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Correct wire size #89024 08/26/04 01:09 PM
Joined: Aug 2004
Posts: 2
C
c3contr Offline OP
Junior Member
Hi,
I'm wiring a new geothermal heat pump system. It requires a minimum of 2 supply circuits. One for the compressor, fan and electronics and another for 4 heater strips.
My question is related to the wire size required for the 19.2KW heater strips. All 4 strips consume a total of 19.2KW @ 230v. The MCA for the heater strip supply circuit is 110amps. The max breaker is 100 amps.
The NEC table 310-16 lists #1AWG wire for 110amps in the 60 degree column and #2AWG wire in the 75 degree column. While I believe the system connections are rated for 75 degree, I don’t see it posted.
Per code, What is the correct wire size for this setup. Are all heat pump/furnace systems rated for 75 degree terminations?
Thanks in advance for all responses!
Chris

2017 / 2014 NEC & Related Books and Study Guides
Re: Correct wire size #89025 08/26/04 05:47 PM
Joined: Jun 2004
Posts: 220
T
trekkie76 Offline
Member
unless you can find on the equipment where it says 75 degrees for the terminations, 110-14(c)1 says to use 60 degree conductors. #1 AWG CU

Re: Correct wire size #89026 08/26/04 06:58 PM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
It is not the wire size that determines the temp-rating you can use.

It is either the size of the terminals or equipment that requires above or below 100 amps that determines the temp column to use unless otherwise marked.

Quote
110-14(C)(1)(b) Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG, shall be used only for one of the following:
(1) Conductors rated 75°C (167°F)
(2) Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors.

I would be very surprised if you can not use the 75 C column.

I am confused with this.

Quote
The MCA for the heater strip supply circuit is 110amps. The max breaker is 100 amps

The minimum circuit ampacity can not be higher than the maximum breaker size.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: Correct wire size #89027 08/26/04 10:59 PM
Joined: Jul 2004
Posts: 9,535
G
gfretwell Offline
Member
19.3kw is 80a @ 240v. 100 would seem right.


Greg Fretwell
Re: Correct wire size #89028 08/27/04 09:42 AM
Joined: Aug 2004
Posts: 2
C
c3contr Offline OP
Junior Member
The MCA of 110a and max breaker size of 100a came straight off of the label. It's my knowledge that the MCA number includes the 125% increase typically applied to HVAC/heat pump compressor systems. As gfretwell talks about in his response, 19.2KW @ 230v equals 83.5amps. You multiply that by 1.25 and get 104.3amps. I'd say the manufacturer is pretty close with the 100 amp breaker.
I'll most likely go with the #1awg and the 100a breaker since I can't find 75 degree termination data.
Thanks again!
C3Contr

Re: Correct wire size #89029 08/27/04 11:27 AM
Joined: Apr 2002
Posts: 558
C
caselec Offline
Member
If these heaters are 19.2KW at 230V then they would have a resistance of 2.755 ohms.
2.755 ohms at 240 = 87 amps.
87 amps times 1.25 = 108.7 amps
The minimum circuit size for this unit would be 110 amps. The manufacture should be contacted about the label.


Curt Swartz
Re: Correct wire size #89030 08/27/04 11:35 AM
Joined: Jul 2004
Posts: 9,535
G
gfretwell Offline
Member
I think you will find the toaster strip is rated at 240v. It will be lower if you have 230 going to it.


Greg Fretwell
Re: Correct wire size #89031 08/27/04 04:49 PM
Joined: Jun 2004
Posts: 220
T
trekkie76 Offline
Member
section 110-14(c)1 says terminatations of 100 amps or less, or 14 thru #1AWG are 60 degree. the original post said 100 amp max breaker. that means 60 degree terminations, unless you can find 75 degree on the equipment. # 1 AWG is the upper limit on the 60 degree terminals, the next section quoted by iwire, starts out with larger than #1 awg, so you can;t use that as the determining factor in un identified terminations.you have to determine what size circuit first.

Re: Correct wire size #89032 08/27/04 04:57 PM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
Trekkie here is a quote from the opening post.

Quote
The MCA for the heater strip supply circuit is 110amps.

MCA = Minimum Circuit Ampacity.

That statement alone means the terminals are good for 75 C.

Or

If the equipment only draws 90 amps but is equipped with lugs that accept larger than 1 AWG that also tells us that we can use the 75 C rating. [Linked Image]

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: Correct wire size #89033 08/27/04 05:00 PM
Joined: Jun 2004
Posts: 220
T
trekkie76 Offline
Member
the manufacter sets the equipment circuit rating with the breaker size of 100 amps. the circuit rating determines if it is under the 60 or 75 degree parts of that section

[This message has been edited by trekkie76 (edited 08-27-2004).]

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