Split units

[electro7]

Hi,

I did a commercial job where we ran feeders to condensing rooftop units for split units with separate circuits to fan coils inside. The HVAC contractor was doing troubleshooting because a bunch of components were causing problems in the units such as melting circuit boards, a compressor went out, etc and fuses and circuit breakers were tripping.

He called to ask me how I wired them and wanted to make sure the wire was big enough. Originally the engineer had on the plans #10 awg cu thwn-2 on 40A breakers and such. I thought that would need to be #8 so I told the engineer. He later came back to me and said that because they were motor loads I could down size the conductors and that he had it right to begin with. I told him I didn't know much about the motor code section so assumed he was correct.

Back to the condenser issues- I ran for example say 3-4 units feeders in a homerun conduit, which say would be 8 current carrying conductors (1 phase no neutral) max which would be an adjustment of 70 percent. One unit for example said max OCPD 50A min OCPD 30A which I ran #8 to. The current the HVAC tech said was normally to be around 24 amps, that they would often run less but these were hitting 28 amps which was frying boards and somehow evetually tripping breakers and blowing fuses. I wanted to make sure that the #8 cu thwn-2 was okay for this application. This particular unit was protected by a 50A breaker and 50A supplementary fuses at the unit. #8 thwn-2 base ampacity is 55 amps. 70% of 55 amps is 38.5 amps. Is this okay for this unit?

The engineer pointed out it is a motor load while the HVAC tech said it was a variable speed and would start at like 3 amps and go up to normal running current but wouldn't take a surge. He also said the manufacturer was having some malfunctioning parts but wanted to make sure the wiring was okay. When he measured voltage it was consistent and didn't have a lag.

I told him I think there is an issues with the units because the wiring wouldn't cause over amperage.

Any thoughts?

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[winnie]

HVAC units generally have data plates which state 'max ocpd' which tells you the maximum allowed circuit breaker, and 'minimum circuit ampacity' which tells you what you need for conductors.

For the unit you describe, #8 on a 50 should be fine.

Jon

 

[electro7]

Great thanks Winnie!

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[Fred B]

I told him I think there is an issues with the units because the wiring wouldn't cause over amperage.

Any thoughts?

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Correct, Wiring itself doesn't "cause" over amperage.
Sounds like either what you said "an issue with the unit", or you are getting transients or surges from POCO or a private xfer, given circuit boards melting down. A surge depending on size and duration can cause any or all of the conditions you are describing. Might consider installing a monitor that will "watch" and record the conditions over a period of time. Then determine if surge protection is appropriate or if need to contact POCO about issue at their end.

 

[ceb58]

Jus

HVAC units generally have data plates which state 'max ocpd' which tells you the maximum allowed circuit breaker, and 'minimum circuit ampacity' which tells you what you need for conductors.

For the unit you describe, #8 on a 50 should be fine.

Jon

Just wired a small chiller min circ amp 35 Max fuse/breaker 50 amp prime example of mfg data plate

 

[electro7]

Thanks guys. Glad to know I got that adjustment factor right.

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[Another C10]

I may be off base but did the voltage maintain a strong 220/ 240v while drawing the 28 amps

 

[electro7]

The HVAC tech said the voltage stayed steady at 208V. They are single phase units on a 3 phase 120/208V wye system.

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[Tulsa Electrician]

Was wondering if they were 240 volt only rated. Since the tech said " stayed at 208" must be ok.
If you think about the current draw at 208 versus 240. It's in line.

24*240=5760/208=27.69
Now it has a transformer tap for the low voltage, ya I have seen them smoke boards. Usually there is a tap that has to be changed for 208.
If no tap than they may be 240 volt only rated. Some boards have a dip switch instead of a transformer tap.
Always start with name plate. Then look at diagram posted on unit on in installation instructions.
I tag all unit out untill a tech takes control of the unit. Then it's there's.

 

[tortuga]

Originally the engineer had on the plans #10 awg cu thwn-2 on 40A breakers and such. I thought that would need to be #8 so I told the engineer. He later came back to me and said that because they were motor loads I could down size the conductors and that he had it right to begin with.

The engineer should not have used article 430 to size a branch circuit for a HVAC appliance. Once its a listed unit you use article 440. (440.6)