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Wire size for 70 Amp furnace

Asked Aug 5, 2010, 08:40 PM — 5 Answers
I've installed a heat pump with an auxiliary heating coil. It requires a 70 amp 2 pole breaker. I was informed I need a #3 wire, yet the connections on the heat pump will only fit a #6. The dealer has told me, he has never seen anything more than a #6 on any installation. What do you think?

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tkrussell's Avatar
tkrussell Posts: 9,673, Reputation: 3698
Senior Electrical & Lighting Expert
  
#2

Aug 6, 2010, 07:15 AM


First I would like to know how the 70 amp breaker is determined.

Assuming that is correct, next we need to detemined the wiring method.

I suspect NM-B cable, AKA Romex.

#6 NM-B cable is only rated to carry 55 amps. #6 NM-B cable cannot be connected to a 70 amp breaker, period. However, I am sure this has been done many times. Does not matter, it is wrong.


If #6 THHn is pulled through conduit or MC cable is used, then the #6 is rated 65 Amps.

If this is the case, #6 THHN or MC cable can be connected to a 70 Amp breaker, as we are allowed to go to the next standard size breaker. 65Amp Cb does not exist, 70 Amp CB does exist.

If you need to use Romex, then #4 NM-B cable will be needed.

Even if the buidling has all Romex as the wiring method, there is no reason not to use MC cable so the # 6 could be used.
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nuttup's Avatar
nuttup Posts: 3, Reputation: 10
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#3

Aug 7, 2010, 03:04 PM
Thanks for your reply.
The Heat pump specifies a breaker size of 70 amp.
I'm using a NMD 90 cable. (#3)
Is a THHn cable a metal shielded cable? I'm thinking of running the NMD90 3.3 to a sub panel and then run an MC #6 from the panel to the furnace. Do you think this would meet code?
Thanks again.
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stanfortyman's Avatar
stanfortyman Posts: 4,897, Reputation: 1331
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#4

Aug 7, 2010, 03:38 PM


Quote:
Originally Posted by nuttup View Post
I'm thinking of running the NMD90 3.3 to a sub panel and then run an MC #6 from the panel to the furnace. Do you think this would meet code?
Code where? We have no idea where you are from.
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nuttup's Avatar
nuttup Posts: 3, Reputation: 10
New Member
  
#5

Aug 8, 2010, 07:55 AM
Quote:
Originally Posted by stanfortyman View Post
Code where? We have no idea where you are from.
This installation is in British Columbian, Canada.
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tkrussell's Avatar
tkrussell Posts: 9,673, Reputation: 3698
Senior Electrical & Lighting Expert
  
#6

Aug 9, 2010, 05:48 AM


We can only quote the National Electric Code, enforced in the USA.

Sounds like your plan will meet CEC code, but you really should checked with your local Code Enforcement Office.

THHN is a type of insulation that is pulled through conduit.
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