Mini Split Cable to Indoor Units & OCP

[hjoseph117]

Looking at manufacturer specs for mini splits. The OCP is wired to the outdoor unit which is then wired to the indoor unit with 14 awg tray cable. For a 35A MOCP mini split, why doesn't there need to be 15A OCP before the change to 14 awg wire?

 

[david luchini]

Looking at manufacturer specs for mini splits. The OCP is wired to the outdoor unit which is then wired to the indoor unit with 14 awg tray cable. For a 35A MOCP mini split, why doesn't there need to be 15A OCP before the change to 14 awg wire?

Because it's fused internally in the outdoor unit.

 

[hjoseph117]

That would make sense, trying to find info on the fuses if they exist. Drawings make it look straight from the terminal block.
38MARBQ36
https://www.shareddocs.com/hvac/docs/1009/Public/06/38MARB-10PD.pdf p73

 

[mtnelect]

This one of our installations.

 

[hjoseph117]

Spoke with someone from Carrier a bit and he talked about there being a fuse on the control board within the OU protecting the tray cable to the IU. Been emailing the "help ticket" path he sent from there and keep getting no where.

Have you seen any fuses protecting the 14 awg tray cable mtnelect in any ductless systems?

 

[tortuga]

Because it's fused internally in the outdoor unit.

I have never seen a outdoor unit with fuses for the indoor units.
I think of it like a motor branch circuit the 30A OCPD provides ground fault and short circuit protection for a single motor tap like [re numbered in the 2023 NEC] 430.53(D)(1) or [2020 NEC] 430.53(D)(2)

 

[david luchini]

Spoke with someone from Carrier a bit and he talked about there being a fuse on the control board within the OU protecting the tray cable to the IU. Been emailing the "help ticket" path he sent from there and keep getting no where.

This is Mitsubishi, but I'd imagine most of the mfgrs are similar. Maybe some aren't?

If you follow S1 & S2 on TB1 back to the control board, you will see they are supplied through fuses F1 and F2 mounted on the control board.

 

[winnie]

Here is a service manual for a Fujitsu system.

https://hvacdirect.com/media/pdf/Fujitsu%20LZAS%20Service%20Manual.pdf

Page 29 shows a connection diagram for the circuit board on the indoor unit. It is not a complete schematic, but shows an outline of a PCB with various terminal connections. A fuse _is_ shown.

Page 30 shows a similar diagram for the main circuit board on the outdoor unit. It does _not_ show a fuse. ....hmmmm....

-Jonathan

 

[hjoseph117]

Please comment on supposition of 430.53(D)1.

External OCP protecting an OU with #10 AWG copper wouldn't have to have a smaller fuse at the IU protecting the #14 AWG tray cable if the cable between the IU and OU was shorter than 25ft. This is not practical for ductless manufacturers because some runs are longer. Therefore they have an internal fuse on the OU PCB protecting the tray cable?

I guess small ductless units where the OU is protected by 15A OCP wouldn't need PCB fuses protecting the tray cable...

Thanks for all the help and the diagrams.

 

[tortuga]

When I looked into this a few years back the concern for me was using #14 UF cable (15A 60C ampacity) instead of the 14 AWG tray cable with a 20A ampacity at 75C .
The units in my case had a 20A OCPD and I could just plainly visually see blue/red jumpers on the terminal block. All they did was go thru a little choke coil. There were no fuses in the units.
I have seen many many units that just have jumpers on the terminal blocks.

 

[mtnelect]

I can only speak for "Daikin" ... They have a very sophisticated system that is monitoring the refrigerant and electrical system at various pre-set points. The system will automatically shut off if only one data point is off. In order to restart the system, you have to turn the breaker off first. The wireless remote will display a code directing you to the specific problem.

 

[hjoseph117]

It still breaks my brain on most small branch circuits ocp is 240.4(D), unless it's hvac or other things than you can do 310.16. Guess the #14 UF package told them only to do a 25ft line set max (shrug...).

I'm curious if the manufacturers could be convinced to add small additional loads to the #14 tray cable such as a larger condensate pump without it jeopardizing a warranty. I'm guessing not.. but wondering what the best strategy would be if I decided to go this way. Seems like the fuse protecting the tray cable is manufacturer specific and not sized in a similar fashion between sizes of mini splits...

 

[tortuga]

It still breaks my brain on most small branch circuits ocp is 240.4(D), unless it's hvac or other things than you can do 310.16. Guess the #14 UF package told them only to do a 25ft line set max (shrug...).

I'm curious if the manufacturers could be convinced to add small additional loads to the #14 tray cable such as a larger condensate pump without it jeopardizing a warranty. I'm guessing not.. but wondering what the best strategy would be if I decided to go this way. Seems like the fuse protecting the tray cable is manufacturer specific and not sized in a similar fashion between sizes of mini splits...

Whats the amp different of the two pumps?
Are they both the same voltage?

 

[hjoseph117]

Same voltage, considering a 100+W pump (thinking a no). And a 14W pump, thinking this would be ok since the 24V interface kit option has a 50va step-down transformer and we won't be using the kit so the wire should be at least good for 14W. Then I started thinking about what's actually protecting the tray cable and here we are.

 

[tortuga]

Whats the wattage of the existing pump?

 

[mtnelect]

When designing a ductless system, I always put the inside unit on an outside wall with a dry sump, so it doesn't drip on the foundation.

 

[kwired]

I can't recall connecting any mini split systems that needed more than 20 amp breaker, in fact most if all only needed a 15 amp breaker.

I've done a larger scale VRF system before with multiple compressors some on 60 or even 80 amp three pole breakers, but the zone valves and room air handling units had low enough current draw that there were multiple units on a single 15 amp 208 volt single phase circuit. Signaling was all via twisted pair at low voltage and interconnected every controlled component of the entire system with this same two wire network, each component had a network address and basically only responded to commands on the system that contained it's address in the command.

 

[hjoseph117]

We broke open a new 2 ton mini split and it has an mca of 25 and mocp of 35. 14 awg tray cable to indoor unit, nothing but a choker between the outdoor unit terminal to indoor unit out terminal. They sold it with a 50ft lineset (capable of going 213 ft)... Not sure why they don't need a smaller fuse (6ish A) to protect the tray cable.

Had trouble finding the info on the existing pump. We pansied out and put the pumps on different circuits.

 

[kwired]

We broke open a new 2 ton mini split and it has an mca of 25 and mocp of 35. 14 awg tray cable to indoor unit, nothing but a choker between the outdoor unit terminal to indoor unit out terminal. They sold it with a 50ft lineset (capable of going 213 ft)... Not sure why they don't need a smaller fuse (6ish A) to protect the tray cable.

Had trouble finding the info on the existing pump. We pansied out and put the pumps on different circuits.

Those figures seem rather high for a modern 2 ton unit. Seems more likely for 2.5 ton though.