Temperature of conductors in EMT 310.16

[vilasman]

I did a job where I have to run wiring from a rooftop solar array to the main circuit breaker box. Went to inspection and the inspector flagged me for using THHN and not THWN-2, citing that because of the heat on the roof, the internal temperature in the EMT could cause a problem with the insulation on the wiring. I'll buy that. He referred me 310.16, the temperature correction factor chart. He also referred me 3.15B-2c which he said deals with outside wiring. I can't find this chart in my 2005 code book, which I believe they are still using here in Maryland.

I have (12) 10 gauge + (1) 8 gauge (green) in 1" EMT with compression connectors
Now here's my central question. As I read 310.16 it states it's referring to 3 current conductors in per stick of EMT. Does this mean that I cant run more than 3 current carrying conductors per stick?

 

[Dennis Alwon]

If the wire is outdoors in conduit it is considered a wet location. It is in art. 300.9 in the 2008NEC-- that is why there needs to be a W in the insulation type.

 

[vilasman]

I can understand that but what about the number of conductors in the pipe affecting the temperature rating?

 

[iwire]

I can understand that but what about the number of conductors in the pipe affecting the temperature rating?

You have 12 current carrying conductors, you have to derate for having more than 3 and you have to derate for ambient temp and if it is running across a roof that increases the ambient tempo you must use.

What size fuse or breaker is protecting the number 10s?

 

[marti smith]

Yes, you can run more than three. Temp adjustment (the lower portion of your table 310.16) then bundle adjustment 310.15 (B)(2)a; your 310.15(B)(2)c is in the '08.

 

[vilasman]

to answer iwire's question... and thanks to all who responded... There are 4 sets of solar cells on the roof, each one is giving off around 1175 watts at 240volts. The inverter is built into the panels. 4 sets of 2 hots and a neutral in 1" EMT #8 is the common ground. Each set goes to a 3 pole non fusible disconnect and then under a 15 amp 2 pole breaker in 8 space main lug panel. Then into 6-3 romex and on to the main panel and a 2 pole 50 amp breaker

 

[vilasman]

I think I understand...attempting the calculation now

 

[kwired]

THHN and THWN-2 are both 90 degree conductors, inspector should not fail for the reason you mention. Being installed outdoors it does need a 'W' in the rating. Most conductor that is sold anymore has more than one rating and likely has both THHN and THWN-2 ratings marked on it. You may need detation for temperature, but first you must find out what temperature you have to derate for, this will depend on distance above the roof as well as where on the planet your installation is located.

 

[vilasman]

Here is what I came up with. 50% reduction because 10-20 current carrying conductors in the pipe. .71 reduction based on a max ambient temperature of 132-140 degrees on the roof. 4 sets of 15 panels, 175 watts per panel. Taking each set of panels individually 175 watts x15=1750 watts/240v= 7.29 amps
Now 10 gauge wire at 90 degrees... (inspector requested 90d wire) max current is 40 amps. 50% of 40 is 20 and .71x20 is is 14.2.
Assuming I did right I am well within tolerances.
But given that I used 10 gauge THHN wire, instead of using the .71 reduction I would use .58 which would give me a value of 11.6 so I would still be within tolerances.

If all this is correct.... He also said I should have used THWN-2 because of the 90 degree rating but if my calculations are correct.... I really didn't need to use THWN-2 and my THHN should be good. Am I understanding things correctly

 

[vilasman]

I used pipe hanger straps so I am .5" or so above the roof... I am in maryland

 

[kwired]

Look at Table 310.15(B)(2)(c)

1/2 inch above the roof means you have to add 33 deg C (66 F) to the ambient temperature. Not sure what ambient temperature is in maryland but here I have to use 94 deg F as a starting ambient temp that means I would have to derate 90 degree conductors in a raceway installed 1/2 from a roof by a factor of 41%, and that is after derating for continuous load or more than three current carrying conductors in a raceway if either of these applies.

 

[vilasman]

310.15(B)(2)(c) isnt in 2005 code is it?

 

[kwired]

Here is what I came up with. 50% reduction because 10-20 current carrying conductors in the pipe. .71 reduction based on a max ambient temperature of 132-140 degrees on the roof.

How did you determine this temperature?

4 sets of 15 panels, 175 watts per panel. Taking each set of panels individually 175 watts x15=1750 watts/240v= 7.29 amps
Now 10 gauge wire at 90 degrees... (inspector requested 90d wire) max current is 40 amps. 50% of 40 is 20 and .71x20 is is 14.2.
Assuming I did right I am well within tolerances.
But given that I used 10 gauge THHN wire, instead of using the .71 reduction I would use .58

Both THHN and THWN-2 are 90 deg conductors - you must use THWN or THWN-2 because you are in a wet location

which would give me a value of 11.6 so I would still be within tolerances.

If all this is correct.... He also said I should have used THWN-2 because of the 90 degree rating but if my calculations are correct.... I really didn't need to use THWN-2 and my THHN should be good. Am I understanding things correctly

From 310.15(B)(2)(c)

(c) Conduits Exposed to Sunlight on Rooftops.Where conductors or cables are installed in conduits exposed to direct sunlight on or above rooftops, the adjustments shown in Table 310.15(B)(2)(c) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.16 and Table 310.18.
FPN: One source for the average ambient temperatures in various locations is the ASHRAE Handbook ? Fundamentals.

You need to derate conductor as you normally would for continuous load or more than three current carrying conductors if they apply, then you need to find out what ambient temperature is from a source such as the one in the FPN, then you need to add the value from table 310.15(B)(2)(C) to the ambient temperature to come up with an ambient temperature which you will use to apply the temperature correction factors at the bottom of table 310.16.

It is easier to use recripocals to do the calculation - instead of starting with a 10 awg and then derating it and finding out it is too small and going to a 8 awg and starting all over take your ampacity needed and multiply by reciprocal of the deration value. (example 80% = 1/.80 = 1.25)

So if you have a conductor that will carry 7.29 amps you need to derate for continuous load (or uprate the ampacity) 7.29 x 1.25 = 9.11.

Next you need to derate for number of current carrying conductors in the raceway (or uprate the ampacity) 50% deration = 1/.5 = 2 times ampacity so we are at 9.11 amps x 2 = 18.22.

Next you need to derate for ambient temperature, not sure of maryland but like I said here I must use 94F as a starting ambient temp and add 60F for a raceway that is only 1/2 inch above the roof. 94+60 = 154 deg F is the ambient temperature I must calculate for. T 310.16 says for a 90 deg conductor @ 154 the deration is .41 (or uprate the ampacity of 18.22 amps by the recripocal of .41 which is 1/.41 = 2.44) 18.22 x 2.44 = 44.46. The ampacity of conductor needed is 45 amps which if 90 degree conductors needs to be 8 AWG minimum. 8 AWG is also minimum for 75 degree conductors.

 

[kwired]

310.15(B)(2)(c) isnt in 2005 code is it?

If you are using 2005 code your calculations are probably OK, that section is new in the 2008 code, and makes a huge difference on what people are used to doing for calculations on rooftops. They decided that temperatures on rooftops are pretty severe. We went from a ambient temp deration of .71 to .41 just because of the change in the 2008 for the same installation. The rooftop is the same temperature no matter what the code is.

 

[DARUSA]

THHN and THWN-2 are both 90 degree conductors, inspector should not fail for the reason you mention. Being installed outdoors it does need a 'W' in the rating. Most conductor that is sold anymore has more than one rating and likely has both THHN and THWN-2 ratings marked on it. You may need detation for temperature, but first you must find out what temperature you have to derate for, this will depend on distance above the roof as well as where on the planet your installation is located.

THHN is a 75 degree in a wet location!Actully in a damp location!! No THHN in a wet location is allowed

 

[kwired]

THHN is a 75 degree in a wet location!Actully in a damp location!! No THHN in a wet location is allowed

You said you have THWN-2 the "-2" means it is rated for 90 degrees even in wet locations.

Look at the conductor types in the header of T310.16, which column is THWN-2 listed under? which column is THWN listed under?

If you have a conductor with multiple ratings you use the rating that applies to your installation.

If in a wet location (a rooftop is a wet location) then you must use a rating that has a W in it.

 

[iwire]

You said you have THWN-2 the "-2" means it is rated for 90 degrees even in wet locations.

Who said they have -2?

The OP states he did not have -2.

 

[RETRAINDAILY]

After a quick search it looks like Maryland is on the 08 code.

http://mdcodes.umbc.edu/dhcd2/List of Applicable Codes for MBPS - August 12, 2010.pdf

All year

 

[BillK-AZ]

Base calculation on the circuit breaker rating

Base calculation on the circuit breaker rating

You have to base your wire size on the rating of the circuit breaker protecting the wire from utility current, not PV inverter current. Sounds like Enphase micro inverters that specify a 15A breaker.

.... 4 sets of 15 panels, 175 watts per panel. Taking each set of panels individually 175 watts x15=1750 watts/240v= 7.29 amps

The above is not correct, 175 x 15 = 2625 watts. 10.93A at a nominal 240V.

MD temperature is in the range of 96-104?F. Table 310.15(B)(2)(c) covers conduit 1/2 inch above the roof means you have to add 33 deg C (66 F) to the ambient temperature. This requires a correction factor of 0.41 per Table 310.16 if there are not more than 3 current carrying conductors. You have two conductors per inverter circuit or 8 current carrying conductors (there is no current in the neutral with Enphase inverters). If you want to use a single conduit you need to apply a further factor of 0.7 to the 0.41 per Table 310.15(B)(2)(a) for a total correction of 0.287. This is applied to the breaker rating of 15A for an ampacity requirement of 52.3A. This would be #8 90?C wire. This will not fit in a 1" EMT with grounds and neutrals. Looks like you need two conduits.

Simply relocating your conduit to be over 1/2" above the roof will drop the temperature enough to allow #10 90?C wire.

Two other points that apply:

1. Check your voltage drop (actually a voltage rise above utility voltage) for both the the runs to the AC combiner and from the inverters. The limit is 1.5%, better to aim at 1%, and this should be for warm wire per the calculations. If the voltage rises too much, inverters will start cutting out and reduce your output. You do not mention conduit lengths.

2. You must label the AC Combiner "NO LOADS TO BE CONNECTED TO THIS PANEL) or the 690.64(B)(2) will apply and the #6 will not be acceptable.

 

[kwired]

Who said they have -2?

The OP states he did not have -2.

You are right I got a little lost but remembered seeing the -2 in the op.

The problem in the OP is the inspector telling him that he must must use THWN-2 instead of THHN (which is right) but then explaining that it is because of the 90 degree rating - Both conductor types are 90 degree rated.