confused about AWG and max Amp for wires

[Maltes]

Can someone help me with this chart?

http://www.powerstream.com/Wire_Size.htm

Someone can explain the difference between "Maximum amps for chassis wiring" and "Maximum amps for
power transmission"? Theres a very big difference between the two numbers.

Asking this for wires inside the projector case.

[CountFunkula]

The chassis wiring column is the rating for very short lengths of interconnect between components/modules within your device. The power transmission column is the rating for longer runs where you are bringing power from mains to a device. Needless to say, this also includes situations like the gauge for the length of interconnect between something like a generator and a distro.

In this case, you'd want to use the chassis wiring column, but it never hurts to do some additional de-rating.

[djeric68]

On that website they use "maximum amps for power transmission" to represent the condition of many wires in a conduit or raceway; i.e. a confined area where heat from the conductors will build up and cause overheating problems. The NEC (National Electric Code) has rules for how much current can flow through a given conductor size. The NEC rules are used to prevent electrical fires. Basically the higher the temperature, the less current the cable can carry.



From http://www.southwire.com/support/DeratingAmpacities.htm

"Derating Ampacities

There are two requirements in the NEC for derating ampacities: (1) ambient temperature and (2) number of conductors in a raceway or cable. There are other code sections that affect the size of conductor to be used (e.g., motor circuits or continuous loads) but only the two requirements above are used to regulate ampacity to keep the conductor within its maximum temperature rating.

NEC Section 310-10 contains the basic rule that, in effect, requires derating for the two reasons above. Section 310-10 states that no conductor shall be used in such a manner that its operating temperature will exceed that for which it is designed. Then, four items are mentioned that are the principal determinants of operating temperature: (1) ambient temperature, (2) heat generated internally in the conductor as a result of current flow, (3) the rate at which generated heat dissipates from the conductor, and (4) adjacent current-carrying conductors. It is items (1) and (4) for which derating is done.

DERATING FOR AMBIENT TEMPERATURE
All ampacity tables necessarily must use some ambient temperature as a basis and NEC Table 310-16 uses 30°C (86°F). At the bottom of the table, correction factors are given for situations where the ambient is expected to be higher or lower than 30°C (86°F). Use of these factors is fairly straightforward; for example, what is the ampacity of a 1/0 AWG, aluminum, type THHN conductor when the ambient temperature is 100°F? The answer is found by taking the ampacity from the table and multiplying it by the appropriate correction factor. In this example, we would have 135 x 0.91 = 122.85 amperes.

DERATING FOR NUMBER OF CONDUCTORS
Note 8 to the ampacity tables in the NEC contains the requirement columns for derating ampacity because of adjacent current-carrying conductors. This note states that when the number of conductors in a raceway or cable exceeds 3, the ampacities are to be reduced by the appropriate percentage. In the 1999 NEC, a column of factors is given. For example, what is the ampacity of twelve No. 12 copper THHN conductors installed in one conduit? From Table 310-16 the ampacity (in the table) is 30 amperes. From Note 8, the derating factor for 12 conductors is 50 percent. 30 x 0.5 = 15 amperes per conductor.

DERATING FOR BOTH AMBIENT TEMPERATURE AND NUMBER OF CONDUCTORS
When derating for both conditions is necessary, then both calculations must be made. For example, what is the ampacity of four 1/0 THW copper conductors when the ambient temperature is expected to reach 110°F? From Table 310-16 the ampacity is 150 amperes, derating factor for 110°F is 0.82, and derating factor from Note 8 for 4 conductors is 80 percent. 150 x 0.82 x 0.8 = 98.4 amperes."

So, the website you linked to takes the NEC ampacity tables, applies the appropriate derating factor(s) and created the table of approximate values. This gives users a "ball park" estimate of the current carrying capacities. Just remember..."As you might guess, the rated ampacities are just a rule of thumb. In careful engineering the voltage drop, insulation temperature limit, thickness, thermal conductivity, and air convection and temperature should all be taken into account"

Hope this helps.

[Maltes]

The chassis wiring column is the rating for very short lengths of interconnect between components/modules within your device. The power transmission column is the rating for longer runs where you are bringing power from mains to a device. Needless to say, this also includes situations like the gauge for the length of interconnect between something like a generator and a distro.

In this case, you'd want to use the chassis wiring column, but it never hurts to do some additional de-rating.

Thank you. I understand that these numbers are a rule of thumb, but wouldn't hurt to know what numbers to assume for very short lenghts and very long. Is it less then one meter? Less than that? More?

[catalanjo]

Thank you. I understand that these numbers are a rule of thumb, but wouldn't hurt to know what numbers to assume for very short lenghts and very long. Is it less then one meter? Less than that? More?

Nothing beats "suck it and see" ....use good quality silicon covered (very- multistrand) wire, use double expected current flow wrapped round a thermometer and if this moves >1C get a bigger wire.(assume that most tables are paranoid albeit with very good reason).

Answer depends on so many parameters including one totally unknown (wire quality) that rule of thumb is unreliable unless from someone's direct experience matching your exact needs.

DTR spent quite while researching this ...why not try sending him a PM.

There is also a guy on LPF who specialises in this wire but can't remember his name (search there for silicon wire).

Cheers

[djeric68]

Thank you. I understand that these numbers are a rule of thumb, but wouldn't hurt to know what numbers to assume for very short lengths and very long. Is it less then one meter? Less than that? More?

Regarding the length: all wire has resistance of xx#ohms/foot. For long runs, the resistance causes a reduction in voltage (voltage drop). When voltage drops (decreases) the current increases. Increased current causes increased heating. NEC requires voltage drop to be less than 5%. Inside a projector this is not a problem to worry about. Another rule of thumb is that you don't need to consider the length of the cable until the length is greater than the voltage carried on the cable - ex. if 200' of wire carrying 120v, check that the size cable can meet the voltage drop requirements.


In general, are you asking about ampacity regarding the power (120/240VAC) wiring inside the projector, the power leads to LD drivers and LD's, or the wire for the low voltage/low current "signals" inside the projector?

I ask because the mains power wire only carries about 5A or less and you will be fine with 16 or 18AWG wire for internal power connections. The mains power cord that comes with projectors is usually 16 or 18AWG.

For the other signal (ILDA) wires there will usually be 0-5V DC on these at very low current - so these should not be to sensitive to the wire size.

Cable size for the power from the scanner power supplies to the galvo amplifiers is important. If the wire is too small, not enough current will be available to the amplifiers and galvo performance will be suffer.

With all of this said, if you use the "chassis wiring" values you will be ok. Derating needs to be accounted for only after the correct size cable is selected, and then it is derated (carries less current) to account for overheating due to bundling cables together, long runs, or runs of multiple conductors in a conduit. None of these conditions apply inside a projector, so using the "chassis wiring" values is appropriate.

[planters]

Higher quality wire will have better conductivity, but the gauge trumps all. Another, practical consideration is the flexibility of the wire. Very fine stranded wire with highly flexible silicone insulation will be much easier to route around corners for a given gauge.

Personally, I believe the recommended gauges, even for chassis wiring is very conservative. Consider the copper leads that are mounted on the diodes. The big blue lasers can safely handle 5A and are case isolated, so they are not being very effectively cooled by the diode flange. I have run these lasers with 26G hook up leads although I routinely use 24G wire "just to be safe".

The length should be irrelevant for our purposes. If you double the length and therefore double the resistance, you have also doubled the surface area for heat loss.

[catalanjo]

@ "If you double the length and therefore double the resistance, you have also doubled the surface area for heat loss. "

This is an important detail that lots of people do not realise...always assuming same circumstances for entire length (no tunnels,knots, sleeves.crossing hot components etc.)
Cheers