24 VDC, 150 Amp Wire Sizing Question

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aanvari3

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Okay so we're installing a battery charger and batteries that needs to feed an existing 24 Volt DC panel that has a 150 amp circuit breaker on it, so the wires from the batteries should be sized to carry 150 amps with less than 3% voltage drop right? Okay, so the place they want to put the battery charger and batteries is about a 100 feet run from the existing 24 VDC panel. Using the wire sizing calculator (from http://www.powerstream.com/Wire_Size.htm) i got size 500 kcmil for a 2.67% voltage drop. Can i parallel say 3 #3/0 wires for the positive and negative runs which would then be looking at 100 feet runs of 50 amps each which leaves a 2.65% voltage drop for each #3/0 wire?

Am i calculating this correctly, is there a better way to wire the panel using smaller wires? I'd like to just keep the battery charger and batteries in the same room as the 24 v dc panel but unfortunately its completely full, and another closer room has no room for a heater/air conditioner in it for the batteries.
 
aanvari3 said:
Okay so we're installing a battery charger and batteries that needs to feed an existing 24 Volt DC panel that has a 150 amp circuit breaker on it, so the wires from the batteries should be sized to carry 150 amps with less than 3% voltage drop right? Okay, so the place they want to put the battery charger and batteries is about a 100 feet run from the existing 24 VDC panel. Using the wire sizing calculator (from http://www.powerstream.com/Wire_Size.htm) i got size 500 kcmil for a 2.67% voltage drop. Can i parallel say 3 #3/0 wires for the positive and negative runs which would then be looking at 100 feet runs of 50 amps each which leaves a 2.65% voltage drop for each #3/0 wire?

Am i calculating this correctly, is there a better way to wire the panel using smaller wires? I'd like to just keep the battery charger and batteries in the same room as the 24 v dc panel but unfortunately its completely full, and another closer room has no room for a heater/air conditioner in it for the batteries.
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I guess part of the answer lies in where you came up with the 3% VD from? As for your calculations, they seem OK.

Most of the time the manufacturer will spec out what the VD can be.
 
090327-0849 EST

I sort of do not understand your question.

Why put the battery 100 feet away from the load (your panel with the 150 A breaker)?

A battery is a relatively low impedance and constant voltage source. Ideally the battery should be next to the load. Then the charger can be as far from the battery as reasonable, a 1000 ft if you want. If the charger's maximum charging current is only 20 A, then you only need charging wires that can handle the 20 A. Pick any other charging current values you desire to determine the charging wire size. Voltage drop on the charging wires should be no concern with the correct charger. To feedback voltage information from the battery to the charger you only need maybe #24 wire, but would likely use #16 for mechanical reasons.

.
 
They want the batteries in a room thats heated and air conditioned, the original room they were in had no place for an air conditioner or heater (this is in wisconsin). The room the 24 VDC panel is in doesn't have space for the batteries either. So they want to put the charger and batteries in this new building thats heated and has room for them, and its really close to the panel but after running up and down and in a cable tray between the two buildings its about a 100 feet run.
 
090327-0919 EST

What is the load on the 24 V panel and what is the voltage range the load can tolerate?

Can you create a much shorter route for the cables?

What is the actual load current and how does it vary with time?

Using arbitrary generalized rules in a special application may not work. Suppose this load draws 600 A for 1 second and is off for 9 seconds repetitively. If the 150 A breaker is a thermal breaker it won't trip, but a voltage drop design based on 100 A continuous design probably would be inadequate.

.
 
petersonra said:
I guess part of the answer lies in where you came up with the 3% VD from? As for your calculations, they seem OK.

Most of the time the manufacturer will spec out what the VD can be.
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After reading some later posts it occured to me that you may wish to look at what the actual load is likely to be off the batteries, rather than relying on the size of the CB. Its quite possible the actual load is much lower than 150A.
 
Is the new battery charger/battery combination functioning as a secondary (or backup) power source, or are you installing a power supply/battery charger combo unit and a set of batteries to replace the existing 24VDC panel power supply?
 
I do this all the time with DC battery plants. You are correct you do not use NEC table 310 to determine the cable size, you design it by voltage drop, then make sure it meets the minimum NEC requirement. FWIW if the run is over 15 feet, it should exceed NEC.

Anyway the formula I use will look familar to most folks.

CM = (22.2 x I X D)/ VD

Where

CM = Circular Mills of cable size needed.
22.2 = Constant
I = Max load current in Amps
D = One-way cable distance
VD = Voltage Drop

Typically on a 24 VDC system Telco design for a maximum voltage drop from the battery terminals to the last distribution panel = 1 volt. That leaves .5 volt on the branch distribution for a total of 1.5 volts max.

As for cable it is common to parallel cable runs to make it easier to work with. Also we use Class H and I cable stranding aka Extra-Flex cable. A lot of the cable used is dual rated DLO/RHH/RHW. You just have to be very careful in termination using long barreled lugs with inspection holes, and a crimper made for the job that uses color coded dies like a TBM-14 hydralic crimper or Burndy product line.

Let you in on a little secret, For the terminal on the battery term plates, use lead plated lugs and coat the conductors with No-Ox-Id before compressing the terminals onto the cables. In fact we require all wire skinners to have No-Ox-Id applied before termination. If you do that you get a fail-proof 50 plus year termination that will pass a 15 micro-ohm or less test.
 
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It seems to me that the charger adjusts it's voltage to reach a certain current and therefore will self compensate for VD.
 
Okay, new question on battery sizing. If the power source for the battery charger dies, we need the batteries to run for 1.5 hours. Again, i'm going to assume full load so i'll use the circuit breaker 150 amps, i know it should never reach that much or it'll trip in the first place. The batteries we have quoted to us are twelve 2 volt cell batteries, i assume they are in series for 24 volts. GNB absolyte IIP valve regulated lead acid batteries. The quote says 256 ah battery provides 214 amps for 30 minutes.

I'm a little iffy on how thats calculated. Is it as easy as saying i need 150 amps for 1.5 hours, so 225 ah?
 
aanvari3 said:
Okay, new question on battery sizing. If the power source for the battery charger dies, we need the batteries to run for 1.5 hours. Again, i'm going to assume full load so i'll use the circuit breaker 150 amps, i know it should never reach that much or it'll trip in the first place. The batteries we have quoted to us are twelve 2 volt cell batteries, i assume they are in series for 24 volts. GNB absolyte IIP valve regulated lead acid batteries. The quote says 256 ah battery provides 214 amps for 30 minutes.

I'm a little iffy on how thats calculated. Is it as easy as saying i need 150 amps for 1.5 hours, so 225 ah?
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yes, however, the amp-hr rating on batteries is usually stated at a certain discharge rate, and at higher discharge rate, the capacity is much lower, as the quote indicates.

214 amps @ 30 minutes = 107 amp-hr for cells rated at 256 ah (probably a ten hour rating).
 
So I think you have answered my previous question - you are using a combination power supply/battery charger as your primary power supply and the new set of batteries as secondary (backup).

Here is the spec document for the GNB batteries you were quoted.

http://industrialenergy.exide.com/exidepdfs/Absolyte_26_10.pdf?t=28957

If your quoted battery is the 90A/6-90A07 (which has a 256Ah rating), it has the following rating:

AMPS to final 1.75 volts per cell at 77 degF (or 21.0 V):

For your size (which might be the 90due to the 256Ah you mentioned),

30 mins at 214A
60 mins at 151A
120 mins at 93A

So, your 1.5hrs would fall between 93A and 151A (it is not linear).

To supply a 150A load for 1.5 hrs, the chart would indicate the next size cell, at 344Ah before adjustments for temperature or a higher final voltage.

Your vendor most likely went by your load info and not the 150A current.

kent
 
iwire said:
It seems to me that the charger adjusts it's voltage to reach a certain current and therefore will self compensate for VD.
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Not really Bob. A commercial rectifier is a constant voltage regulator of 27.03 volts

Here is the poop. A 24 volt battery plant under normal operating conditions operates @ 27 VDC, and around 29 VDC max in equalize condition. But forget that for a minute because it is not important for this discussion.

A battery plant like this in its purest form is a UPS, where the clock keeps on ticking even when the power takes a licking or goes off OK? Battery plants are designed to keep the equipment running for X amount of hours when the power fails.

The equipment like any other equipment has an input operating voltage range. Typically to meet Bellcore (telephone companies) specs is a very wide window of 20 to 30 volts input before it shuts down. At first appearance that seems like a huge window, and it is, but imagine that tuff spec in the AC world.

As soon as the power goes off the voltage immediately drops from 27 to 24 volts or a little less at the battery terminals. Now factor in the voltage drop of say 1.5 volts or more at the equipment, which means it sees 22.5 volts or less and the equipment dies at 20 volts. You do not have much room to work with.

Ok time goes on and the battery voltage decays. As the battery voltage drops, the equipment is constant power, so the current goes up as the voltage drops. Seeing the picture?

OK a battery at full charge is about 2 volts per cell, and at 1.75 volts at 100% fully discharge. A 24 volt plant has 12 cells. So now as time passes and the batteries discharge to 1.75 volts per cell or 21 volts at the battery terminals, if you do not control the voltage drop to the equipment, the equipment fails long before the batteries are exhausted.

So if you have a 3 volt drop on the cable (way too high on 24 volt systems), and the equipment dies at 20 volts, batteries operating range on discharge is 24 down to 21, do the math. So normal operating voltage does not mean squat. It is the discharge or lights out that means everything so NEC and normal logic goes out the window.
 
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iwire said:
It seems to me that the charger adjusts it's voltage to reach a certain current and therefore will self compensate for VD.
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As noted above, not really but... the charger voltage sense lines can connect to the battery lugs to get the actual battery voltage, not the voltage output by the charger 100 ft of copper away. Some chargers make this easy, others, well, no hope at all. But remote sensing does solve the "remote charger" problem.
 
dbuckley said:
As noted above, not really but... the charger voltage sense lines can connect to the battery lugs to get the actual battery voltage, not the voltage output by the charger 100 ft of copper away. Some chargers make this easy, others, well, no hope at all. But remote sensing does solve the "remote charger" problem.
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DB it depends on the plant configuration, and where the discharge buss is located. Most configurations the charge and discharge buss are located directly above the and no load current flows to or from the batteries other than an amp or two to overcome self dischage. It is only when the rectifiers turn off that currents flows from the batteries.
 
dbuckley said:
As noted above, not really but... the charger voltage sense lines can connect to the battery lugs to get the actual battery voltage, not the voltage output by the charger 100 ft of copper away. Some chargers make this easy, others, well, no hope at all. But remote sensing does solve the "remote charger" problem.
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But, as the battery approaches full charge, the current (and thus, the voltage drop) reduces, so the voltage at the charger terminals more closely reflects the battery voltage, and full charge should be sensed fairly accurately.
 
It's funny how we all think, I hear battery charger with a 150 amp 24 VDC output and I think of something like this

Used%20Fork%20Battery%20Charger


Which is a battery charger for electric industrial vehicles. And with a unit like this I do believe it would take care of voltage drop by simply by applying more voltage to the circuit and as the battery became charged the VD would drop to insignificant values. There is only charging going on with this, no discharging.

On the other hand Dereck hears battery charger and he immediately thinks Telco plant. In this case I do believe Dereck is right and I was wrong. (Not that it surprises me :smile:)
 
iwire said:
It's funny how we all think, I hear battery charger with a 150 amp 24 VDC output and I think of something like this


On the other hand Dereck hears battery charger and he immediately thinks Telco plant.
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Yes we all have our little box we think inside of. But from the description the OP gave sure sounded like a Telco like battery plant with dirstibution.

FWIW I do have a hot rod electric golf cart I modified, and it uses a charger simular to the pic you show of a fork lift battery charger.

If you are interested the type of charger you have is for things like forklifts. Those types are constant current meaning the voltage is whatever it takes to push a set amount of current up to a point when the voltage reaches full charge potential, then the voltage regulates, or constant voltage, and the the current tapers back until the timer goes off and the unit shuts down or just reamins in float voltage until turned off. Once the battery is fully charged and the charger is in float voltage mode current is almost nothing so voltage drop is not an issue.
 
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Okay, third question. In the NEC 480.9(A) it says that valve regulated battereis are referred to as "sealed" but require ventilation because they still emit gas. Okay, so the room we are putting the batteries has ventilation to outside, but the room opposite the ventilation has an air condition. We are planning on cutting a hole through the rooms to place a vent to allow the air condition in the opposite room to cool/heat the battery room. So the battery room will have a vent to outside, and a vent to another room that will be blowing cool/hot air in. This okay?
 
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