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Author Topic: ac or dc cable runs  (Read 809 times)
steve
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« on: September 01, 2019, 08:47:10 PM »

I hope someone can help with this this problem. I am about to fit 15 no 275 watt panels to my shed which is about 110 metres from my consumer unit.
I wonder if anyone can tell me the best way to to connect back to the house. AC or DC ?
Also is it best to wire all the panels as a single string if DC is the best cabling option or is it best to have a string of 7 and 8 connected in parallel then split again at the invertor ?
Or is having the invertor at the shed end and AC the way to go ?
Help would be greatly appreciated
thank Steve 
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TT
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« Reply #1 on: September 01, 2019, 09:12:21 PM »

Personally I would run AC
Swa direct buried is easy to do.
Keeps DC close to the panels and inverter.



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nowty
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« Reply #2 on: September 01, 2019, 09:29:27 PM »

Personally I would run AC
Swa direct buried is easy to do.
Keeps DC close to the panels and inverter.


We have had this many times before and the consensus was to run DC because you risk a large voltage drop which makes the inverter trip out with AC overvoltage. You can still run DC down a buried SWA cable.

If the inverter can handle DC 600v+, then ideally do it as a single string, if not then two strings but you cannot run unequal strings in parallel into the same MPPT. If inverter has two MPPTs then you can have two separate unequal strings.
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JohnS
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« Reply #3 on: September 01, 2019, 09:33:55 PM »

Have you measured your AC voltage?   What size cable are you considering?  What I the max DC voltage of your inverter?

These factors will help a decision.

In general AC voltage is around 230v, DC max around 600v.  Higher voltage and thus less current gives less voltage drop and losses.

However, for a marginal benefit, I would follow TT's recommendation.

Edited after Nowty's post.

If your AC supply voltage is high, you have a higher risk of over voltage as the permissable voltage drop is smaller.  In general, the last house on a long supply line will be more likely to suffer as the local transformer will have been set high to minimise under voltage at times of high load.
« Last Edit: September 01, 2019, 09:41:24 PM by JohnS » Logged

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TT
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« Reply #4 on: September 02, 2019, 12:48:13 PM »

2 sets of strings for a DC run is then a 4 core cable.

Running Ac it's just a 2 core,
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Nickel2
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« Reply #5 on: September 02, 2019, 04:10:38 PM »

Using the power-model below:

https://photovoltaic-software.com/solar-tools/dc-ac-drop-voltage-calculator

The following figures can be obtained:

AC single-phase. 230V @ 20A = 4140W at max panel output.
Using 110m  buried 6mm SWA cable the losses are as follows:
AC volt drop = 11.15V = 4.85%
Energy loss = 244.35W = 5.90%
Using 110m  buried 10mm SWA cable the losses are as follows:
AC volt drop = 6.75V = 2.93%
Energy loss = 146.61W = 3.54%

For the same PV power production as DC, these figures can be obtained:
DC 600V @ 6.9A = 4140W at max panel output.
Using 110m  buried 6mm SWA cable the losses are as follows:
DC volt drop = 4.21V = 0.70%
Energy loss = 17.45W = 0.42%
Using 110m  buried 10mm SWA cable the losses are as follows:
AC volt drop = 2.52V = 0.42%
Energy loss = 17.45W = 0.42%

This assumes a ground temperature of 15 deg C.
The heating effect at worst case = 2.2W per metre has been ignored.
Is this any help?
(Personally I'd be happier with the AC option for safety grounding reasons, although the losses are higher)
All comments welcome  Smiley
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nowty
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« Reply #6 on: September 02, 2019, 07:25:08 PM »

AC single-phase. 230V @ 20A = 4140W at max panel output.
Using 110m  buried 6mm SWA cable the losses are as follows:
AC volt drop = 11.15V = 4.85%
Energy loss = 244.35W = 5.90%
Using 110m  buried 10mm SWA cable the losses are as follows:
AC volt drop = 6.75V = 2.93%
Energy loss = 146.61W = 3.54%

This is what I was talking about, if you do it in 6mm AC, the inverter will be running at 11.15v above your mains incoming voltage and in practice it will be a few volts higher than that.

For example, SMA recommend that their 4kw inverters using 10mm cable, that the max length is 38.8m.

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Heat storage of 15+ kWh.
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Nickel2
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« Reply #7 on: September 02, 2019, 08:00:27 PM »

Nowty - Point registered and understood Smiley
How does one get on with the safety aspect of a possible 600V DC lurking about without a ground reference? If one does ground either side, or if a leak develops, would there be galvanic corrosion problems?
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1.140kW mono south-facing at 49*
EpEver 4210A at 24v
24V 400 Ah battery. (4x200Ah FLA)
EpEver STI1000-24-230 pure sine inverter
Of course it'll work. (It hasn't caught fire yet).
Iain
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« Reply #8 on: September 02, 2019, 08:38:33 PM »

Hi

Quote
How does one get on with the safety aspect of a possible 600V DC lurking about without a ground reference?

(On a transformer inverter, As long as the DC cable stays earth free.)

Probably safer that way. Bit like building sites where they have floating supplies (yellow transformers).

If you touch one live wire your body just drags it down to earth potential, (the other goes up) therefore safe. Just don't touch the other wire at the same time !!

Iain

« Last Edit: September 02, 2019, 08:54:58 PM by Iain » Logged

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gnarly
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« Reply #9 on: September 02, 2019, 08:48:00 PM »

If it's a TL (transformerless) inverter then there actually is a complex connection between the incoming mains and the DC string connections.  So potentially you'd still get a belt if you touched just one of the wires !
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camillitech
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« Reply #10 on: September 03, 2019, 08:52:51 AM »

I hope someone can help with this this problem. I am about to fit 15 no 275 watt panels to my shed which is about 110 metres from my consumer unit.
I wonder if anyone can tell me the best way to to connect back to the house. AC or DC ?
Also is it best to wire all the panels as a single string if DC is the best cabling option or is it best to have a string of 7 and 8 connected in parallel then split again at the invertor ?
Or is having the invertor at the shed end and AC the way to go ?
Help would be greatly appreciated
thank Steve 

If you are putting 15 panels on the roof I'm guessing it's a decent sized shed? You don't say if the shed already has power, if not then make sure it does and just connect your inverter at the shed. HVDC is the best for long cable runs for sure but a 'man shed' with power is even better  Grin

Cheers, Paul
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steve
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« Reply #11 on: September 03, 2019, 11:58:28 AM »

thanks for all the help.

My house voltage is 232 and the invertor i am using is a sunnyboy 4000TL-20.

 Its sounding a bit like the invertor won't work on that sort of cable run on ac or do I have to go with an absolutley massive cable !

cheers Steve
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nowty
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« Reply #12 on: September 03, 2019, 03:23:21 PM »

Your 4000TL-20 inverter can only take 550v so you cannot do a single string as the voltage will kill the inverter.

It should be a dual MPPT one though so no problems having two unequal strings.
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11kW+ of PV installed and 54+ MWh generated.
Lithium battery storage of 50+ kWh.
Hot water storage of 15+ kWh.
Heat storage of 15+ kWh.
6kW Ground source heatpump.
EV BMW i3 (another 30+ kWh's of storage).
250,000+ litres of water harvested from underground river.
Home grown Fruit and Veg.
TT
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« Reply #13 on: September 03, 2019, 05:17:42 PM »

Isn't it max voltage of 750 volts?
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nowty
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« Reply #14 on: September 03, 2019, 09:38:50 PM »

Isn't it max voltage of 750 volts?

No, only the later model 4000TL-21 has that higher limit.

I run both a 4000TL-20 and 4000TL-21.

I also note that my 4000TL-20 is more susceptible to tripping out over voltage, it seems to have some extra voltage drop internally.
« Last Edit: September 03, 2019, 09:43:34 PM by nowty » Logged

11kW+ of PV installed and 54+ MWh generated.
Lithium battery storage of 50+ kWh.
Hot water storage of 15+ kWh.
Heat storage of 15+ kWh.
6kW Ground source heatpump.
EV BMW i3 (another 30+ kWh's of storage).
250,000+ litres of water harvested from underground river.
Home grown Fruit and Veg.
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