Correct wire size for length 2

[paintballJim]

This is probably a trivial question for those of you with experience but I would like to avoid a dumb mistake. I am attempting to select the correct wire gage to run a 12V motor. This is a 1//2 hp 45 amp (FLA) with a 30 min. duty cycle. The power cord will be 25'. The American Wire Gage table shows that a 10 ga wire is rated to handle 55 amps and since this is a chassis wiring application I read that the table values are a little over conservative so i may be able to push the limits a bit.
The thing that has me a little concerned is that our vendor that we buy the pump and motor from was supplying a 20' 8ga power cord but this fall switched from 8ga to 10ga. Using the table and calculator supplied at Power Stream AWG Table I see that using 10ga wire will give them a 3.6V drop on their 20' cord and us a 4.6V drop with a 25' cord. I am assuming that my pump will run slightly slower, but will there be any other ramifications? I would be inclined to go with the 8ga to be safe but the 10ga is 1/3 the cost and I am being pushed to keep costs down as usual.
what calculations should a mechanical learn to be able to make an intelligent decision on this issue? Or am I just worrying over nothing?

Thanks guys.
Jim S

 

[VEBill]

Your instincts are correct.

In addition to the minimum wire gauge for the current (heat/safety), you also need to perform a Voltage Drop Analysis to make sure that the motor will be happy with the supply voltage (at the motor).

This should also consider start-up current.

 

[MiketheEngineer]

And your wire might heat up a bit and since the mfg suggested a size you do not want to use - he may void any warranties.

And how much money are we talking about??? a few bucks

Buy the right cable and suck it it up..

 

[paintballJim]

Mike,
You misunderstand. The manufacturer stepped down to a THINNER gage wire. I want the wire to be 5 feet longer than what he is providing so I am attempting to figure out if I can follow his lead and use the lighter gage wire or if I should stay with the heavier wire. The difference is $40, which for one or two applications is not much. But for the projected hundreds of units a year it could add up. You know, the old, a billion here and a billion there and pretty soon you are talking real money.

 

[IRstuff]

Why do you need to put your 12V source so far away from the motor?

TTFN
faq731-376

 

[paintballJim]

We are manufacturing a trailer to haul fuel to tractors and equipment in the field. The pump and motor will be on the trailer pulled behind a pickup. We could mount a battery on the trailer, but it still needs to be charged. So the standard operation is to have a cord that runs from the pump to the battery on the pickup so that the alternator can keep the battery charged up. some manufacturers make the customer unhook the trailer once it is close to the tractor and turn it around so the pickup battery is close to the motor and pump but we would like to be a little more user friendly.

Jim

 

[OperaHouse]

Is speed of fueling an issue? If not smaller wire would be fine. Have you ever measured the actual current? You are pumping just to overcome head pressure. The currents are likely less than you think.

 

[paintballJim]

VE1BLL,
How do I determine if the motor is able to handle the voltage drop. I have determined that the drop will be 4.6V. I also know that the manufacturer must be comfortable with a 3.6V drop as that is what the power cord he is providing will produce. I am obtaining the same cord as the manufacturer is using from the same wiring vendor.

 

[VEBill]

You either consult the detailed specifications on the motor where it hopefully provides this level of detail (they often don't), or you build a prototype and run some tests.

You wrote that there's a battery on the trailer and this cable under discussion is to keep the trailer battery topped up from the track's electrical system. This raises all sorts of interesting questions and failure modes.

As you have a power source at both ends, the cable should be protected with an appropriate value fuse at both ends.

If the local battery on the trailer has enough capacity to pump one load of fuel start to finish, then the recharging system could be current limited (electronics) and then you could optimize the recharging system and perhaps use smaller gauge wire. There would have to be some analysis of usage patterns, duty cycle, recharging time, cost trade offs, etc. Probably thicker wire is best overall; copper isn't as expensive as it was a few years ago.

If the local battery doesn't have enough capacity to complete one pumping operation, then the truck's electrical system must take over halfway through the job, then what's the local battery for? Acknowledge that there might be a mix of small jobs and large jobs.

 

[waross]

The simple answer is that if you need the total design capacity of the pump then you won't get it with low voltage.
The complex answer depends on the size and condition of the local battery and the size and condition of the truck battery, and on the duty cycle.
We can give you tips but we don't have enough information to engineer the system for you.
But consider: With a local battery connected to the truck battery through a cable both batteries will contribute to the load current.
With a decent sized local battery the current through the cable will be much less than full motor current, but will increase as the local battery is depleted. The current will continue after pumping is completed until the local battery is recharged.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[IRstuff]

It's still a bit hazy how long the battery has to supply its 40A, how long does it get to rest, and how much of a day is it used in that manner.

Without that, it is difficult to come up with an optimum configuration.

TTFN
faq731-376

 

[ScottyUK]

If you can mount the pump battery locally to the pump then the volt-drop problem will be much less because the charging current will be lower than the load current.

This sounds very similar to an RV battery application where two batteries charge from a common source but power two independent systems. Might be worth a look at how that industry has solved its problems. Word of warning: from a quick look at Google there are some RV-ers who I wouldn't permit to wire up an electric chair because they'd make it too dangerous...

 

[VEBill]

If you can mount the pump battery locally to the pump...

It is, right?

...then the volt-drop problem will be much less because the charging current will be lower than the load current.

If the local (trailer mounted) battery is totally flat when it's connected to the truck (being a trailer this can easily happen), then the charging current may be extremely high (potentially far more than 40A). Hopefully a fuse will pop before anything burns up.

If the pump is also switched on while the local battery is still presenting a near dead short to the truck's electrical system, then add another 45 amp for the pump.

The charging current will only be 'lower' if there's something to limit it. The OP hasn't mentioned anything about active circuits to limit the current.

 

[LionelHutz]

The motor can handle the voltage drop just fine. It's a DC motor so it will just run slower and produce less output power as the voltage is lowered. The question is will your pump be useful at the slower speed?

Actually, after reading your explanation again, I'm now seeing that this system does not have it's own battery. You are supposed to pop the hood and connect the power wire to the truck battery when doing fueling operations. If yes, then why not use the trailer plug power and a trailer mounted battery and allow the wiring through the trailer plug to recharge the trailer battery between fueling operations? I suppose this still comes down to the duty cycle. FYI, "with a 30 min. duty cycle" is not giving a duty cycle since you forgot the amount of off time. A trailer mounted battery could also allow the trailer to be dropped off somewhere and some amount of fueling to still occur. You really didn;t give enough information to make a decision on this - such as these trailers typically being left outside unused all winter with the battery left connected and then expecting the battery to still hold a charge in the spring.

Alternatively, what are you using as the power cord? Is it the typical multi-strand cabtyre type wire? If yes, check into using a cable with more conductors and paralleling them. Look at using a 4 or 6 conductor 12awg cable.

 

[paintballJim]

Thank you for your input guys. I understand that you can't do the design work for me. I guess my question is more "where do I look to determine how to engineer it myself". Being a mechanical I don't have the ready resources and background at my finger tips.
The local battery is an option but I would prefer not to have it as I think it causes as much work as it solves. The motor will run between 10 to 30 min. if it is running at full capacity. The cable from the pump to the pickup battery would only be attached during the pumping operation and the rest of the time recharging a local battery would depend on the
pickup's wiring harness and an inline trickle charger which would not charge enough during the down times and could easily over tax the pickup harness. I think what it comes down to is to use VE1BLL's suggestion and find out if the motor can handle the voltage drop and run a test knowing that the capacity will be reduced if we go with the smaller gage wire.

Thanks

Jim

 

[paintballJim]

Lionel,
Yes you have the plan. I am concerned that the pickup wiring will not be sufficent to handle the load of recharging the battery in the length of time needed. I can control the wire gage if the power cord, but I have no control over what the different pickups will have. plus, the battery would cost more than simply using the heavier gage of wire. It would have the ability to run the pump if the trailer is left in the field as you suggest, so it will be offered as an option but I would prefer not to rely on it. I can foresee a case where the pump is run for 30 min. in the field and then the tank is refilled and needs to pump again in less than 2 hours so I do not foresee the inline chargers I have found being able to recharge the battery in that amount of time.

 

[TomDOT]

Okay, if 8ga costs 3x as much as 10ga, and we're thinking that 10ga is marginal...

I'm not an electrical guy, but is there any reason you can't just double up on the 10ga? 2x 2x10ga gives you both higher capacity, and lower cost than single 8ga, with some marginal added installation time and hassle.

 

[waross]

Just supply #8 AWG as an $80 option. Most units will work with the #10 AWG. Heavy use installations which need the fastest pumping cycle should go for the cable upgrade.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[paintballJim]

Bill,
I think this is the way we will be going. We are offering several different options as to how the system is setup so I think this should work. It would appear we are on the edge and the system will work with the lighter wire but we just won't reach the maximum rated gpm.