3 phase - 208V HLT control panel with 6kW element (HELP)

[Mike Gordy]

I am trying to control a 3 phase 208V 6kW element with a PID. I have 3 phase 208V 40 amps to feed the element. I drew up the schematic posted here and ran it past an electrcian that does control wiring. He suggested changing all the controls to 24 volts. Can anyone elaborate if they have done this type of build and give some suggestions? Thank you.

 

[MintonHVAC]

I am trying to control a 3 phase 208V 6kW element with a PID. I have 3 phase 208V 40 amps to feed the element. I drew up the schematic posted here and ran it past an electrcian that does control wiring. He suggested changing all the controls to 24 volts. Can anyone elaborate if they have done this type of build and give some suggestions? Thank you.

24v controls with SSR's is what I use for a 8.5kw setup. I can't seem to open up your PDF. Would really like to check out what you have in store.

 

[Mike Gordy]

24v controls with SSR's is what I use for a 8.5kw setup. I can't seem to open up your PDF. Would really like to check out what you have in store.

Its a PDF. How do i embbed it into the thread. I did notice that in order to view the file i attached, you would have to save to desktop and then open.

 

[John Eberly]

Couple of comments from an electrical engineer-

I don’t see any ground fault protection, I would recommend it. Three pole gfi breakers are not cheap, but protect from a lot of potential trouble, especially important when you are not experienced with building electrical equipment.

The grounding conductor should be sized with the 40 amp feed, not with your control power 120 circuit.

You could eliminate the 120 volt separate circuit by tapping your control power from one of the three phase legs, load side of the input breaker. This way the three pole breaker would kill power to the whole panel - two sources into a panel are not good practice in the professional world, you should either have one device kill all power or have a prominent label stating that there are multiple power sources inside. Your heater load is only about 17 amps, so the 40 amp breaker has capacity. The imbalance in current would not be a problem.

You might consider eliminating the contactor and switch and just using the three pole breaker as your power switch - this would save some money and help pay for a gfi breaker.

 

[Mike Gordy]

Couple of comments from an electrical engineer-

I don’t see any ground fault protection, I would recommend it. Three pole gfi breakers are not cheap, but protect from a lot of potential trouble, especially important when you are not experienced with building electrical equipment.

The grounding conductor should be sized with the 40 amp feed, not with your control power 120 circuit.

You could eliminate the 120 volt separate circuit by tapping your control power from one of the three phase legs, load side of the input breaker. This way the three pole breaker would kill power to the whole panel - two sources into a panel are not good practice in the professional world, you should either have one device kill all power or have a prominent label stating that there are multiple power sources inside. Your heater load is only about 17 amps, so the 40 amp breaker has capacity. The imbalance in current would not be a problem.

You might consider eliminating the contactor and switch and just using the three pole breaker as your power switch - this would save some money and help pay for a gfi breaker.

The 40 amp feed is coming from a 40 amp disconnect that has GFCI breaker in main 200 amp panel.
I will have to check if the electrician brought 3 or 4 wires from the disconnect.
good idea to tap off the main feed for the pump and the rest of the 120 volt power needs.
As for the rest of the drawing, how does the flow look? fairly sound?
I will be using the EZ-Boil PID from Auber Instruments.

So the 3 pole breaker in the control panel could be downsized?

 

[John Eberly]

Maximum continuous load on a breaker is typically 80%, so your 40A unit will carry 32 Amps reliably. I would leave the breaker as shown and run #8 wire from your panel. You’ll have a little more capacity available and the wire and breaker will run cooler and last forever.