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Old 01-06-2010, 08:59 AM   #1
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wire size?


Hey Guys!
I am running a 240v, 90amp, 40ft run from the box to my furnace. (Actually from my box to a 90amp disconnect then to the furnace) I want to double check on the wire size needed. I have consulted 2 electricians and the wire calculators online and have 3 answers,lol! The answers have run from 3ga to 6ga. this is a 2 wire with ground application. The furnace is a 15k unit. Thanks

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Old 01-06-2010, 09:06 AM   #2
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wire size?


Can you post the model number of the furnace? Where is it going to be located?

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Old 01-06-2010, 10:38 AM   #3
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wire size?


The furnace nameplate should have the MCA (minimum circuit amps) and MOCP (maximum breaker)


Using only the information you supplied at 15KW (assuming this is the heat strips plus fan motor) served by a single branch circuit would be calculated at continuous load. 15000/240 = 62.5 amps ... 1.25 x 62.5 = 78 amps. So you would need #4 THHN with 75C ampacity of 85 amps if using conduit or #3 cable like SER 60C ampacity of 85 amps. Your breaker is 1.25 times the continuous load of 62.5 so you need an 80 amp double pole breaker. Your ground should be #8 copper. Do not use aluminum wire.

Is there any supplemental overcurrent protection at the heating unit? Because you need to be sure this isn't a two circuit Furnace.

It is typical for 15 kw to have 2 overcurrent devices at the unit itself. The supply wiring is usually 60 amps and the other one is 30 from your panel..
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Last edited by Stubbie; 01-06-2010 at 10:46 AM.
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Old 01-06-2010, 11:12 AM   #4
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wire size?


you need to run No.3 copper THWN conductors to achieve your 90Amps. does the name plate on your furnace call for 90Amps?
the reason im asking is becouse to achieve 15k you only need 62.5 Amps x 125% = 78 amps. just wondered were you got the 90Amps from.

Last edited by richgriffith; 01-06-2010 at 04:19 PM.
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Old 01-06-2010, 03:16 PM   #5
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Quote:
Originally Posted by richgriffith View Post
you need to run No.4 copper conductors to achieve your 90Amps. .
What kind of #4 wire are we talking about?
#4 copper SE is rated for 70 Amps.
#4 NM (romex) is also rated 70 Amps.
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Old 01-06-2010, 04:13 PM   #6
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No.3 THWN copper good for 100Amp. NO.4 THWN is good for 85Amps. My mistake was looking in 90 degree column.
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Old 01-06-2010, 08:27 PM   #7
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wire size?


For anything under 100 amps don't you have to use the 60degree column
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Old 01-06-2010, 09:33 PM   #8
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Quote:
Originally Posted by Paelectrican View Post
For anything under 100 amps don't you have to use the 60degree column
Not necessarily There are some exceptions. However if your using NM cable, UF cable, and SE or SER indoors you have to use the 60 degree column.
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Old 01-08-2010, 02:41 AM   #9
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wire size?


You really need to looka at the nameplate on the furnace. If the furnace nevermind We need to know!!
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Old 01-09-2010, 07:11 AM   #10
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wire size?


The furnace is a ruud UHQA1215J
14.4 kw
HTR amp 60/53
min circuit amps 80/70
the internal fuse block is rated 90amp
I want to get all the wiring run , then I will get one of our electricians to do the final hook up.
Since 2 electriciands have given me 2 different answers, i was trying to get a consensus
worst comes to worst, I'll just run service entrance wire and be done with it
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Old 01-09-2010, 07:27 PM   #11
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Quote:
Originally Posted by mellowe4 View Post
The furnace is a ruud UHQA1215J
14.4 kw
HTR amp 60/53
min circuit amps 80/70
the internal fuse block is rated 90amp
One thing you need to know is it 208/240 or 240/208?
It looks like you have a 2 circuit furnace possibily. this is a 15 Kw furnace. Some are wired with 2 cir's and some are wires with 1. Need more info.
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Old 01-10-2010, 09:37 AM   #12
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208/240 single phase
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Old 01-10-2010, 09:48 AM   #13
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So your system is 208/240 volts Minimum circuit ampacity 80/70 Amps.
Is this right. If so at 240 Volts you need wire that can handle 70 Amps.

Which would be either #4 copper SE or NM / or #2 Al. SE
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Old 01-10-2010, 10:09 AM   #14
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thanks for the info , i have added some pics for verification

http://www.diychatroom.com/members/m...nace-question/
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Old 01-10-2010, 10:16 AM   #15
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Good Pics. Your furnace Mfg gave you the wire sizes right on the fuse block they require #2 Copper or 1/0 Al. Theres your answer.

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