Subpanel calculations for old conduit and heat pump?

[diamond29]

I’m trying to get a heat pump installed for my pool.

I have an old subpanel fed by 3/4 inch pvc conduit thats about 140 foot long. It’s got #10 wire in it currently. I’d really like to avoid retrenching this conduit, and pull new #6 through here.

I have had a handful of electricians out - a couple have said #6 is too small and redigging the trench has to be done, a couple have said that they won’t pull wire through an old conduit like mine, and a couple said they would pull new wire for a 50 amp breaker.

I have a 1hp pool pump, a salt water chlorine generator, some lights, an automation/wifi panel, and a service outlet for the cleaning robot. I added up the load here to about 12 amps @ 240 volts.

The heat pump I’m looking at has a compressor and fan load of about 30 amps, and an MCA of 42.5.

I used southwire’s minimum gauge calculator and it looks like I can get about 55 amps with <3% voltage drop on #6 conductors at that distance.

When i add the other loads with the MCA of the heat pump, i see that it’s safely under 55 amp.

Not sure what to think - I’d love to save about $3500 on digging the trench and risking hurting a huge old tree, but I also understand its always better to have a bigger subpanel.

Appreciate any feedback if others have been in similar situations - all the electricians seem relatively trustworthy to me.

 

[ajw22]

Welcome to TFP.

You can put 4 6AWG wires in 3/4' conduit. 2 hots, neutral, and ground wires. I don't think that is your issue.

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I have a 1hp pool pump, a salt water chlorine generator, some lights, an automation/wifi panel, and a service outlet for the cleaning robot. I added up the load here to about 12 amps @ 240 volts.

The heat pump I’m looking at has a compressor and fan load of about 30 amps, and an MCA of 42.5.

You are skirting the proper load calculations. For outlets you need to assume full rated load is connected to the outlet as you have no control over what can be plugged in there in the future. So the outlet should be a 15A or 20A load.

Instead of 12 amps you should be using 22 to 30 amps for your devices.

NEC says no circuit should be loaded to more then 80% of capacity or 40 amps on a 50 amp breaker circuit.

So with a 30 amp HP circuit you have 10 amps left for your other devices. That really does not fit.

No matter how you look at it a 50amp circuit is questionable for your setup.

 

[JamesW]

What is the heat pump model number?

 

[JamesW]

#6 awg is probably too small.

A 48 amp load on a 60 amp breaker would be about the maximum for #6 AWG wire.

The heat pump might be about 34 amps.

That leaves about 14 amps for everything else.

What are the amps for everything else?

Can you show the labels on each piece of equipment that show the current draw?

You probably need #4 wire.

You can probably use a smaller ground and neutral.

If you used (2) #4 for the hot wires and (2) #8 for neutral and ground, the fill percentage would be 31.29% + 14.04 % = 45.33%, which is too much.

Maximum fill for more than two conductors is 40% of the conduit.

With #4 wire, you can do up to about 64 amps on an 80 amp breaker.





Most likely, you will need a larger conduit with #4 AWG wire.

Note: Get all design approved by a licensed electrician before deciding what to do.

(2017 NEC) 215.2(A)(2) reference sizing based on 250.122, and amperage of grounded conductor "neutral" would be based on unbalanced load and can be sized accordingly allowing a smaller neutral than the current carrying conductors on your feeder, but not smaller than the EGC required for entire load. Also per 215.1(A)(1) ex#3 the grounded conductor "neutral" is permitted to be sized to 100% of the continuous plus noncontinous loads, not the 125% otherwise required. Also if you selectively choose to upsize your "neutral" the EGC must also be upsized in proportion to the increase in "neutral" size according to 250.122(B)
So overall a true load calculation would need to be done inorder to 1. Determine minimum conductor size, 2. to determine minimum "neutral" size, 3. that also will dictate the minimum EGC size. All this to determine minimum conduit size required.
Most just opt to go with whatever current carrying conductor size is what they'll use for the "neutral".

Downsize neutral to sub panel? (need to fit in existing conduit)

Happy New Year! I'm a contractor getting several different answers from my electricians so as usual--coming here for a tie breaker. I have a situation where a customer decided they want to add some outdoor heaters to an existing pool sub panel. The panel cable is currently undersized for the...

forums.mikeholt.com

Conduit Fill Calculator | Southwire

Get accurate conduit fill percentages with Southwire's Conduit Fill Calculator. Perfect for electricians, engineers, and contractors.

www.southwire.com

 

[JamesW]

Outlets are calculated at a minimum of 180 VA, which is 1.5 amps or 125% of the continuous load plus 100% of the noncontinuous load whichever is higher.

So, you really need to know what the outlet will be used for.

(I) Receptacle Outlets.
Except as covered in 220.41 and 220.14(J), receptacle outlets shall be calculated at not less than 180 volt-amperes for each single or for each multiple receptacle on one yoke.

The branch-circuit rating shall not be less than the noncontinuous load plus 125 percent of the continuous load.

 

[JamesW]

I have a 1hp pool pump, a salt water chlorine generator, some lights, an automation/wifi panel, and a service outlet for the cleaning robot. I added up the load here to about 12 amps @ 240 volts.

Pump 5 amps.
SWG 1 amp.
Outlet 1.5 amp minimum.
Robot
Automation.

A 48 amp load on a 60 amp breaker would be about the maximum for #6 AWG wire.

The heat pump might be about 34 amps.

That leaves about 14 amps for everything else.

You might be able to get it done with #6 AWG wires.

Note: Get all design approved by a licensed electrician before deciding what to do.

 

[diamond29]

I appreciate the feedback. The outlets are not used for anything besides the robot, and in the fall maybe some yard stuff (when the pool is not being used).

James your numbers match the equipment I have except the pump load is slightly higher at 6.5 amps. The heat pump is a thermeau s125

Signature Pool Heat Pump Models | THERMEAU®

I’m getting the sense that this is cutting it very close…if it doesn’t work out should I expect failing equipment? Or fire at my main panel?

 

[ajw22]

I’m getting the sense that this is cutting it very close…

Yup, I think you are flying too close to the sun.

To me, you have too many assumptions that have to always be right for there not to be problems.

if it doesn’t work out should I expect failing equipment? Or fire at my main panel?

If you are lucky then it will just be the circuit breaker tripping.

Pulling too many amps over wire stuffed into a 3/4" conduit can overheat the wires in the conduit. That can lead to insulation failure, charred wires, and tripping the CB.

There are enough margins in electrical circuits that an actual fire is a remote possibility.

But in trying to save a few $'s now you are setting yourself, or the next owner of your house, to spend more money as things get flaky and then need to replace the wiring.

My philosophy is doing it right the first time is always the least expensive way. Some folks like to spend the least amount now even when it will cost them more over the long term. Depends the way you want to live your life.

 

[JamesW]

Note: Get all design approved by a licensed electrician before deciding what to do.

A 48 amp load on a 60 amp breaker would be about the maximum for #6 AWG wire.

It looks close, but you might be able to get it in under 48 amps.

Only you can decide if you want to try it.

If you try it, measure the actual load to make sure that it is not over 48 amps.

Heat pump 30 amp.
Pump 7 amps.
SWG 1 amp.
Outlet 1.5 amp minimum. Robot 5.
Automation.
Total. 43.

The outlets are not used for anything besides the robot,

Can you show the power supply label for the robot?

(2) #4 AWG and (2) #8 AWG would probably work, but the fill percentage is just over the limit.

If you used (2) #4 for the hot wires and (2) #8 for neutral and ground, the fill percentage would be 31.29% + 14.04 % = 45.33%, which is too much.

Maximum fill for more than two conductors is 40% of the conduit.

 

[JamesW]

if it doesn’t work out should I expect failing equipment? Or fire at my main panel?

48 amps of continuous current might result in wire temperatures of up to about 124 to 132 degrees F with low ambient temperature.

48 amps of continuous current might result in wire temperatures of up to about 152 degrees F with high ambient temperature.

44°C (111.2°F) is the maximum temperature people can hold without pain and damage.

Experiments by Moritz and Henriques (1947) revealed that burns will not occur if the temperature is below 44 °C. At 44 °C or above, tissue injury will occur after several hours. Above 70 °C, full-thickness tissue destruction occurs in seconds (Arturson, 1996). An estimation of thermal injury according to time and temperature is given in Table 2.

Table 2. Summary of contact time with temperature in causing partial-thickness burns

Temperature (°C)	45	50	55	60	65
Time taken	3 h	4 min	30 s	5 s	1 s

 

[JamesW]

New Study Finds Attic Temperatures Can Pose Serious Risk.

If you've ever been up in your attic on a sunny summer day, you know the meaning of hot.

If you have a dark color roof on your house, the heat buildup in the attic can be unbearable.

Well, pity your home's electrical wiring - it has to perform in this superheated environment day after day.

The heat doesn't affect the copper conductors in the wiring.

They can handle far higher temperatures than those found in attics.

The problem is the plastic insulation and jacketing that surround the wires.

These are usually rated to withstand up to 194°F, but temperatures that approach this limit are not recommended.

Two factors combine to heat up the wire.

The first is the heat in the surrounding, or ambient, air in the attic.

Recent research carried out by the Copper Development Association has shown that ambient temperatures in attics can easily be 30° to 50°F higher than the outdoor temperature.

Remember, the outdoor temperature that the weatherman quotes is measured in the shade.

As if that weren't enough, consider the second factor - the electrical current running through the wires heats them up even more.

The CDA research has shown that temperatures of open wires in attics can get perilously close to the 194°F limit.

If the wires are buried in attic insulation, pass over light fixtures or, worst of all, are arranged in tight bundles, they get even hotter than if they're out in the open air.

Wires so hot you can't touch them aren't good.

One way to alleviate the heat is to use larger diameter wires - for example, use 12 gage instead of 14 gage, or 10 gage instead of 12 gage (larger wires have smaller gage numbers).

Because larger wires offer less resistance to electrical current, they permit more current flow while staying cooler.

Is Your Electrical Wiring Too Hot?

Copper In Your Home: Home Planning Series Article - Is Your Electrical Wiring Too Hot?

copper.org

 

[JamesW]

Power = 5.938 x 48 = 285 watts.

Power = 48^2 x 0.0724 = 167 watts.

Power lost will be about 167 to 285 watts, which is radiated out from the wire as it reaches operating temperature.

 

[JamesW]

Going to #4 AWG wire will result in about 110 to 185 watts of radiated power.

At 48 amps, the temp of the wires might be about 120 degrees F.