# Metric?? Electricians don't need metric! **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [March 12, 2019, 8:45pm UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025 "2019-03-12T20:45:32Z")\ **Posts on this page:** 12\ **Page:** 2
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [March 13, 2019, 9:03pm UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025/21 "2019-03-13T21:03:24Z")
> [@Doctor\_Jackson](#): > > Honest question - Why would the manufacturer label the unit as “5000 mA” instead of “5 A”? That’s like a store pricing an item at “500,000 cents” instead of $5,000" - I mean, they mean the same, but… No, it’s not really like that. In metric, it’s not all that unusual to use 4-digit numbers. The elevation of Mt Everest is 8848 meters. AA rechargeable batteries are usually around 2000 mAh capacity. The [Toyota Prius](https://en.wikipedia.org/wiki/Toyota_Prius_(XW50)) is 4540 mm long and weighs 1365 kg. Although they really should stop at 4 digits - battery packs with 13000 mAh or even [26800 mAh](https://www.amazon.com/Anker-PowerCore-Portable-Double-Speed-Recharging/dp/B01JIWQPMW/ref=sr_1_2_sspa?keywords=usb+power+bank&qid=1552510736&s=gateway&sr=8-2-spons&psc=1) are ridiculous. ---
**Author:** ![Balthisar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/balthisar/32/2825_2.png) [@Balthisar](https://boards.straightdope.com/u/Balthisar)\ **Post date:** [March 14, 2019, 11:27am UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025/22 "2019-03-14T11:27:58Z")
> [@Doctor\_Jackson](#): > > Honest question - Why would the manufacturer label the unit as “5000 mA” instead of “5 A”? That’s like a store pricing an item at “500,000 cents” instead of $5,000" - I mean, they mean the same, but… I would think that it’s meant to address the situation of the OP, but in reverse. It’s for people who have mA-labelled devices they need to power, and don’t realize that 5A is sufficient for their 500mA load. ---
**Author:** ![somepumpkin](https://avatars.discourse-cdn.com/v4/letter/s/7feea3/32.png) [@somepumpkin](https://boards.straightdope.com/u/somepumpkin)\ **Post date:** [March 14, 2019, 12:43pm UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025/23 "2019-03-14T12:43:38Z")
> [@AngelSoft](#): > > […] He’s in his late 50s so maybe that has something to do with it? :eek: ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [March 14, 2019, 12:49pm UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025/24 "2019-03-14T12:49:29Z")
> [@Doctor\_Jackson](#): > > Honest question - Why would the manufacturer label the unit as “5000 mA” instead of “5 A”? That’s like a store pricing an item at “500,000 cents” instead of $5,000" - I mean, they mean the same, but… Because the unit normally used is mA. If every item uses not just the same unit system but the same units, it’s a lot easier to compare, combine and in general use multiple items. It’s not just about labeling the specific item, but about making sure that it will get properly arranged between those which are 4800mA and 5250mA, rather than right beside the 5mA because “well, this one is 5 and this one is 5 so it’s the same, right?” (Disclaimer: I once tried to teach 4th-grade arithmetic to people who couldn’t understand that 1/3 is smaller than 1/2. 3 is larger than 2, therefore one third is larger than one half, QED, and utter confusion when doing pie-slicing exercises. They could do the pie-slicing but they couldn’t relate it to the fractions.) ---
**Author:** ![swampspruce](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/swampspruce/32/261_2.png) [@swampspruce](https://boards.straightdope.com/u/swampspruce)\ **Post date:** [March 14, 2019, 2:04pm UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025/25 "2019-03-14T14:04:48Z")
> [@Crafter\_Man](#): > > But there _are_ some goofy things in electrical engineering: > > 1. When analyzing a DC electrical circuit, we place an arrow next to the wire to signify the direction of current. But the electrons in the wire move in the _opposite_ direction. > > 2. The symbol for a physical quantity, and the symbol for its unit, are usually different. An example for mass is m = 40 kg (m and kg are different). An example for time is t = 20 s (t and s are different). And example for current is I = 45 A (I and A are different). But the symbol for voltage, and the symbol for its unit, are the _same_. 😕 An example is V = 30 V. That’s because the arrow represents the holes moving not the electrons…😃 That’s not necessarily true either. E=IR is V=AΩ ---
**Author:** ![N9IWP](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/n9iwp/32/3154_2.png) [@N9IWP](https://boards.straightdope.com/u/N9IWP)\ **Post date:** [March 14, 2019, 5:23pm UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025/26 "2019-03-14T17:23:28Z")
(could have sworn I posted this earlier) Sometimes (often?) Capacitors are rated at say 4700µF instead of the equivalent 4.7mF Brian ---
**Author:** ![Balance](https://avatars.discourse-cdn.com/v4/letter/b/ccd318/32.png) [@Balance](https://boards.straightdope.com/u/Balance)\ **Post date:** [March 14, 2019, 10:48pm UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025/27 "2019-03-14T22:48:24Z")
As a rule of thumb, the preferred prefix is usually the largest prefix that can be used without incorporating a decimal for most common items in whatever set is being measured. Decimals are harder to read than digits for many people, especially in tiny print (or in an obscure color code). Since it’s not unusual to have power supplies rated at less than 1 amp, milliamps are the unit of choice. > [@N9IWP](#): > > Sometimes (often?) Capacitors are rated at say 4700µF instead of the equivalent 4.7mF Capacitors tend to get broken up into ranges–you will see some rated in nanoFarads and picoFarads, but otherwise µF is dominant. (I have seen special-application caps rated in Farads, though.) Inertia plays a part; once people are in the habit of using a particular prefix in rating a type of item, they tend to stick to it unless it becomes really annoying. Saying “Point zero-zero-zero-five microFarads” gets annoying very quickly, though. ---
**Author:** ![Doctor\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/doctor_jackson/32/32_2.png) [@Doctor\_Jackson](https://boards.straightdope.com/u/Doctor_Jackson)\ **Post date:** [March 15, 2019, 12:49pm UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025/28 "2019-03-15T12:49:52Z")
OK, **Balance** , that makes some sense. The whole concept “avoid decimals if possible” is new to me. Being in the US, my most common use of the metric system is mostly around tools, where it is not at all uncommon to see a wrench/socket sized, for example, 7.5mm. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [March 15, 2019, 1:00pm UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025/29 "2019-03-15T13:00:01Z")
To be clear, it’s the decimal point which is hard to pick up when printed on the side of a tiny ceramic cap. But you need a chart, or a capacitance meter, or a ton of experience to read random capacitor ratings. ---
**Author:** ![Balance](https://avatars.discourse-cdn.com/v4/letter/b/ccd318/32.png) [@Balance](https://boards.straightdope.com/u/Balance)\ **Post date:** [March 15, 2019, 2:09pm UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025/30 "2019-03-15T14:09:44Z")
> [@Doctor\_Jackson](#): > > OK, **Balance** , that makes some sense. The whole concept “avoid decimals if possible” is new to me. Being in the US, my most common use of the metric system is mostly around tools, where it is not at all uncommon to see a wrench/socket sized, for example, 7.5mm. Well, manufacturers are probably more inclined to avoid decimals when the print is smaller than you typically see stamped on tools. I grabbed a random power supply from a drawer and a socket from the garage and measured the print, and the type on the socket was 3 times larger. Disclosure, however: the power supply rating made a liar out of me–or at least highlighted that labeling is inconsistent–as it was rated as “0.5A”. It’s still not easy to see a decimal in a 3.5-point font, but the leading 0 made it obvious that it was there. My rule-of-thumb may be wishful thinking. 😛 ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [March 15, 2019, 2:22pm UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025/31 "2019-03-15T14:22:22Z")
> [@Balance](#): > > Capacitors tend to get broken up into ranges–you will see some rated in nanoFarads and picoFarads, but otherwise µF is dominant. (I have seen special-application caps rated in Farads, though.) Inertia plays a part; once people are in the habit of using a particular prefix in rating a type of item, they tend to stick to it unless it becomes really annoying. Saying “Point zero-zero-zero-five microFarads” gets annoying very quickly, though. Don’t forget the weirdos who measure capacitance in centimeters. :eek: ---
**Author:** ![california\_jobcase](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@california\_jobcase](https://boards.straightdope.com/u/california_jobcase)\ **Post date:** [March 15, 2019, 11:33pm UTC](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025/32 "2019-03-15T23:33:18Z")
> [@Crafter\_Man](#): > > But there _are_ some goofy things in electrical engineering: > > 1. When analyzing a DC electrical circuit, we place an arrow next to the wire to signify the direction of current. But the electrons in the wire move in the _opposite_ direction. > > 2. The symbol for a physical quantity, and the symbol for its unit, are usually different. An example for mass is m = 40 kg (m and kg are different). An example for time is t = 20 s (t and s are different). And example for current is I = 45 A (I and A are different). But the symbol for voltage, and the symbol for its unit, are the _same_. 😕 An example is V = 30 V. What? No more E for electromotive force, as in E=I x R, or P=I x E? [Previous page](https://boards.straightdope.com/t/metric-electricians-dont-need-metric/831025.md?page=1)