI breaker ($200) arc'ed and the electrician is distancing himself from the situation. I'm trying to be fair and not just an angry homeowner.

The 240v/20amp is a GFI breaker (jacuzzi blower).
The 120v/20amp is a GFCI receptacle (jacuzzi heater).
The 240v/40amp (air conditioning) breaker is off - ie., winter.

The jacuzzi's 240v/20amp GFI breaker kept tripping , even with no load (tub unplugged). The electrician inspected the breaker and it had burns - ie., it arc'ed. He installed a non-GFI 240v/20amp (for testing purposes) and the continuity, polarity and voltage at the tub was correct.
Neither the conduit or neutral showed voltage. The jacuzzi ran fine (nobody in it, of course) so we can eliminate it being a problem with the tub's circuitry.

I have two suspicions:

1) The GFI breaker was installed incorrectly. The end of pigtail on the GFI breaker did not appear to be crimped when the electrician took it out. I
could be wrong, however. Also, I looked the neutral wire and the wire's end
appears to be crimped toward the neutral-bus bar. This tells me the
electrician (perhaps) wired the neutral to the bus instead of the GFI.

What are the implications of incorrectly wiring a GFI breaker's neutral incorrectly? Will that arc the breaker - or must there still be some kind of a short situation?

2) They cut a wire during pulling. That is why I asked about the conduit load requirements. The only problem with this theory is that the non-GFI breaker did Not trip during testing. If the wire was cut, it seems the test breaker would have also arc'ed -- GFI or not.

Stubbie 03-25-2008 01:19 PM

Capndave

9 wires in conduit requires a deration in ampacity to make sure the heat will not effect the required wire ampacity for the loads. First you have to move up to 3/4 emt conduit as 1/2 is way too small. Then we need the wire insulation type...thhn, etc.... Then we need the ampacity of the a/c compressor and fan. The 14's and 12's if they are supplying an individual load we would need those loads in amps. If supplying receptacles and lights then we would calculate based on 15 and 20 amp branch circuit ratings.

capndave 03-25-2008 01:26 PM

Wire is THHN.

capndave 03-25-2008 01:31 PM

The circuit requirements, from the jacuzzi manual.

(1) Dedicated 120VAC, 20AMP
(1) Dedicated 240VAC, 20AMP

Both those are satisfied.

The A/C is off (winter) and I'd need to lookup the load requirements. It
should also be noted that the A/C circuit breaks off at 75 feet via a junction box - then the jacuzzi circuits carry another 10 feet to the receptacles.

InPhase277 03-25-2008 01:36 PM

capndave,

If the neutral wasn't installed properly on the breaker, then this could have been the source of the trip. However, this would mean that the breaker never would have stayed on for long enough to arc. If the neutral was damaged during installation, this could also cause the GFCI breaker to trip, and when the non-GFCI breaker was installed everything would still work properly, because a neutral-to-ground short won't cause an ordinary breaker to trip. This is what I suspect the problem is.

Another thing, if the breaker cost you $200, then someone is really ripping someone off. All electricians put a mark up on materials, but yikes!

InPhase277

capndave 03-25-2008 02:01 PM

Thanks InPhase277.

Just one point of clarification. You suspect, the neutral wire was compromised during installation and say this could cause the breaker
to Trip. How about Arc? The GFI breaker was fried at ONE of the hot
contact points. If I am mistakenly calling that burn an "Arc" - please
correct me.

> if the breaker cost you $200 ...
It's the GE THQL2120GFI. I've found it for $125 + s/h. So he made about $75
bucks off the breaker, which I'm OK with.

Stubbie 03-25-2008 02:09 PM

I would agree with Inphases comments. Only thing I would add is a short from a hot phase to neutral or hot phase to ground and the 240 gfci is located in the panel, this is likely what arced the gfci and destroyed it. I've watched this happen many times in seminars and in technical classes during demonstrations. I've also done it myself when giving demonstrations when I worked for Ford. This involves dead shorting or ground faulting with a toggle switch and connecting the neutral or ground with a hot phase on the load side of the gfci through the switch. When you throw the switch you create the fault. With the lighting in the lab turned down the arcing at the gfci is quite impressive and the gfci about 9 times out of ten will be destroyed. This would be no different than a short to neutral or ground (emt) load side of the gfci somewhere in the conduit.

Hope this helps.

Stubbie 03-25-2008 02:13 PM

I wanted to add that you were not clear where the 240 gfci was located. Was it in the jacuzzi or the service panel? I am assuming at the service panel which would support what i just said in the previous post, plus you mentioned you paid 200 dollars for it, so that is impling to me you installed it at the service panel.

capndave 03-25-2008 02:21 PM

Big thanks to the inventors of GFI breakers. I only used the jacuzzi once,
for about 15 minutes, and suspect it arc'ed while I was in the tub.

For academic purposes - What do you guys think would have happened in this
specific situation if it was a non-GFI breaker, with a neutral short?

capndave 03-25-2008 02:27 PM

Quote:

Originally Posted by Stubbie (Post 110767)
I wanted to add that you were not clear where the 240 gfci was located. Was it in the jacuzzi or the service panel? I am assuming at the service panel which would support what i just said in the previous post, plus you mentioned you paid 200 dollars for it, so that is impling to me you installed it at the service panel.

Per the jacuzzi manual, two (2) dedicated circuits are required to be fully operational.

(1) Dedicated 240VAC, 20AMP

The 240v/20amp is a GFI breaker which, is at the panel of course. The receptacle for this circuit is a 20amp double pole, with the neutral wired
to the receptacles green/grounding screw.

(2) Dedicated 120VAC, 20AMP
The 120v/20amp is a GFCI receptacle -- which is at the tub. The breaker
is a standard 20amp (ie., $5 bucks).


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