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#89024 - 08/26/04 10:09 AM Correct wire size
c3contr Offline
Junior Member

Registered: 08/26/04
Posts: 2
Loc: Imperial, MO. USA
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

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#89025 - 08/26/04 02:47 PM Re: Correct wire size
trekkie76 Offline
Member

Registered: 06/19/04
Posts: 219
Loc: baileyville, maine, usa
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

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#89026 - 08/26/04 03:58 PM Re: Correct wire size
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
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

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#89027 - 08/26/04 07:59 PM Re: Correct wire size
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
19.3kw is 80a @ 240v. 100 would seem right.
_________________________
Greg Fretwell

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#89028 - 08/27/04 06:42 AM Re: Correct wire size
c3contr Offline
Junior Member

Registered: 08/26/04
Posts: 2
Loc: Imperial, MO. USA
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

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#89029 - 08/27/04 08:27 AM Re: Correct wire size
caselec Offline
Member

Registered: 04/14/02
Posts: 557
Loc: San Jose, CA
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.
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Curt Swartz

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#89030 - 08/27/04 08:35 AM Re: Correct wire size
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
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

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#89031 - 08/27/04 01:49 PM Re: Correct wire size
trekkie76 Offline
Member

Registered: 06/19/04
Posts: 219
Loc: baileyville, maine, usa
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.

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#89032 - 08/27/04 01:57 PM Re: Correct wire size
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
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.

Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#89033 - 08/27/04 02:00 PM Re: Correct wire size
trekkie76 Offline
Member

Registered: 06/19/04
Posts: 219
Loc: baileyville, maine, usa
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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