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
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.
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.
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
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 08/27/0410:27 AM08/27/0410:27 AM
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.
Re: Correct wire size#89030 08/27/0410:35 AM08/27/0410:35 AM
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.